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Food Web: Concept and Applications

Introduction

There are two types of food chains: the grazing food chain, beginning with autotrophs, and the detrital food chain, beginning with dead organic matter (Smith & Smith 2009). In a grazing food chain, energy and nutrients move from plants to the herbivores consuming them, and to the carnivores or omnivores preying upon the herbivores. In a detrital food chain, dead organic matter of plants and animals is broken down by decomposers, e.g., bacteria and fungi, and moves to detritivores and then carnivores.

Food web offers an important tool for investigating the ecological interactions that define energy flows and predator-prey relationship (Cain et al. 2008). Figure 1 shows a simplified food web in a desert ecosystem. In this food web, grasshoppers feed on plants; scorpions prey on grasshoppers; kit foxes prey on scorpions. While the food web showed here is a simple one, most feed webs are complex and involve many species with both strong and weak interactions among them (Pimm et al. 1991). For example, the predators of a scorpion in a desert ecosystem might be a golden eagle, an owl, a roadrunner, or a fox.

The idea to apply the food chains to ecology and to analyze its consequences was first proposed by Charles Elton (Krebs 2009). In 1927, he recognized that the length of these food chains was mostly limited to 4 or 5 links and the food chains were not isolated, but hooked together into food webs (which he called "food cycles"). The feeding interactions represented by the food web may have profound effects on species richness of community, and ecosystem productivity and stability (Ricklefs 2008).

Types of Food Webs

Applications of food webs, food webs are constructed to describe species interactions (direct relationships)..

The fundamental purpose of food webs is to describe feeding relationship among species in a community. Food webs can be constructed to describe the species interactions. All species in the food webs can be distinguished into basal species (autotrophs, such as plants), intermediate species (herbivores and intermediate level carnivores, such as grasshopper and scorpion) or top predators (high level carnivores such as fox) (Figure 1).

These feeding groups are referred as trophic levels. Basal species occupy the lowest trophic level as primary producer. They convert inorganic chemical and use solar energy to generate chemical energy. The second trophic level consists of herbivores. These are first consumers. The remaining trophic levels include carnivores that consume animals at trophic levels below them. The second consumers (trophic level 3) in the desert food web include birds and scorpions, and tertiary consumers making up the fourth trophic level include bird predators and foxes. Grouping all species into different functional groups or tropic levels helps us simplify and understand the relationships among these species.

Food webs can be used to illustrate indirect interactions among species.

Indirect interaction occurs when two species do not interact with each other directly, but influenced by a third species. Species can influence one another in many different ways. One example is the keystone predation are demonstrated by Robert Paine in an experiment conducted in the rocky intertidal zone (Cain et al. 2008; Smith & Smith 2009; Molles 2010). This study showed that predation can influence the competition among species in a food web. The intertidal zone is home to a variety of mussels, barnacles, limpets, and chitons (Paine 1969). All these invertebrate herbivores are preyed upon by the predator starfish Pisaster (Figure 3). Starfish was relatively uncommon in the intertidal zone, and considered less important in the community. When Paine manually removed the starfish from experimental plots while leaving other areas undisturbed as control plots, he found that the number of prey species in the experimental plots dropped from 15 at the beginning of the experiment to 8 (a loss of 7 species) two years after the starfish removal while the total of prey species remained the same in the control plots. He reasoned that in the absence of the predator starfish, several of the mussel and barnacle species (that were superior competitors) excluded the other species and reduced overall diversity in the community (Smith & Smith 2009). Predation by starfish reduced the abundance of mussel and opened up space for other species to colonize and persist. This type of indirect interaction is called keystone predation.

Food webs can be used to study bottom-up or top-down control of community structure.

Top-down control occurs when the population density of a consumer can control that of its resource, for example, predator populations can control the abundance of prey species (Power 1992). Under top-down control, the abundance or biomass of lower trophic levels depends on effects from consumers at higher trophic levels. A trophic cascade is a type of top-down interaction that describes the indirect effects of predators. In a trophic cascade, predators induce effects that cascade down the food chain and affect biomass of organisms at least two links away (Ricklefs 2008). Nelson Hairston, Frederick Smith and Larry Slobodkin first introduced the concept of top-down control with the frequently quoted "the world is green" proposition (Power 1992; Smith & Smith 2009). They proposed that the world is green because carnivores depress herbivores and keep herbivore populations in check. Otherwise, herbivores would consume most of the vegetation. Indeed, a bird exclusion study demonstrated that there were significantly more insects and leaf damage in plots without birds compared to the control (Marquis & Whelan 1994).

Food webs can be used to reveal different patterns of energy transfer in terrestrial and aquatic ecosystems.

As a diagram tool, food web has been approved to be effective in illustrating species interactions and testing research hypotheses. It will continue to be very helpful for us to understand the associations of species richness/diversity with food web complexity, ecosystem productivity, and stability.

References and Recommended Reading

Cain, M. L., Bowman, W. D. & Hacker, S. D. Ecology . Sunderland MA: Sinauer Associate Inc. 2008.

Cebrian, J. Patterns in the fate of production in plant communities. American Naturalist 154 , 449-468 (1999)

Cebrian, J. Role of first-order consumers in ecosystem carbon flow. Ecology Letters 7 , 232-240 (2004)

Elton, C. S. Animal Ecology . Chicago, MI: University of Chicago Press, 1927, Republished 2001.

Knight, T. M., et al. Trophic cascades across ecosystems. Nature 437 , 880-883 (2005)

Krebs, C. J. Ecology 6 th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.

Marquis, R. J. & Whelan, C. Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects. Ecology 75 , 2007-2017 (1994)

Molles, M. C. Jr. Ecology: Concepts and Applications 5 th ed. New York, NY: McGraw-Hill Higher Education, 2010.

Paine, R. T. The Pisaster-Tegula interaction: Prey parches, predator food preferences and intertide community structure. Ecology 60 , 950-961 (1969)

Paine, R. T. Food web complexity and species diversity. The American Naturalist 100 , 65-75 (1966)

Paine, R. T. Food webs: Linkage, interaction strength and community infrastructure. Journal of Animal Ecology 49 , 667-685 (1980)

Pimm, S. L., Lawton, J. H. & Cohen, J. E. Food web patterns and their consequences. Nature 350 , 669-674 (1991)

Power, M. E. Top-down and bottom-up forces in food webs: do plants have primacy? Ecology 73 , 733-746 (1992)

Schoender, T. W. Food webs from the small to the large. Ecology 70 , 1559-1589 (1989)

Shurin, J. B., Gruner, D. S. & Hillebrand, H. All wet dried up? Real differences between aquatic and terrestrial food webs. Proc. R. Soc. B 273 , 1-9 (2006) doi:10.1098/rspb.2005.3377

Smith, T. M. & Smith, R. L. Elements of Ecology 7 th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.

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Module 26: Ecology and the Environment

Food chains and food webs, learning outcomes.

• Differentiate between food chains and food webs and recognize the importance of each

The term “food chain” is sometimes used metaphorically to describe human social situations. In this sense, food chains are thought of as a competition for survival, such as “who eats whom?” Someone eats and someone is eaten. Therefore, it is not surprising that in our competitive “dog-eat-dog” society, individuals who are considered successful are seen as being at the top of the food chain, consuming all others for their benefit, whereas the less successful are seen as being at the bottom.

Figure 1. These are the trophic levels of a food chain in Lake Ontario at the United States-Canada border. Energy and nutrients flow from photosynthetic green algae at the bottom to the top of the food chain: the Chinook salmon.

The scientific understanding of a food chain is more precise than in its everyday usage. In ecology, a food chain is a linear sequence of organisms through which nutrients and energy pass: primary producers, primary consumers, and higher-level consumers are used to describe ecosystem structure and dynamics. There is a single path through the chain. Each organism in a food chain occupies what is called a trophic level . Depending on their role as producers or consumers, species or groups of species can be assigned to various trophic levels.

In many ecosystems, the bottom of the food chain consists of photosynthetic organisms (plants and/or phytoplankton), which are called primary producers . The organisms that consume the primary producers are herbivores: the primary consumers . Secondary consumers are usually carnivores that eat the primary consumers. Tertiary consumers are carnivores that eat other carnivores. Higher-level consumers feed on the next lower tropic levels, and so on, up to the organisms at the top of the food chain: the apex consumers . In the Lake Ontario food chain shown in Figure 1, the Chinook salmon is the apex consumer at the top of this food chain.

One major factor that limits the length of food chains is energy. Energy is lost as heat between each trophic level due to the second law of thermodynamics. Thus, after a limited number of trophic energy transfers, the amount of energy remaining in the food chain may not be great enough to support viable populations at yet a higher trophic level.

The loss of energy between trophic levels is illustrated by the pioneering studies of Howard T. Odum in the Silver Springs, Florida, ecosystem in the 1940s (Figure 2). The primary producers generated 20,819 kcal/m 2 /yr (kilocalories per square meter per year), the primary consumers generated 3368 kcal/m 2 /yr, the secondary consumers generated 383 kcal/m 2 /yr, and the tertiary consumers only generated 21 kcal/m 2 /yr. Thus, there is little energy remaining for another level of consumers in this ecosystem.

Figure 2. The relative energy in trophic levels in a Silver Springs, Florida, ecosystem is shown. Each trophic level has less energy available and supports fewer organisms at the next level.

There is a one problem when using food chains to accurately describe most ecosystems. Even when all organisms are grouped into appropriate trophic levels, some of these organisms can feed on species from more than one trophic level; likewise, some of these organisms can be eaten by species from multiple trophic levels. In other words, the linear model of ecosystems, the food chain, is not completely descriptive of ecosystem structure. A holistic model—which accounts for all the interactions between different species and their complex interconnected relationships with each other and with the environment—is a more accurate and descriptive model for ecosystems. A food web is a graphic representation of a holistic, non-linear web of primary producers, primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics (Figure 3).

Figure 3. This food web shows the interactions between organisms across trophic levels in the Lake Ontario ecosystem. Primary producers are outlined in green, primary consumers in orange, secondary consumers in blue, and tertiary (apex) consumers in purple. Arrows point from an organism that is consumed to the organism that consumes it. Notice how some lines point to more than one trophic level. For example, the opossum shrimp eats both primary producers and primary consumers. (credit: NOAA, GLERL)

A comparison of the two types of structural ecosystem models shows strength in both. Food chains are more flexible for analytical modeling, are easier to follow, and are easier to experiment with, whereas food web models more accurately represent ecosystem structure and dynamics, and data can be directly used as input for simulation modeling.

