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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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19.1: Introduction to and Components of Food Webs

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Introduction

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 1). 

Figure 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 2), but no food web model can capture all of the complexity found within a natural ecosystem. 

Figure 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 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 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.

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.

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 licenced 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). 

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.

Decomposers, Detritivores, and Scavangers

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. 

Figure 4: 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.

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). There are many kinds of invertebrates, vertebrates and plants that carry out coprophagy. By doing so, all these detritivores contribute to decomposition and the nutrient cycles. Detritivores are usually arthropods and help in the process of remineralization. 

Plant tissues are made up of resilient molecules (cellulose, chitin, lignin and xylan) that decay at a much lower rate than other organic molecules. Detritivores perform the first stage of remineralization, by fragmenting the dead plant matter, allowing decomposers to perform the second stage of remineralization (Keddy 2017). The activity of detritivores are the reason why we do not see an accumulation of plant litter in nature (Keddy 2017; Sagi et al. 2019). 

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). 

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 polychaetes such as worms of the family Terebellidae.

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). 

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".

Earthworms are soil-dwelling detritivores.

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 Levels 

Figure 7: 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. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it is from the start of the chain. A food web starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, and typically finish with 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). 

Trophic levels can be represented by numbers, starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.

• 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 8: 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 9: 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 \(\PageIndex{4}\)

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 10: 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).

Ecological pyramids place the primary producers at the base. They can depict different numerical properties of ecosystems, including numbers of individuals per unit of area, biomass (g/m2), and energy (k cal m−2 yr−1). The emergent pyramidal arrangement of trophic levels with amounts of energy transfer decreasing as species become further removed from the source of production is one of several patterns that is repeated amongst the planet’s ecosystems (Pimm et al. 1991; Raffaelli 2002; Proulx et al. 2005). The size of each level in the pyramid generally represents biomass, which can be measured as the dry weight of an organism (Rickleffs 1996). Autotrophs may have the highest global proportion of biomass, but they are closely rivaled or surpassed by microbes (Whitman et al. 1998; Grommbridge and Jenkins 2002). 

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 11: 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.

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• Length: 45 Minutes

Life Science

Students construct possible food webs for six different ecosystems and learn about producers, consumers, herbivores, carnivores, and decomposers. Student sheets are provided in English and in Spanish.

This activity is from The Science of Food Teacher's Guide. Although it is most appropriate for use with students in grades 3–5, the lessons are easily adaptable for other grade levels. The guide is also available in print format.

Teacher Background

Objectives and standards, materials and setup, procedure and extensions, handouts and downloads.

Environments, such as oceans, forests, lakes, and deserts, are homes to different communities of organisms. Within each distinct environment, plants, animals, and other living things must find ways to obtain water, food, and other necessary resources. Different kinds of organisms have different needs. As seen in the previous activities, plants need air, water, nutrients (usually from soil), and light. Animals need air, water, and food.

All animals depend on plants and other producers. Some animals eat plants for food. Other animals eat animals that eat the plants, and so on. Some organisms even feed on waste and dead material. The general sequence of who eats whom in an ecosystem is known as a food chain. Energy is passed from one organism to another at each step in the chain. Most organisms, however, have more than one food source. Thus, a web, which depicts all of the different foods eaten by each animal, is a more accurate model of interactions within an ecosystem.

This activity lets students construct possible food webs for different ecosystems, as they learn about the roles of different kinds of living organisms.

Producers make all the molecules they need from simple substances and energy from the sun.

All other living things depend on producers for food.

Living things that must eat other organisms as food are known as consumers.

Food webs show all of the different interactions among producers and consumers in an ecosystem.

Science, Health, and Math Skills

Integrating information

Drawing conclusions

Materials per Student Group (see Setup)

set of crayons: one each of blue, green, red, and yellow

set of Ecosystem Cards representing one ecosystem

sheet of white construction or drawing paper, 9 in. x 12 in.

Make copies of the six sets of Ecosystem Cards for students in advance. Each group of students will receive one set of the cards.