Two general types of food webs are often shown interacting within a single ecosystem. A grazing food web (such as the Lake Ontario food web in Figure 3) has plants or other photosynthetic organisms at its base, followed by herbivores and various carnivores. A detrital food web consists of a base of organisms that feed on decaying organic matter (dead organisms), called decomposers or detritivores. These organisms are usually bacteria or fungi that recycle organic material back into the biotic part of the ecosystem as they themselves are consumed by other organisms. As all ecosystems require a method to recycle material from dead organisms, most grazing food webs have an associated detrital food web. For example, in a meadow ecosystem, plants may support a grazing food web of different organisms, primary and other levels of consumers, while at the same time supporting a detrital food web of bacteria, fungi, and detrivorous invertebrates feeding off dead plants and animals.

Consequences of Food Webs: Biological Magnification

One of the most important environmental consequences of ecosystem dynamics is biomagnification. Biomagnification is the increasing concentration of persistent, toxic substances in organisms at each trophic level, from the primary producers to the apex consumers. Many substances have been shown to bioaccumulate, including classical studies with the pesticide d ichloro d iphenyl t richloroethane (DDT), which was published in the 1960s bestseller, Silent Spring , by Rachel Carson. DDT was a commonly used pesticide before its dangers became known. In some aquatic ecosystems, organisms from each trophic level consumed many organisms of the lower level, which caused DDT to increase in birds (apex consumers) that ate fish. Thus, the birds accumulated sufficient amounts of DDT to cause fragility in their eggshells. This effect increased egg breakage during nesting and was shown to have adverse effects on these bird populations. The use of DDT was banned in the United States in the 1970s.

Figure 4. This chart shows the PCB concentrations found at the various trophic levels in the Saginaw Bay ecosystem of Lake Huron. Numbers on the x -axis reflect enrichment with heavy isotopes of nitrogen (15N), which is a marker for increasing trophic level. Notice that the fish in the higher trophic levels accumulate more PCBs than those in lower trophic levels. (credit: Patricia Van Hoof, NOAA, GLERL)

Other substances that biomagnify are polychlorinated biphenyls (PCBs), which were used in coolant liquids in the United States until their use was banned in 1979, and heavy metals, such as mercury, lead, and cadmium. These substances were best studied in aquatic ecosystems, where fish species at different trophic levels accumulate toxic substances brought through the ecosystem by the primary producers. As illustrated in a study performed by the National Oceanic and Atmospheric Administration (NOAA) in the Saginaw Bay of Lake Huron (Figure 4), PCB concentrations increased from the ecosystem’s primary producers (phytoplankton) through the different trophic levels of fish species. The apex consumer (walleye) has more than four times the amount of PCBs compared to phytoplankton. Also, based on results from other studies, birds that eat these fish may have PCB levels at least one order of magnitude higher than those found in the lake fish.

Other concerns have been raised by the accumulation of heavy metals, such as mercury and cadmium, in certain types of seafood. The United States Environmental Protection Agency (EPA) recommends that pregnant women and young children should not consume any swordfish, shark, king mackerel, or tilefish because of their high mercury content. These individuals are advised to eat fish low in mercury: salmon, tilapia, shrimp, pollock, and catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even influencing the food we eat.

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High school biology

Course: high school biology   >   unit 9.

• Flow of energy and matter through ecosystems

Food chains & food webs

• Example identifying roles in a food web
• Energy flow and primary productivity
• Trophic levels review
• Trophic levels

Key points:

• Producers , or autotrophs, make their own organic molecules. Consumers , or heterotrophs, get organic molecules by eating other organisms.
• A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another.
• In a food chain, each organism occupies a different trophic level , defined by how many energy transfers separate it from the basic input of the chain.
• Food webs consist of many interconnected food chains and are more realistic representation of consumption relationships in ecosystems.
• Energy transfer between trophic levels is inefficient (with a typical efficiency around 10 % ‍   ). This inefficiency limits the length of food chains.

Introduction

Autotrophs vs. heterotrophs.

• Photoautotrophs , such as plants, use energy from sunlight to make organic compounds (sugars) out of carbon dioxide in photosynthesis . Other examples of photoautotrophs include algae and cyanobacteria.
• Chemoautotrophs use energy from chemicals to build organic compounds out of carbon dioxide (or similar molecules). This is called chemosynthesis . For instance, there are hydrogen sulfide-oxidizing chemoautotrophic bacteria found in undersea vent communities (where no light can reach).

Food chains

• At the base of the food chain lie the primary producers . The primary producers are autotrophs, and are most often photosynthetic organisms (such as plants, algae, or cyanobacteria).
• The organisms that eat the primary producers are called primary consumers . Primary consumers are usually herbivores (plant-eaters), though they may be algae or bacteria eaters.
• The organisms that eat the primary consumers are called secondary consumers . Secondary consumers are generally meat-eaters ( carnivores ).
• The organisms that eat the secondary consumers are called tertiary consumers . These are carnivore-eating carnivores, like eagles or big fish.
• Some food chains have additional levels, such as quaternary consumers (carnivores that eat tertiary consumers). Organisms at the very top of a food chain are called the apex consumers .

Decomposers

Grazing vs. detrital food webs, energy transfer efficiency limits food chain lengths.

• In each trophic level, a significant amount of energy is dissipated as heat, as organisms carry out cellular respiration and go about their daily lives.
• Some of the organic molecules an organism eats cannot be digested and leave the body as feces (poop) rather than being used.
• Not all of the individual organisms in a trophic level will get eaten by organisms in the next level up. Some instead die without being eaten.

Attribution

• " Ecology of ecosystems ," by Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .
• " Energy flow through ecosystems ," by OpenStax College, Concepts of Biology, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .
• " Energy flow through ecosystems ," by OpenStax College, Biology, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .
• " Flow of energy ," by CK-12 Foundation, CC BY-NC 3.0 .

Works cited

• F. Stuart Chapin III, Pamela A. Matson, and Harold A. Mooney, "Trophic Dynamics," in Principles of Terrestrial Ecosystem Ecology (New York: Springer-Verlag, 2002), 250-251.
• Peter H. Raven, George B. Johnson, Kenneth A. Mason, Jonathan B. Losos, and Susan R. Singer, "The Flow of Energy in Ecosystems," in Biology , 10th ed., AP ed. (New York: McGraw-Hill, 2014), 1216.

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Food Web and Food Network: Role of Food System Ecological Interconnectedness in Achieving the Zero Hunger Goal

• Living reference work entry
• First Online: 31 July 2019
• Cite this living reference work entry

• Michele F. Fontefrancesco 7 &
• Henry Sidsaph 8  

Part of the book series: Encyclopedia of the UN Sustainable Development Goals ((ENUNSDG))

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1 Citations

Ecological pyramids ; Food chains ; Food cycle ; Food networks

A food web is a representation of the food relationships that link all the different living organisms and nonliving components that constitute an ecosystem (Jackson 2013 ). The concept was developed as a theoretical tool in natural sciences, but it found use in the ongoing debate of the social sciences and education in particular to raise awareness concerning the themes of sustainability and environmental resilience.

From Ecology to Food Webs

In order to understand the concept of a food web, it is necessary to look at its conceptual basis, an ecosystem. Moving from a dichotomic understanding of the environment that tended to divide living and nonliving, human and nonhuman components, and consider them as different and distinct aspects of the geography of a place (Holt-Jesen 2018 ), since the 1930s (Jackson 2013 ), the concept of the ecosystem has become common knowledge. It refers to a particular...

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Fontefrancesco, M.F., Sidsaph, H. (2019). Food Web and Food Network: Role of Food System Ecological Interconnectedness in Achieving the Zero Hunger Goal. In: Leal Filho, W., Azul, A., Brandli, L., Özuyar, P., Wall, T. (eds) Zero Hunger. Encyclopedia of the UN Sustainable Development Goals. Springer, Cham. https://doi.org/10.1007/978-3-319-69626-3_23-1

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Graph Theory and Ecological Food Webs

An useful and interesting application of graph theory to ecology is the idea of a food web, which are networks that represent the flow of energy in an ecosystem expressed in terms of “what eats what.” Unlike the friendship graphs we’ve examined in class, food webs are directed graphs, with a directed edge pointing from node X to node Y indicating that “organism X eats Y.” I will be discussing the concepts of trophic level and food chain length, and some of their implications, in this blog post. As we will come to see, knowing things about the structure of food webs can tell us many useful things about the ecosystems they model.

One of the more useful concepts in food web theory is the idea of “trophic level,” which is a numerical representation of an organism’s position in a food web. There’s not really an analogous concept for friendship graphs, since the idea of trophic level really only makes sense for a directed graph. It is defined recursively so primary producers (organisms which make their own energy, usually from sunlight) have a trophic level of one, herbivores that eat primary producers have a trophic level of two, carnivores that eat those herbivores have a trophic level of three, higher-level carnivores that eat those lower-level carnivores have a trophic level of four, and so on.

The idea of trophic level has many interesting applications in ecology, one of the most notable is the use of the mean trophic level, that is, the trophic level averaged over all the organisms in an ecosystem, as a measure of the quality of ecosystems (especially fisheries.) In general, the quality of catches in fisheries is positively correlated with mean trophic level; since fisheries prefer to catch fish at higher trophic levels, which are in general bigger and worth more to the fisheries. This led Daniel Pauly to claim in 1998 that this has led to a decline in the quality of fisheries over time in a process he called “fishing down the food web.” [1] Fisheries tend to overfish at higher trophic levels as a result of demand for higher trophic level fish. This leads to a decrease in the mean trophic level as fisheries now have to fish at lower and lower trophic levels to catch the same amount of fish. However, this claim has been disputed in recent ecological literature, a 2010 paper by Trevor Branch demonstrated findings that despite decreases in the biodiversity of fisheries, the mean trophic level has actually increased somewhat. [2] This calls into question exactly how useful mean trophic level alone is as a measure of fishery quality.

Another similar concept is the idea of food chain length, which is the difference between the lowest (primary producers, which have a value of 1) and the highest (apex predators) trophic levels in an ecosystem. In general, food webs with a lower value of food chain length tend to be more stable in the face of perturbations (like droughts or wildfires). This is a result called the dynamic stability hypothesis. [3] This is because, with a larger food chain length, there are more ways for a disturbance at any trophic level to affect all other trophic levels in the food web. If the apex predators at the top of a food web have a sudden die-out, then the mesopredators they feed on will become overly abundant and overfeed on herbivores and lower predators, which may then cause the number of primary producers to spiral out of control. Similarly, if a drought or an unusually cold winter causes primary producers to die out, now herbivores won’t be able to get enough food, meaning low-level predators also begin to die out, resulting in a drastic decrease in the number of apex predators in the ecosystems.

References: [1] https://www.science.org/doi/abs/10.1126/science.279.5352.860 [2] https://www.nature.com/articles/nature09528 [3] https://www.nature.com/articles/268329a0

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What Is a Food Web? Definition, Types, and Examples

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Food Web Definition

Trophic levels in a food web, energy movement, food web vs. food chain, types of food webs, importance of the study of food webs.

A food web is a detailed interconnecting diagram that shows the overall food relationships between organisms in a particular environment. The simplest explanation is that food webs are "who eats whom" diagrams showing the complex feeding relationships for a specific ecosystem.

The study of food webs is important, as such webs can show how energy flows through an ecosystem . It also helps us understand how toxins and pollutants become concentrated within a particular ecosystem. Examples include mercury bioaccumulation in the Florida Everglades and mercury accumulation in the San Francisco Bay.

Food webs can also help us study and explain how species diversity is related to how they fit within the overall food dynamic. They may also reveal critical information about the relationships between invasive species and those native to a particular ecosystem.

Key Takeaways: What Is a Food Web?

• Think of a food web as a "who eats whom" diagram showing an ecosystem's complex feeding relationships.
• Knowing the interconnectedness of organisms in energy transfer within an ecosystem is vital to understanding food webs and how they apply to real-world science.
• An increase in toxic substances, like man-made persistent organic pollutants (POPs), can profoundly impact ecosystem species.
• By analyzing food webs, scientists can study and predict how substances move through the ecosystem to help prevent the bioaccumulation and biomagnification of harmful substances.

The concept of a food web, previously known as a food cycle, is typically credited to Charles Elton, who first introduced it in his book Animal Ecology, published in 1927. He is considered one of the founders of modern ecology and his book is a seminal work. In this book, he also introduced other important ecological concepts like niche and succession .

In a food web, organisms are arranged according to their trophic level. An organism's trophic level refers to how it fits within the food web and is based on how it feeds. 

There are two main designations: autotrophs and heterotrophs. Autotrophs make their food, while heterotrophs do not. Within this broad designation are five main trophic levels: primary producers, primary consumers, secondary consumers, tertiary consumers, and apex predators.

A food web shows how the different trophic levels within various food chains interconnect and how energy flows through them within an ecosystem.

Primary Producers

Primary producers make their food via photosynthesis, which uses the sun's energy to make food by converting its light energy into chemical energy. Examples of primary producers include plants and algae. These organisms are also known as autotrophs.

Primary Consumers

Primary consumers are animals that eat the primary producers. They are named as such because they are the first organisms to eat the primary producers who make their own food. Primary consumers are also known as herbivores. Examples of animals in this designation are rabbits, beavers, elephants , and moose.

Secondary Consumers

Secondary consumers consist of organisms that eat primary consumers. Since secondary consumers are animals that eat the animals that eat the plants, they are called carnivorous or omnivorous. Carnivores eat animals, while omnivores consume both other animals and plants. Bears are an example of a secondary consumer.

Tertiary Consumers

Similar to secondary consumers, tertiary consumers can be carnivorous or omnivorous. The difference is that secondary consumers eat other carnivores. An example is an eagle.

Apex Predators

Lastly, the final level is composed of apex predators . Apex predators are at the top because they do not have natural predators. Lions are an example.

Decomposers and Detritivores

Additionally, organisms known as decomposers consume dead plants and animals and break them down. Fungi are examples of decomposers. Other organisms known as detritivores consume dead organic material. A vulture is an example of a detrivore.

Energy flows through the different trophic levels. It begins with the sun's energy, which autotrophs use to produce food. This energy is transferred up the levels as the different organisms are consumed by members of the levels above them.

Approximately 10% of the energy transferred from one trophic level to the next is converted to biomass—the overall mass of an organism or the mass of all the organisms that exist in a given trophic level.

Since organisms expend energy to move around and go about their daily activities, only a part of the energy consumed is stored as biomass.

VectorMine / Getty Images

While a food web contains all constituent food chains in an ecosystem, food chains are a different construct. A food web can be composed of multiple food chains, some very short and others much longer. Food chains follow the flow of energy as it moves through the chain. The starting point is the energy from the sun, and this energy is traced as it moves through the food chain. This movement is typically linear, from one organism to another.

For example, a short food chain may consist of plants that use the sun's energy to produce their food through photosynthesis and the herbivore that consumes these plants. This herbivore may be eaten by two different carnivores, which are a part of this food chain. When these carnivores are killed or die, the decomposers in the chain break down the carnivores, returning nutrients to the soil that can be used by plants.

This brief chain is one of many parts of the overall food web that exists in an ecosystem. Other food chains in the food web for this particular ecosystem may be very similar to this example or may be much different. 

Since it is composed of all of the food chains in an ecosystem, the food web shows how the organisms in an ecosystem interconnect.

Blueringmedia / Getty Images

There are several types of food webs, which differ in how they are constructed and what they show or emphasize about the organisms within the particular ecosystem depicted.

Scientists can use connectance and interaction food webs, along with energy flow, fossil, and functional food webs, to depict different aspects of the relationships within an ecosystem. They can also further classify the types of food webs based on the ecosystem being depicted in the web.

Connectance Food Webs

In a connectance food web, scientists use arrows to show one species being consumed by another. All of the arrows are equally weighted. The degree of strength of the consumption of one species by another is not depicted.

Interaction Food Webs

Like in connectance food webs, scientists also use arrows in interaction food webs to show one species being consumed by another. However, the arrows used are weighted to show the degree or strength of consumption of one species by another.

The arrows depicted in such arrangements can be wider, bolder, or darker to denote the strength of consumption if one species typically consumes another. If the interaction between species is weak, the arrow can be very narrow or nonexistent.

Energy Flow Food Webs

Energy flow food webs depict the relationships between organisms in an ecosystem by quantifying and showing the energy flux between organisms.

Fossil Food Webs

Food webs can be dynamic, and the food relationships within an ecosystem can change over time. Scientists attempt to reconstruct the relationships between species in a fossil food web based on available evidence from the fossil record.

Functional Food Webs

Functional food webs depict the relationships between organisms in an ecosystem by depicting how different populations influence the growth rate of other populations within the environment.

Food Webs and Type of Ecosystems

Scientists can also subdivide the above types of food webs based on the type of ecosystem. For example, an energy-flow aquatic food web would depict the energy flux relationships in an aquatic environment. In contrast, an energy-flow terrestrial food web would show such relationships on land.

Food webs show us how energy moves through an ecosystem, from the sun to producers to consumers. The interconnectedness of how organisms are involved in this energy transfer within an ecosystem is vital to understanding food webs and how they apply to real-world science.

Just as energy can move through an ecosystem, other substances can also move through. There can be devastating effects when toxic substances or poisons are introduced into an ecosystem.

Bioaccumulation and biomagnification are important concepts. Bioaccumulation is the accumulation of a substance, like poison or a contaminant, in an animal. Biomagnification refers to the buildup and increase in the concentration of said substance as it is passed from trophic level to trophic level in a food web.

This increase in toxic substances can profoundly impact species within an ecosystem. For example, man-made synthetic chemicals often do not break down easily or quickly and can build up in an animal's fatty tissues over time. These substances are known as persistent organic pollutants (POPs).

Marine environments are common examples of how these toxic substances can move, such as from phytoplankton to zooplankton, then to fish that eat the zooplankton, then to other fish (like salmon) who eat those fish, and up to orca who eat salmon. Orcas have a high blubber content so the POPs can be found at very high levels. These levels can cause several issues like reproductive problems, developmental issues with their young as well as immune system issues.

By analyzing and understanding food webs, scientists can study and predict how substances may move through the ecosystem. They can then better help prevent the bioaccumulation and biomagnification of these toxic substances in the environment through intervention.

• “ Food Webs and Networks: the Architecture of Biodiversity .” Life Sciences at the University of Illinois at Urbana-Champaign , Biology Department.
• “ 11.4: Food Chains and Food Webs .” Geosciences LibreTexts , Libretexts.
• “ Terrestrial Food Webs .” Smithsonian Environmental Research Center.
• “ Bioaccumulation and Biomagnification: Increasingly Concentrated Problems! ” CIMI School.
• Food Chains and Food Webs: Learn the Difference
• Energy Flow in Ecosystems
• What Are Biotic and Abiotic Factors in an Ecosystem?
• Why Choosing Nectar-Rich Plants for a Garden Is So Important
• How Does Mercury Get in Fish?
• What Is an Indicator Species? 10 Key Examples
• Why Flowering Meadows Are Better Than Lawns
• What Is a Trophic Cascade? Definition and Ecological Impact
• What Is a Pioneer Species?
• Lichens in a Garden and What They Tell You
• Understanding the Sustainable Seafood Industry
• Types of Forests: Definitions, Examples, and Importance
• What Is Glitter? Environmental Impact and Sustainable Alternatives
• 10 Stunning Plants and Sea Creatures on the Ocean Floor
• Male Spiders Fight Less When There Are More Females Around
• 3 Types of Biodiversity: Overview and Importance

Food chain, food web and ecological pyramids

June 13, 2020 Gaurab Karki Biodiversity 0

• The food chain is an ideal representation of flow of energy in the ecosystem.
• In food chain, the plants or producers are consumed by only the primary consumers, primary consumers are fed by only the secondary consumers and so on.
• The producers that are capable to produce their own food are called autotrophs.
• Any food chain consists of three main tropic levels, viz., producers, consumers and decomposers.
• The energy efficiency of each tropic level is very low. Hence, shorter the food chain greater will be the accessibility of food.
• Food webs are more complex and are interrelated at different tropic levels.
• Organisms have more than one choice for food and hence can survive better.
• Hawks don’t restrict their food to snakes, snakes eat animals other than mice, and mice eat grass as well as grasshoppers, and so on.
• A more realistic illustration of feeding habits in an ecosystem is called a food web.

 Food web:

• Charles Elton presented the food web concept in year 1927, which he termed as food cycle.
• Charles Elton described the concept of food web as:
• The carnivore animals prey on the herbivores.
• These herbivores obtain the energy from sunlight.
• The later carnivores may also be preyed upon by other carnivores.
• Until a reach where an animal has no enemies it forms a terminus on this food cycle.
• There are chains of animals that are related together by food, and all are dependent on plants in the long run.
• This is termed as a food chain and all the food chains in a community is known as the food web.
• A food web is a graphical depiction of feeding connections among species of an ecological community.
• Food web includes food chains of a particular ecosystem.
• The food web is an illustration of various techniques of feeding that links the ecosystem.
• The food web also explains the energy flow through species of a community as a result of their feeding relationships.
• All the food chains are interconnected and overlapping within an ecosystem and they constitute a food web.
• In natural environment or an ecosystem, the relationships between the food chains are interrelated.
• These relationships are very complex, as one organism may be a part of multiple food chains.
• Hence, a web like structure is formed in place of a linear food chain.
• The web like structure if formed with the interlinked food chain and such matrix that is interconnected is known as a food web.
• Food webs are an inseparable part of an ecosystem; these food webs permit an organism to obtain food from more than one type of organism of the lower trophic level.
• Every living being is responsible and is a part of multiple food chains in the given ecosystem.

Ecological pyramids:

• The trophic levels of different organisms based on their ecological position as producer to final consumer is represented by ecological pyramid.
• The food producer is present at the base of the pyramid and on the top.
• Other consumer trophic levels are present in between.
• The pyramid includes a number of horizontal bars presenting specific trophic levels.
• The length of each bar stands for the total number of individuals or biomass or energy at each trophic level in an ecosystem.
• An ecological pyramid is a graphical representation outlined to show the biomass or bio productivity at each trophic level in a given ecosystem.
• These are trophic pyramid, energy pyramid, or sometimes food pyramid.
• Biomass is the quantity of living or organic matter present in an organism.
• Biomass pyramids represent the amount of biomass, and how much of it is present in the organisms at each trophic level.
• The productivity pyramids shows the production or turnover in biomass.
• Ecological pyramids initiates with producers on the bottom such as green plants and proceed through the various trophic levels such as herbivores that feed on plants, then carnivores that feed on herbivores, then carnivores that feed those carnivores, and so on.
• The highest level is shown at the top of the chain.
• An ecological pyramid of biomass represents the relationship between biomass and trophic level by quantifying the biomass present at each trophic level of an ecological community at a particular time.
• It is a graphical representation of biomass present in per unit area in different trophic levels.
• Flow of energy through the food chain will be in a predictable way, entering at the base of the food chain, by photosynthesis in primary producers, and then moving up the food chain to higher trophic levels.
• The transfer of energy from one trophic level to the next is not efficient.
• It may also be useful and productive to analyse how the number and biomass of organisms differs across trophic levels.
• Both the number and biomass of organisms at each trophic level should be affected by the amount of energy joining that trophic level.
• When there is a direct correlation between energy, numbers, and biomass then biomass pyramids and numbers pyramids will be formed.
• However, the relationship between energy, biomass, and number can be complex by the growth form and size of organisms and ecological relationships occurring among trophic levels.

 Types of pyramids:

• Pyramid of numbers.
• Pyramid of biomass.
• Pyramid of energy or productivity.

1. Pyramid of numbers:

• Pyramid of numbers represents the population of trophic level as the total number of individuals of different species present at each trophic level.
• Pyramid of numbers may be upright and or completely inverted depending upon count of individual present and so.
• The pyramid of number does not completely define the trophic structure for an ecosystem as it is very tough to count all the organisms present there.
• Pyramid of number- upright: grassland ecosystem
• In this pyramid, the number of individuals is decreased from lower level to higher trophic level.
• The examples of pyramid of numbers are Grassland ecosystem and pond ecosystem.
• In grass ecosystem, at base (lowest trophic level) grass is present in plentiful amount.
• The next higher trophic level is primary consumer i.e. herbivore (example – grasshopper).
• The number count of grasshopper is less than that of grass.
• The next energy level is primary carnivore (example: rat). The number of rats are less than grasshopper, because, they feed on grasshopper.
• The next higher trophic level is secondary carnivore (example: snakes). They feed on rats.
• The next higher trophic level is the top carnivore. (example – Hawk).
• As we reach each higher trophic level, the numbers of individual decreases from lower to higher trophic level.
• Pyramid of numbers – inverted: tree ecosystem
• In this type of pyramid, the number of individuals is increased from lower level to higher trophic level. Example, tree ecosystem.

2. Pyramid of biomass:

• Pyramid of biomass represents the total dry weight of organisms.
• It is usually determined by collecting all organisms inavding each trophic level separately and measuring their dry weight.
• This will serve to solve the size difference problem because all kinds of organisms at a trophic level are weighed.
• The unit for measurement of biomass is g/m2.
• The biomass of a species is expressed in terms of fresh or dry weight.
• Measurement of biomass in terms of dry weight is considered more accurate.
• Certain mass of living material of each trophic level at a particular time called as standing crop.
• The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
• pyramid of biomass: upright
• The pyramid of biomass on land contains a large base of primary producers with a lesser trophic level present on top.
• The biomass of producer termed as autotrophs is at the maximum trophic level.
• The biomass of next trophic level from base, i.e., primary consumers is less than the producers.
• The biomass of next higher trophic level, i.e., secondary consumers is less than the primary consumers.
• The top, high trophic level consists very less amount of biomass.
• On other hand, in many aquatic ecosystems, the pyramid of biomass may be present in an inverted form whereas pyramid of numbers for aquatic ecosystem is upright.
• It is because the producers are small phytoplankton that grow and reproduce very rapidly.
• Here, the pyramid of biomass has a small base as compared to the consumer biomass at any instant actually exceeding the producer biomass and the pyramid is represent in inverted shape.

3. Pyramid of energy:

• The pyramid of energy represents the flow of energy from lower trophic level to higher trophic level.
• During the flow of energy from one organism to other, there is remarkable loss of energy.
• This loss of energy is in the form of heat.
• The primary producers like the autotrophs contain more amount of energy available.
• The least energy is available in the tertiary consumers.
• Thus, shorter food chain has more amount of energy available even at the highest trophic level.
• An energy pyramid is regarded most suitable to compare the functional roles of the trophic levels in an ecosystem.
• An energy pyramid represents the amount of energy at each trophic level and loss of energy taking place during transfer to another trophic level.
• Hence the pyramid is always upward, with a large energy base at the bottom.
• Suppose an ecosystem receives 1000 calories of light energy in a given day.
• Most of the energy is not absorbed by plants; some amount of energy is reflected back to space.
• Green plants utilise only a small portion of that absorbed energy, out of which the plant uses up some for respiration and of the 1000 calories, only 100 calories (10%) are stored as energy rich materials.
• Now, suppose an animal eats the plant containing 100 calorie of food energy, that animal uses some of it for its own metabolism and stores only 10 calorie as food energy.
• A lion that eats that animal gets an even smaller amount of energy.
• Thus, usable energy decreases while passing from sunlight to producer to herbivore to carnivore. Therefore, the energy pyramid will always be upright.
• ecological pyramid

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ENCYCLOPEDIC ENTRY

A food web consists of all the food chains in a single ecosystem.

Biology, Ecology

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A  food web   consists of all the  food chains in a single  ecosystem . Each living thing in an ecosystem is part of  multiple   food chains . Each food chain is one possible path that  energy  and  nutrients may take as they move through the ecosystem . All of the interconnected and overlapping food chains in an ecosystem make up a food web . Trophic Levels Organisms in food webs are grouped into categories called  trophic levels . Roughly speaking, these levels are divided into  producers (first trophic level ),  consumers , and  decomposers (last trophic level ). Producers Producers make up the first trophic level . Producers , also known as  autotrophs , make their own food and do not depend on any other organism for nutrition. Most autotrophs use a process called  photosynthesis  to create food (a nutrient called  glucose ) from sunlight ,  carbon dioxide , and water. Plants are the most familiar type of autotroph , but there are many other kinds.  Algae , whose larger forms are known as  seaweed , are autotrophic .  Phytoplankton , tiny organisms that live in the ocean, are also autotrophs . Some types of  bacteria  are autotrophs . For example, bacteria living in active  volcanoes use  sulfur , not carbon dioxide , to produce their own food. This process is called  chemosynthesis . Consumers The next trophic levels are made up of animals that eat producers . These organisms are called consumers . Consumers can be  carnivores (animals that eat other animals) or  omnivores (animals that eat both plants and animals). Omnivores , like people, consume many types of foods. People eat plants , such as  vegetables and fruits . We also eat animals and animal products, such as meat, milk, and eggs. We eat  fungi , such as mushrooms. We also eat algae , in  edible   seaweeds like  nori (used to wrap  sushi  rolls) and  sea lettuce  (used in salads). Bears are omnivores , too. They eat berries and mushrooms, as well as animals such as salmon and deer. Primary consumers are  herbivores . Herbivores eat plants , algae , and other producers . They are at the second trophic level . In a  grassland   ecosystem , deer, mice, and even elephants are herbivores . They eat grasses,  shrubs , and trees. In a  desert   ecosystem , a mouse that eats  seeds and  fruits is a primary consumer . In an ocean ecosystem , many types of fish and turtles are herbivores that eat algae and  seagrass . In  kelp forests , seaweeds known as giant kelp provide shelter and food for an entire ecosystem .  Sea urchins are powerful primary consumers in kelp forests . These small herbivores eat dozens of kilograms (pounds) of giant kelp every day. Secondary consumers eat herbivores . They are at the third trophic level . In a desert ecosystem , a secondary consumer may be a snake that eats a mouse. In the kelp forest , sea otters are secondary consumers that hunt sea urchins . Tertiary consumers eat the secondary consumers . They are at the fourth trophic level . In the desert ecosystem , an owl or eagle may prey on a snake. There may be more levels of consumers before a chain finally reaches its  top predator . Top predators , also called  apex predators , eat other consumers . They may be at the fourth or fifth trophic level . They have no natural enemies except humans. Lions are apex predators in the grassland ecosystem . In the ocean, fish like the great white shark are apex predators . In the desert , bobcats and mountain lions are top predators . Detritivores and Decomposers Detritivores and decomposers make up the last part of food chains . Detritivores are organisms that eat nonliving plant and animal  remains . For example,  scavengers such as vultures eat dead animals. Dung beetles eat animal  feces . Decomposers , like fungi and bacteria , complete the food chain . Decomposers turn  organic wastes , such as  decaying plants , into inorganic materials, such as nutrient -rich soil. They complete the cycle of life, returning nutrients to the soil or oceans for use by autotrophs . This starts a whole new series of food chains . Food Chains Food webs connect many different food chains , and many different trophic levels . Food webs can support food chains that are long and complicated, or very short. For example, grass in a forest clearing produces its own food through photosynthesis . A rabbit eats the grass. A fox eats the rabbit. When the fox dies, decomposers such as worms and mushrooms break down its body, returning it to the soil where it provides nutrients for plants like grass. This short food chain is one part of the forest 's food web . Another food chain in the same ecosystem might involve completely different organisms. A caterpillar may eat the leaves of a tree in the forest . A bird such as a sparrow may eat the caterpillar. A snake may then prey on the sparrow. An eagle, an apex predator , may prey on the snake. Yet another bird, a vulture, consumes the body of the dead eagle. Finally, bacteria in the soil decompose the remains . Algae and plankton are the main producers in marine ecosystems . Tiny shrimp called  krill  eat the microscopic plankton. The largest animal on Earth, the blue whale, preys on thousands of tons of krill every day. Apex predators such as orcas prey on blue whales. As the bodies of large animals such as whales sink to the seafloor, detritivores such as worms break down the material. The nutrients released by the decaying flesh provide chemicals for algae and plankton to start a new series of food chains . Biomass Food webs are defined by their  biomass . Biomass is the energy in living organisms. Autotrophs , the producers in a food web , convert the sun's energy into biomass . Biomass decreases with each trophic level . There is always more biomass in lower trophic levels than in higher ones. Because biomass decreases with each trophic level , there are always more autotrophs than herbivores in a healthy food web . There are more herbivores than carnivores . An ecosystem cannot support a large number of omnivores without supporting an even larger number of herbivores , and an even larger number of autotrophs . A healthy food web has an  abundance  of autotrophs , many herbivores , and relatively few carnivores and omnivores . This balance helps the ecosystem maintain and  recycle   biomass . Every link in a food web is connected to at least two others. The biomass of an ecosystem depends on how balanced and connected its food web is. When one link in the food web is threatened, some or all of the links are weakened or  stressed . The ecosystems biomass   declines . The loss of plant life usually leads to a decline in the herbivore population, for instance. Plant life can decline due to  drought , disease, or human activity. Forests are cut down to provide  lumber  for construction. Grasslands are paved over for shopping malls or parking lots. The loss of biomass on the second or third trophic level can also put a food web out of balance. Consider what may happen if a  salmon run  is  diverted . A salmon run is a river where salmon swim. Salmon runs can be diverted by  landslides and  earthquakes , as well as the construction of  dams and  levees . Biomass is lost as salmon are cut out of the rivers. Unable to eat salmon, omnivores like bears are forced to rely more heavily on other food sources, such as ants. The area's ant population shrinks. Ants are usually scavengers and detritivores , so fewer nutrients are broken down in the soil. The soil is unable to support as many autotrophs , so biomass is lost. Salmon themselves are predators of insect larvae and smaller fish. Without salmon to keep their population in check,  aquatic  insects may  devastate  local plant communities. Fewer plants survive , and biomass is lost. A loss of organisms on higher trophic levels , such as carnivores , can also disrupt a food chain . In  kelp forests , sea urchins are the primary consumer of kelp . Sea otters prey on urchins. If the sea otter population shrinks due to disease or hunting, urchins devastate the kelp forest . Lacking a community of producers , biomass   plummets . The entire kelp forest disappears. Such areas are called  urchin barrens . Human activity can reduce the number of predators. In 1986, officials in Venezuela dammed the Caroni River, creating an  enormous  lake about twice the size of Rhode Island. Hundreds of hilltops turned into islands in this lake. With their habitats reduced to tiny islands, many terrestrial predators weren’t able to find enough food. As a result, prey animals like howler monkeys, leaf-cutter ants, and iguanas flourished. The ants became so numerous that they destroyed the rainforest , killing all the trees and other plants . The food web surrounding the Caroni River was destroyed. Bioaccumulation Biomass declines as you move up through the trophic levels . However, some types of materials, especially  toxic  chemicals, increase with each trophic level in the food web . These chemicals usually collect in the  fat  of animals. When an herbivore eats a plant or other autotroph that is covered in  pesticides , for example, those pesticides are stored in the animal’s fat . When a carnivore eats several of these herbivores , it takes in the pesticide chemicals stored in its prey . This process is called  bioaccumulation . Bioaccumulation happens in aquatic ecosystems too.  Runoff  from  urban areas or  farms can be full of  pollutants . Tiny producers such as algae , bacteria , and seagrass absorb minute amounts of these pollutants . Primary consumers , such as sea turtles and fish, eat the seagrass . They use the energy and nutrients provided by the plants , but store the chemicals in their fatty tissue. Predators on the third trophic level , such as sharks or tuna, eat the fish. By the time the tuna is consumed by people, it may be storing a  remarkable  amount of bio accumulated toxins. Because of bioaccumulation , organisms in some polluted ecosystems are unsafe to eat and not allowed to be  harvested .  Oysters in the  harbor  of the United States' New York City, for instance, are unsafe to eat. The pollutants in the harbor   accumulate in its oysters , a filter feeder . In the 1940s and 1950s, a pesticide called  DDT  (dichloro-diphenyl-trichloroethane) was widely used to kill insects that spread diseases. During  World War II , the  Allies  used DDT to  eliminate typhus  in Europe, and to control  malaria  in the South Pacific. Scientists believed they had discovered a miracle drug. DDT was largely responsible for eliminating malaria in places like Taiwan, the Caribbean, and the Balkans . Sadly, DDT bio accumulates in an ecosystem and causes damage to the environment. DDT accumulates in soil and water. Some forms of DDT decompose slowly. Worms, grasses, algae , and fish accumulate DDT . Apex predators , such as eagles, had high amounts of DDT in their bodies, accumulated from the fish and small mammals they prey on. Birds with high amounts of DDT in their bodies lay eggs with extremely thin shells. These shells would often break before the baby birds were ready to hatch. DDT was a major reason for the decline of the bald eagle, an apex predator that feeds primarily on fish and small rodents. Today, the use of DDT has been restricted. The food webs of which it is a part have recovered in most parts of the country.

Lost Energy Biomass shrinks with each trophic level. That is because between 80% and 90% of an organism's energy, or biomass, is lost as heat or waste. A predator consumes only the remaining biomass.

A Million to One Marine food webs are usually longer than terrestrial food webs. Scientists estimate that if there are a million producers (algae, phytoplankton, and sea grass) in a food web, there may only be 10,000 herbivores. Such a food web may support 100 secondary consumers, such as tuna. All these organisms support only one apex predator, such as a person.

Out for Blood One of the earliest descriptions of food webs was given by the scientist Al-Jahiz, working in Baghdad, Iraq, in the early 800s. Al-Jahiz wrote about mosquitoes preying on the blood of elephants and hippos. Al-Jahiz understood that although mosquitoes preyed on other animals, they were also prey to animals such as flies and small birds.

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Desert Food Chain

A desert food chain is a graphical representation showing who eats whom and thus the flow of energy in the desert biome. Like other food chains, there are two main types of organisms in a desert food chain: producers and consumers.

Producers are organisms that make their food. Usually, plants and microorganisms are producers. In contrast, consumers feed on producers for their livelihood. Based on their position in the food chain, consumers are divided into primary, secondary, tertiary, and quaternary consumers or apex predators.

A desert food chain is unique due to its harsh environment.

Desert Food Chain Examples

Some of the common desert food chains around the world based on their geographic location are found in:

• The Sahara Desert
• The Sonoran Desert
• The Gobi Desert
• The Australian Desert
• Atacama Desert
• The Arizona Desert

Like all ecosystems, organisms in deserts can be organized in different trophic levels:

Like other food chains, desert food chains also start with producers making their food. In deserts, date palms, cacti, acacia, sagebrush, desert milkweed, and desert willow are the producers. These plants store water in their bodies due to water scarcity in the environment.

Plants like cacti are the keystone species of the desert biome. Without them, the whole food chain will collapse.

Primary Consumers (Herbivores)

Insects and small mammals like Kangaroo rats, desert tortoises, ground squirrels, Arabian camels, and some insects feed only on plants to survive. These groups of herbivorous animals are the primary consumers of the desert food chain.

Secondary Consumers (Omnivores)

Secondary consumers are mostly omnivores ( plant and animal eaters), although a few may be carnivores (animal eaters). Animals like lizards, coyotes, rattlesnakes, mongooses, tarantulas, and scorpions are some examples of secondary consumers in the desert.

Tertiary and Apex Consumers (Carnivores)

Tertiary and apex consumers are carnivorous animals that reside on the top two trophic levels of the food chain.

The organisms in this group include predators like the striped hyena, sand cat, fox, hawks and eagle, and cheetah. They feed on primary and secondary consumers. Those organisms perform the role of a tertiary consumer or an apex predator based on their participation in the food chain.

For example, the rattlesnake is the apex consumer in the food chain: Brittlebush -> Squirrel -> Rattlesnake. In contrast, the snake is the tertiary consumer in the food chain: Brittlebush -> Grasshopper -> Grasshopper mouse -> Rattlesnake -> Hawk.

In the desert, humans are the ultimate predators. Other apex predators compete with humans for food and survival.

Decomposers

Finally, decomposers in the soil, like fungi, bacteria , and worms, decompose the dead organic matter of plants and animals to recycle the nutrients stored in animal flesh and decaying plant matter and return them to the soil.

When a group of overlapping and interconnecting desert food chains is represented, it forms a desert food web . A desert food web shows the nature of interdependence of the organisms in a desert over a wide area.

• Desert Food Chain: Examples | What is a Desert Biome Food Chain? – https://Study.com
• Desert Food Chains – Digital-desert.com
• Desert Biome Food Chain – Npshistory.com
• Desert Food Chain – Desert Food Web – Desertusa.com

Article was last reviewed on Monday, August 21, 2023

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Relationships Between Organisms: Food Chains, Webs, and Pyramids

Let’s get started.

Let's explore the various tools biologists use to study and monitor the movement of matter and energy in an ecosystem. Food chains, food webs, and ecological pyramids help us understand who eats whom and how changes in a population of organisms can impact predators and prey. Before you get started, don’t forget to print out the OnTRACK Biology Journal . 

TEKS Standards and Student Expectations

B(12) The student knows that interdependence and interactions occur within an environmental system. The student is expected to:

B(12)(C)  analyze the flow of matter and energy through trophic levels using various models, including food chains, food webs, and ecological pyramids

Learning Objective

Describe and apply the tools scientists use to study the movement of matter and energy within environmental systems, including food chains, food webs, and ecological pyramids.

Essential Questions

How do matter and energy move through an ecosystem?

What happens when an energy resource in an ecosystem becomes depleted?

Check your prior knowledge. Before completing this resource, check the boxes of the vocabulary words that you already understand in your OnTRACK Biology Journal .

• Photosynthesis
• Heterotroph
• Secondary Consumer
• Tertiary Consumer
• Quaternary Consumer
• Trophic Level
• Kilocalorie
• Ecological Pyramid
• Pyramid of Energy
• Pyramid of Numbers
• Pyramid of Biomass
• Biodiversity
• Invasive Species

Energy, Producers, and Consumers

Now view the video to review the components of a food chain and see a real life example in the Everglades ecosystem, which is a region of tropical wetlands found in the southern portion of Florida. Use the graphic organizer on page 4 in your OnTRACK Biology Journal to take notes from the video.

Compare your notes to those of a classmate. Make sure you both have a complete set of notes that includes all the terms from the video. If you don’t, view the video again.  

Ecosystem Research An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment (things like air, water, and soil), interacting as a system.

In the video, the Everglades ecosystem was used to show how energy and matter flow from one organism to another. What other kinds of ecosystems do you know about? In Texas, we have many ecosystems, such as deserts, mountains, plains, prairies, seacoasts, timberlands, and marshes. Choose one type of ecosystem you are curious about and brainstorm at least three questions to research. Write them on page 5 in your OnTRACK Biology Journal . Include some of the new terms you have learned in your questions by referring to page 4 of your OnTRACK Biology Journal . Use available resources to research and answer your questions and write them in your journal. You will use this information later.

Energy Flow in Ecosystems

In every ecosystem, organisms are linked through feeding relationships. There are a great many feeding relationships in any ecosystem, but energy always flows from primary producers to various consumers. These feeding relationships are represented by food chains and food webs. 

A food chain is a sequence in which organisms transfer energy by eating and being eaten. Here is an example of a food chain from the video.

Notice that the arrows point in the direction of the energy flow. The point of the arrow goes to who is doing the eating.

In most ecosystems, feeding relationships are much more complicated than the relationships shown in a food chain. The network of feeding interactions is called a food web. View the video to see an example of a food web from the Everglades ecosystem.

Compare and contrast a food chain and a food web on the graphic organizer on page 6 of your OnTRACK Biology Journal . Use the Venn diagram to list the features of a food chain and food web as well as what the two have in common.  

Check Your Learning Drag the circle icon and place it over the arrow that is pointing in the correct direction in the following diagram.

Make Your Own Food Web Now that you have seen an example of a food web, refer to the ecosystem you researched in the first section of this resource and create a food web to show how matter and energy flow between organisms that are likely to live in that ecosystem. You can show your food web on a poster or through a digital graphic organizer created online such as in Lucidchart or in Google Drive with Google Drawings . Remember to draw the arrows moving in the direction that the matter and energy are flowing.

Once your food web is complete, label each of the following:

• Decomposers

Additionally, complete the following and post near or on your food web diagram:

Describe the food web and the places on Earth where this ecosystem and these organisms might be found.

Trophic Levels

Have you ever heard the phrase, “top of the food chain”? What does this phrase mean? What kinds of organisms are typically at the top of the food chain?

Look at the food web you created at the end of the previous section, Energy Flow in Ecosystems. Count how many organisms are between the top of the food chain and the sun. Keep this activity in mind as you complete the next section. Be sure to take notes on any new concepts on page 7 of your OnTRACK Biology Journal .

Each step in a food chain or food web is called a trophic level. Trophic is derived from the Greek word for nutrition. In ecology, it means of or relating to feeding and nutrition . Therefore, a trophic level is a feeding level. Primary producers always make up the first trophic level as they are the basis of all other nutrition. Various consumers occupy every other level.

The shape of a pyramid is used to show the difference in the amount of energy at each trophic level in a food web. We call these diagrams ecological pyramids. Most of the energy is found at the bottom of the food web where the producers are found, and more and more energy is lost as it travels from organism to organism. On the following pyramid, place the term in the correct trophic level on this ecological pyramid by dragging and dropping them into the blanks next to the organism’s photo.

As we explained before, most of the energy in a food web is found at the bottom of the ecological pyramid where producers are found. These producers obtain their energy from the sun. As these plants are consumed by primary and secondary consumers, most of the energy is used by the animal, and very little is passed to the next consumer.

Refer back to the food web you created at the end of the previous section. Can you identify producers, first level or primary consumers, secondary consumers, third level or tertiary consumers, and fourth level or quaternary consumers in your food web? Use these organisms to create an ecological pyramid on page 7 of your OnTRACK Biology Journal . Find a friend to share your pyramid with!

Ecological Pyramids

Consider the difference between the temperature in the room and the temperature inside your body. Why do you think your body is able to stay so much warmer than the air around it? Did your response have anything to do with the fact that much of the energy we get from food is used to keep us warm?

We're going to learn how energy moves up the food chain, the different ways it can be measured, and how most of that energy is lost as heat at each level.

You've already learned about ecological pyramids. There are three main types of ecological pyramids: pyramid of energy, pyramid of numbers, and pyramid of biomass.  

Pyramid of Energy A pyramid of energy shows the relative amount of energy available at each trophic level of a food chain or web. On average, about 10 percent of the energy available within one trophic level is transferred to the next level. The other 90 percent is given off as heat during metabolic processes that allow our bodies to function . In this pyramid, the energy is expressed in Kcal or kilocalories. In this pyramid of energy, you will see the primary producer and consumers from the videos. What do you notice when you look at the number of kilocalories at each level?

Pyramid of Energy Interactive - Image Sources:

Mosquito Larvae

Gambusia Affinis

Myakka River Blue Heron

American Alligator

Pyramid of Biomass A pyramid of biomass shows the relative amount of organic matter, or living tissue, available at each trophic level in an ecosystem. Biomass is usually measured in grams or kilograms of organic matter per unit area.

Not all ecological pyramids are uniform in structure. For example, look at the pyramid below. Notice the bottom level is smallest. This represents a pyramid of biomass from the ocean ecosystem. Phytoplankton is the producer. They are so small and weigh so little that there is very little biomass.

Pyramid of Numbers A pyramid of numbers shows the relative number of individual organisms at each trophic level in an ecosystem.

Again, not all pyramids of numbers have the largest amount at the bottom. Look at this pyramid of numbers in a forest. One oak tree provides nutrition for 2000 insects, which feed 90 sparrows and one hawk.

Why Does It Matter?

Biodiversity is essential to all organisms living in an ecosystem. Picture these organisms on a balance scale. If one of the organisms is removed or its population decreases or increases drastically, the scale is thrown off the balance. For a real life example of how this can happen, listen to the following podcast about the Florida Everglades ecosystem and respond to the comprehension questions on page 10 of your OnTRACK Biology Journal .

Ecologists and biologists keep a watchful eye on human activity in threatened ecosystems like the Everglades. Choose a type of ecological pyramid. Respond to the following prompt on page 11 of your OnTRACK Biology Journal :

• How could this pyramid help biologists study and track the balance of matter and energy in an ecosystem? Provide an example.  

Invasive Species Invasive species are plants, animals, or pathogens that are non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause harm.

View the linked video about several invasive species found in the Everglades.

//www.nps.gov/media/video/view.htm?id=F7B580CC-1DD8-B71C-07EE48C133240AE7  

Complete the ecological pyramid for before and after the Burmese Python was introduced to the wetlands on page 9 of your OnTRACK Biology Journal . Respond to the following prompts in your journal:

• In what ways did the introduction of the Burmese Python impact the organisms native to the ecosystem?
• What are some reasons the Burmese Python thrives in the Everglades ecosystem?
• What are some reasons humans are responsible for responding to these problems?

Extend Your Knowledge To learn more, visit the National Invasive Species Information Center . Identify an invasive species in Texas that has a negative impact on the flow of matter and energy in an ecosystem. Create a presentation to share your understanding. Your presentation should demonstrate how this species fits into the ecosystem’s food web, the consequences of the introduction of this species to the ecosystem, and current efforts to respond to the problem as well as any actions citizens can take to help.

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19.1: Introduction to and Components of Food Webs

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All living things require energy in one form or another. Energy is required by most complex metabolic pathways (often in the form of adenosine triphosphate, ATP), especially those responsible for building large molecules from smaller compounds, and life itself is an energy-driven process. Living organisms would not be able to assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their monomeric subunits without a constant energy input.

Food webs illustrate how energy flows directionally through ecosystems, including how efficiently organisms acquire it, use it, and how much remains for use by other organisms of the food web.

Food Chains and Food Webs

In ecology, a food chain is a linear sequence of organisms through which nutrients and energy pass: primary producers, primary consumers, and higher-level consumers are used to describe ecosystem structure and dynamics. There is a single path through the chain. 

Food chains do not accurately describe most ecosystems. Even when all organisms are grouped into appropriate trophic levels, some of these organisms can feed on species from more than one trophic level; likewise, some of these organisms can be eaten by species from multiple trophic levels. In other words, the linear model of ecosystems, the food chain, is not completely descriptive of ecosystem structure. A holistic model—which accounts for all the interactions between different species and their complex interconnected relationships with each other and with the environment—is a more accurate and descriptive model for ecosystems. A food web is a graphic representation of a holistic, nonlinear web of primary producers, primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics ( Figure \(\PageIndex{1}\) ). 

Figure \(\PageIndex{1}\): Example of simplified food chains (a) and food webs (b) of terrestrial and marine ecosystems. Developed by LadyofHats and licensed under CC0. 

Though more complex than a food chain, a food web remains a simplified illustration of the direct and indirect trophic interactions among species in an ecosystem. Food webs often aggregate many species into trophic groups, which are functional groups of species that have the same predators and prey in a food web. Software can be used to model more complex interactions ( Figure \(\PageIndex{2}\) ), but no food web model can capture all of the complexity found within a natural ecosystem. 

Figure \(\PageIndex{2}\): An example of a more complex food web developed by Hoover et al. 2021 using a program called Ecopath. This food web depicts trophic relationships among species in the Canadian Beaufort Sea. Horizontal lines represent trophic level. Image licensed under CC-BY 4.0. 

Components of a Food Web

The three basic ways in which organisms get food are as producers, consumers, and decomposers.

Producers (autotrophs) are typically plants or algae. Plants and algae do not usually eat other organisms, but pull nutrients from the soil or the ocean and manufacture their own food using photosynthesis. For this reason, they are called primary producers. In this way, it is energy from the sun that usually powers the base of the food chain (Cengage Learning 2002). An exception occurs in deep-sea hydrothermal ecosystems, where there is no sunlight. Here primary producers manufacture food through a process called chemosynthesis (van Dover 2000).

Consumers (heterotrophs) are species that cannot manufacture their own food and need to consume other organisms. Animals that eat primary producers (like plants) are called herbivores. Animals that eat other animals are called carnivores, and animals that eat both plants and other animals are called omnivores.

Decomposers (detritivores) break down dead plant and animal material and wastes and release it again as energy and nutrients into the ecosystem for recycling. Decomposers, such as bacteria and fungi (mushrooms), feed on waste and dead matter, converting it into inorganic chemicals that can be recycled as mineral nutrients for plants to use again.

Energy is acquired by living things in three ways: photosynthesis, chemosynthesis, and the consumption and digestion of other living or previously living organisms by heterotrophs.

Photosynthetic and chemosynthetic organisms are both grouped into a category known as autotrophs : organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as a carbon source). Photosynthetic autotrophs ( photoautotrophs ) use sunlight as an energy source, whereas chemosynthetic autotrophs ( chemoautotrophs ) use inorganic molecules as an energy source. Autotrophs are critical for all ecosystems. Without these organisms, energy would not be available to other living organisms and life itself would not be possible.

Photoautotrophs, such as plants, algae, and photosynthetic bacteria, serve as the energy source for a majority of the world’s ecosystems. Photoautotrophs harness the solar energy of the sun by converting it to chemical energy in the form of ATP (and NADP). The energy stored in ATP is used to synthesize complex organic molecules, such as glucose.

Chemoautotrophs are primarily bacteria that are found in rare ecosystems where sunlight is not available, such as in those associated with dark caves or hydrothermal vents at the bottom of the ocean ( Figure \(\PageIndex{3}\) ). Many chemoautotrophs in hydrothermal vents use hydrogen sulfide (H2S), which is released from the vents as a source of chemical energy. This allows chemoautotrophs to synthesize complex organic molecules, such as glucose, for their own energy and in turn supplies energy to the rest of the ecosystem.

Figure \(\PageIndex{3}\): Swimming shrimp, a few squat lobsters, and hundreds of vent mussels are seen at a hydrothermal vent at the bottom of the ocean. As no sunlight penetrates to this depth, the ecosystem is supported by chemoautotrophic bacteria and organic material that sinks from the ocean’s surface. This picture was taken in 2006 at the submerged NW Eifuku volcano off the coast of Japan by the National Oceanic and Atmospheric Administration (NOAA). The summit of this highly active volcano lies 1535 m below the surface.

Not Your Average Food Web: Deep Sea \(\PageIndex{1}\)

Food webs in the deep sea vary depending on proximity to seamount, hydrothermal vents, and trenches. In areas near hydrothermal vents, chemosynthetic bacteria are the major primary producers. These chemoautotrophs are what provides energy for the rest of the trophic levels in this system.

Figure \(\PageIndex{4}\): A comparison of photosynthetic (left) vs. chemosynthetic (right) food webs. Diagram developed by GRID-Arendal and licensed under CC-SA-NC.

Species in deep-sea ecosystems have adapted to interact with each other in many ways. One key interaction is the symbiosis between many species and chemosynthetic bacteria in hydrothermal vent systems. These bacteria live within the body of species like tubeworms, which are dependent on the bacteria to survive, similar to the relationship between zooxanthellae and coral. Another important type of deep sea community develops when a dead whale (or other large marine organism) carcass sinks to the ocean floor and provides an influx of nutrients. The communities support scavengers like hagfish, opportunists like bristle worms, and eventually enter a sulfophilic stage that appears similar to a hydrothermal vent community.

Figure \(\PageIndex{5}\): Whale falls serve as an extremely important influx of nutrients to the sun-starved deep ocean. This photo shows a Whale skeleton submerged in Monterey Bay National Marine Sanctuary, covered in octopuses and several other species. Photo by National Marine Sanctuaries is licensed under CC 2.0. 

Heterotrophs

Unlike autotrophs, heterotrophs consume rather than produce biomass energy as they metabolize, grow, and add to levels of secondary production. A food web depicts a collection of polyphagous heterotrophic consumers that network and cycle the flow of energy and nutrients from a productive base of self-feeding autotrophs (Pimm et al. 1991; Odum and Barrett 2005; Benke 2010). Autotrophs and heterotrophs come in all sizes, from microscopic to many tonnes - from cyanobacteria to giant redwoods, and from viruses to blue whales.

A gradient exists between trophic levels running from complete autotrophs that obtain their sole source of carbon from the atmosphere, to mixotrophs (such as carnivorous plants) that are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete heterotrophs that must feed to obtain organic matter. 

There are different kinds of feeding relations that can be roughly divided into herbivory, carnivory, scavenging and parasitism. Some of the organic matter eaten by heterotrophs, such as sugars, provides energy. An often overlooked but key component of food webs are the decomposers. 

Not Your Average Food Web: Wasp-Waist Ecosystems \(\PageIndex{2}\)

Food webs can be controlled by top-down mechanisms (predator abundance determines the abundance of lower trophic levels), bottom-up mechanisms (primary producer abundance determines the abundance of higher trophic levels), or a combination of both. In wasp-waist food webs, population dynamics are controlled by planktivorous lower trophic level species such as sardine, anchovy, and small squids rather than the bottom or the top (Cury et al. 2011). These lower trophic level species often have high abundance but low diversity. The term “wasp-waist” describes the shape of these food webs, with many species existing at lower trophic levels (i.e., the plankton) and at higher trophic level (i.e., the predators), but very few lower trophic level species linking the plankton and the predators. These lower trophic level species exert top-down control on zooplankton and bottom-up control on top predators, with environmental factors largely affecting their abundance (Cury et al. 2000; Cury et al. 2003). Wasp-waist ecosystems are highly vulnerable to collapse when forage fish decline due to the critical energetic links that they provide between highly available zooplankton and larger predators (Shannon 2000). 

Figure \(\PageIndex{6}\): A diagram showing the structure of a wasp-waist model for the California Current Large Marine Ecosystem. Arrows indicate inputs of a trophic group to another. Figure modified from Madigan et al. 2012.

Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators (Tan and Corlett 2011). While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding behavior (Getz 2011). Scavengers play a fundamental role in the environment through the removal of decaying organisms, serving as a natural sanitation service (Ogada et al. 2011). While microscopic and invertebrate decomposers break down dead organisms into simple organic matter which are used by nearby autotrophs, scavengers help conserve energy and nutrients obtained from carrion within the upper trophic levels, and are able to disperse the energy and nutrients farther away from the site of the carrion than decomposers (Olson et a. 2016). Decomposers and detritivores complete this process, by consuming the remains left by scavengers. Scavengers are not typically thought to be detritivores, as they generally eat large quantities of organic matter. 

Decomposers are often left off food webs, but if included, they mark the end of a food chain (Hutchinson 2013). Thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level (Kane et al. 2016; Pahl and Ruedas 2021). 

Detritivores (also known as detrivores , detritophages , detritus feeders , or detritus eaters ) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces) (Wetzel 2001). By doing so, detritivores contribute to decomposition and to nutrient cycles. Plant tissues are made up of resilient molecules (cellulose, chitin, lignin and xylan) that decay at a much lower rate than other organic molecules. The activity of detritivores is the reason why we do not see an accumulation of plant litter in nature (Keddy 2017; Sagi et al. 2019).  C oprophagy  is a specific case of detritivory used to describe animals that eat feces (their own, from another individual of their species, or from another species).

Detritivores are an important aspect of many ecosystems. They can live on any type of soil with an organic component, including marine ecosystems, where they are termed interchangeably with bottom feeders. Typical detritivorous animals include millipedes, springtails, woodlice, dung flies, slugs, many terrestrial worms, sea stars, sea cucumbers, fiddler crabs, and some sedentary polychaete worms.

Decomposers are organisms that break down dead or decaying organisms; they carry out decomposition, a process possible by only certain kingdoms, such as fungi (NOAA 2014). Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. While the terms decomposer and detritivore are often interchangeably used, detritivores ingest and digest dead matter internally, while decomposers directly absorb nutrients through external chemical and biological processes (Keddy 2017). Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are technically detritivores, not decomposers, since they must ingest nutrients - they are unable to absorb them externally (Sagi et al. 2019).

Figure \(\PageIndex{7}\): Fungi are the primary decomposers in most environments, illustrated here Mycena interrupta . Only fungi produce the enzymes necessary to decompose lignin, a chemically complex substance found in wood.

Not Your Average Food Web: Detrital Web \(\PageIndex{3}\)

Detritus is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts communities of microorganisms that colonize and decompose (i.e. remineralize) it. In terrestrial ecosystems it is present as leaf litter and other organic matter that is intermixed with soil, which is denominated "soil organic matter". The detritus of aquatic ecosystems is organic material that is suspended in the water and accumulates in depositions on the floor of the body of water; when this floor is a seabed, such a deposition is denominated "marine snow".

Figure \(\PageIndex{8}\): Earthworms are soil-dwelling detritivores.

Figure \(\PageIndex{9}\): Two Adonis blue butterflies lap at a small lump of feces lying on a rock.

In a detrital web, plant and animal matter is broken down by decomposers, e.g., bacteria and fungi, and moves to detritivores and then carnivores (Gönenç et al. 2007). There are often relationships between the detrital web and the grazing web. Mushrooms produced by decomposers in the detrital web become a food source for deer, squirrels, and mice in the grazing web. Earthworms are detritivores that consume decaying leaves and are then consumed by a variety of wildlife, especially birds.

Trophic Level 

Figure \(\PageIndex{10}\):   Food web diagram showing the various ways in which organism roles can be differentiated. Developed by N. Gownaris. 

The trophic level of an organism is the position it occupies in a food web. The trophic level of an organism is the number of steps it is from the start of a food chain. A food web starts at trophic level 1 with primary producers, followed by herbivores at level 2, carnivores at level 3 or higher, and apex predators at level 4 or 5. The path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths. The word trophic derives from the Greek τροφή (trophē) referring to food or nourishment (merriam-webster.com, 2017). 

• Level 1: Plants and algae make their own food and are called producers.
• Level 2: Herbivores eat plants and are called primary consumers.
• Level 3: Carnivores that eat herbivores are called secondary consumers.
• Level 4: Carnivores that eat other carnivores are called tertiary consumers.
• Apex predators by definition have no predators and are at the top of their food web.

Figure \(\PageIndex{11}\): Examples of species found at each trophic level of a terrestrial ecosystem. 

The trophic level concept was introduced in a historical landmark paper on trophic dynamics in 1942 by Raymond L. Lindeman. The basis of trophic dynamics is the transfer of energy from one part of the ecosystem to another (Odum and Heald 1975; Cortés 1999). The trophic dynamic concept has served as a useful quantitative heuristic, but it has several major limitations including the precision by which an organism can be allocated to a specific trophic level. Omnivores, for example, are not restricted to any single level. Nonetheless, recent research has found that discrete trophic levels do exist, but "above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores.” (Pauly et al. 1998).   

Figure \(\PageIndex{12}\): Killer whales (orca) are apex predators but they are divided into separate populations that hunt specific prey, such as tuna, small sharks, and seals.

The fisheries scientist Daniel Pauly sets the values of trophic levels to one in plants and detritus, two in herbivores and detritivores (primary consumers), three in secondary consumers, and so on. The definition of the trophic level, TL, for any consumer species is (Pauly and Palomares 2005)

\[ T L_{i}=1+\sum_{j}\left(T L_{j} \cdot D C_{i j}\right) \nonumber\]

where 

\[ T L_{j} \nonumber\]

is the fractional trophic level of the prey j, and 

\[ D C_{i j}  \nonumber\]

represents the fraction of j in the diet of i. That is, the consumer trophic level is one plus the weighted average of how much different trophic levels contribute to its food.

In the case of marine ecosystems, the trophic level of most fish and other marine consumers takes a value between 2.0 and 5.0. The upper value, 5.0, is unusual, even for large fish (Cortés 1999), though it occurs in apex predators of marine mammals, such as polar bears and orcas (Pauly et al. 1998). 

Not Your Average Food Web: Microbial Loop \(\PageIndex{4}\)

Figure \(\PageIndex{13}\): Simplified microbial food web in the sunlit ocean by Anders et al. is licensed under CC-BY-SA 4.0. Left side: classic description of the carbon flow from photosynthetic algae to grazers and higher trophic levels in the food chain. Right side: microbial loop, with bacteria using dissolved organic carbon to gain biomass, which then re-enters the classic carbon flow through protists. Based on DeLong & Karl (2005).

The microbial food web refers to the combined trophic interactions among microbes in aquatic environments. These microbes include viruses, bacteria, algae, heterotrophic protists (such as ciliates and flagellates) (Mostajir et al. 2015). Scientists have relatively recently begun to appreciate the importance of this microscopic food web to the functioning of higher trophic levels. 

In aquatic environments, microbes constitute the base of the food web. Single celled photosynthetic organisms such as diatoms and cyanobacteria are generally the most important primary producers in the open ocean. Many of these cells, especially cyanobacteria, are too small to be captured and consumed by small crustaceans and planktonic larvae. Instead, these cells are consumed by phagotrophic protists which are readily consumed by larger organisms. Viruses can infect and break open bacterial cells and (to a lesser extent), planktonic algae (a.k.a. phytoplankton). Therefore, viruses in the microbial food web act to reduce the population of bacteria and, by lysing bacterial cells, release particulate and dissolved organic carbon (DOC). DOC may also be released into the environment by algal cells. The microbial loop describes a pathway in the microbial food web where DOC is returned to higher trophic levels via the incorporation into bacterial biomass.

Ecological pyramids

Figure \(\PageIndex{14}\): Illustration of a range of ecological pyramids, including top pyramid of numbers, middle pyramid of biomass, and bottom pyramid of energy. The terrestrial forest (summer) and the English Channel ecosystems exhibit inverted pyramids. Note: trophic levels are not drawn to scale and the pyramid of numbers excludes microorganisms and soil animals. Abbreviations: P=Producers, C1=Primary consumers, C2=Secondary consumers, C3=Tertiary consumers, S=Saprotrophs (Odum and Barrett 2005).  

Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy (Odum and Barrett 2005). In a pyramid of numbers, the number of consumers at each level decreases significantly, so that a single top consumer, (e.g., a polar bear or a human), will be supported by a much larger number of separate producers. There is usually a maximum of four or five links in a food chain, although food chains in aquatic ecosystems are more often longer than those on land. Eventually, all the energy in a food chain is dispersed as heat (Odum and Barrett 2005).

Pyramid structure can vary across ecosystems and across time. In some instances biomass pyramids can be inverted. This pattern is often identified in aquatic and coral reef ecosystems. The pattern of biomass inversion is attributed to different sizes of producers. Aquatic communities are often dominated by producers that are smaller than the consumers that have high growth rates. Aquatic producers, such as planktonic algae or aquatic plants, lack the large accumulation of secondary growth as exists in the woody trees of terrestrial ecosystems. However, they are able to reproduce quickly enough to support a larger biomass of grazers. This inverts the pyramid. Primary consumers have longer lifespans and slower growth rates that accumulate more biomass than the producers they consume. Phytoplankton live just a few days, whereas the zooplankton eating the phytoplankton live for several weeks and the fish eating the zooplankton live for several consecutive years (Spellman 2008). Aquatic predators also tend to have a lower death rate than the smaller consumers, which contributes to the inverted pyramidal pattern. Population structure, migration rates, and environmental refuge for prey are other possible causes for pyramids with biomass inverted. Energy pyramids, however, will always have an upright pyramid shape if all sources of food energy are included and this is dictated by the second law of thermodynamics (Odum and Barrett 2005; Wang et al. 2009). 

Figure \(\PageIndex{15}\): A pyramid of biomass shows the total biomass of the organisms involved at each trophic level of an ecosystem. These pyramids are not necessarily upright. There can be lower amounts of biomass at the bottom of the pyramid if the rate of primary production per unit biomass is high.

The graphical representation of the number of organisms at all the levels of a food chain is ______________

Ecological pyramid: the ecological pyramid was discovered by g. evelyn hutchinson and raymond lindeman. it is the graphical presentation of the relationship between the different living organisms at different trophic levels. there are 3 types of ecological pyramids- pyramid of numbers; pyramid of biomass; and pyramid of energy. final answer: the graphical representation of the number of organisms at all the levels of a food chain is the ecological pyramid..

Give one word for the graphic representation of the number of individuals at different trophic levels in a food chain.

A graphical representation of number of individuals of different species belonging to a particular trophic level in an ecosystem is

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It’s a McDouble by name — but almost a McTriple by price.

A shocking graphic exposes how the cost of individual McDonald’s items has surged over the past decade, with the price of a McDouble up an astonishing 168%.

The visual, created by Carbon Finance , is based on a study by FinanceBuzz released last month, which cited average price hikes at the franchise nationwide.

Back in 2014, a McDouble cost $1.19 on average, but consumers can now expect to fork out $3.19 for the small sandwich.

Meanwhile, the graphic reveals how the price of a McChicken sandwich has soared a staggering $199% from $1.00 to $2.99.

A medium serve of McDonald’s French fries will now set you back an average of $3.79, up 138% from the $1.79 they cost 10 years ago.

A Quarter Pounder Meal has surged 122%, while the cost of an Oreo McFlurry has risen 88%.

In its study, FinanceBuzz acknowledged that it is “difficult to accurately source historical data to compare to the present” since “McDonald’s franchisees are given a high level of autonomy in setting menu prices for individual locations.”

They thus declared: “As such, our team collected additional historical data points related to McDonald’s and applied certain adjustments to the final data to create a reasonable representation of national pricing trends over time for the chain.”

However, in a statement, McDonald’s told The Post: “As the report itself notes, pricing is set by individual franchisees and varies by restaurant. This is not an accurate representation of historical or current pricing at McDonald’s restaurants, and the 2024 average prices listed are significantly inflated.”

They added: “Value is part of McDonald’s DNA, and we’re committed to offering customers great value through everyday affordable pricing plus special offers and deals on our App and through the MyMcDonald’s Rewards program.”

However, many customers have complained about forking out more for fast food after the visual went viral on Reddit .

“I quit going there months ago,” one enraged Redditor wrote. “No more supporting this bulls–t.”

“Top it off with longer wait times,” another disgruntled consumer cried. “Once upon a time it was ok because the food was cheap and you could go through the drive-through pretty quickly. Price and speed made up for quality. Now, I could just as easily go to a sit-down restaurant for almost the same cost and time with the benefit of better food. We very rarely go there now.”

A third fumed: “It’s cheaper to get a Double Double Cheeseburger Meal at In ‘N Out (where there’s way more employees working) than it is to get a Quarter Pounder Meal with kiosks everywhere… it’s not even real food. So f–king processed.”

According to the Finance Buzz, McDonald’s was only one of a number of fast food franchises whose prices had risen more than the rate of inflation.

While McDonald’s was the worst offender — tripling the rate of inflation, according to FinanceBuzz — Popeyes, Taco Bell, Chipotle and Jimmy John’s raised the prices of their menu items at more than double the actual inflation rate, the study found.

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