Have students work in teams of 4.

Remind students of the previous activity in which they explored plants that people eat. Ask, Do people only eat one kind of food? What kinds of food do people eat? Explain to students that most other animals also have several food sources, although not all animals are omnivores (eat plants and animals).

Discuss with students the different kinds of consumers: Herbivores (primary consumers) feed on plants and other producers. Cows, camels, caterpillars, and aphids are herbivores. Carnivores (secondary consumers) feed on other animals. Most consumers are animals, but a few are plants that trap and digest insects. There can be several levels of carnivores in a food chain. Lions, owls, and lobsters are carnivores. Omnivores eat plants and animals. Pigs, dogs, humans, and cockroaches all are omnivores. Decomposers and scavengers feed off the dead remains and waste of other organisms at any step along a food chain. Scavengers, such as vultures and flies, feed on remains of animals that have been killed or that die naturally. Decomposers live off waste products and parts of dead organisms. Many kinds of bacteria and fungi (molds and mushrooms) are decomposers. The decomposers themselves are important food sources for other organisms that live in soil, such as worms and insects.

Give each group of students a different set of Ecosystem Cards. Each set consists of six cards depicting producers and consumers typically found within a given environment.

Have students in each group read the information on the cards.

Ask students to identify which organisms are the producers in their ecosystems. Next, have the members of each group identify which cards represent different kinds of consumers (herbivores, carnivores,and scavenger/decomposers).

Once students have identified the producers and different kinds of consumers in their ecosystems, have them discuss “who might eat whom” among the organisms depicted on their cards. For example, in the Freshwater Pond set of cards, the bluegill fish (carnivore) might eat dragonfly nymphs and snails. The snail (decomposer/scavenger) might eat the green algae, as well as waste or dead body parts from all of the other organisms in the system. Have students consider possible food sources for each of the organisms in their ecosystem.

Give each group a sheet of drawing paper. Instruct students to write the names of each of the organisms in their ecosystems around the edges of the sheet. Have them write the names of the producers in green, the herbivores in yellow, the carnivores in blue, and the decomposers and scavengers in red.

Next, have students draw lines to connect each consumer to all of its food sources. They will find that there are many ways to connect even as few as six organisms within an ecosystem.

Encourage students to think about the complex relationships within ecosystems by asking questions such as, What would happen if there were no producers in your ecosystem? No decomposers? Where would humans fit in your food web? Do humans also depend on many different plants and animals?

Have students (individually or in groups) draw pictures of their ecosystems, including the organisms they used to construct their food webs.

Have students conduct additional research about the ecosystems and/or organisms that they used for the food webs by consulting resources available at the library, on the internet, or from sources such as DVD/CD collections.

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Estimated Class Time for the Engagement: 20-30 minutes

EXPLORATION

This student-centered station lab is set up so students can begin to explore food webs. Four of the stations are considered input stations where students are learning new information about food webs, and four of the stations are output stations where students will be demonstrating their mastery of the input stations.  Each of the stations is differentiated to challenge students using a different learning style.  You can read more about how I set up the station labs here .

EXPLORE IT!

Students will be working in pairs to recreate the engagement activity that they went over at the beginning of the food webs lesson. Students will have 2 images that they will be looking at. The first image will require the students to list 7 organisms that live within a desert ecosystem. Students will explain how those organisms are related. The second image is the same as the first, however, includes a desert ecosystem food web. Students will explain how plants get their energy, the direction of the arrows, and what would the impact be if the hawk was removed from the web.

WATCH IT!

At this station, students will be watching a 4-minute video describing how wolves change rivers. Students will then answer questions related to the video and record their answers on their lab station sheet. For example, name 2 impacts the wolves had on the deer population at Yellowstone, how did the re-introduction impact tree populations, and how wolves impacted the flow of rivers in Yellowstone.

RESEARCH IT!

The research station will allow students to get online and participate in an interactive website about food webs. Students will read about food webs and what a trophic level means. Students interact with a food web game where they will drag and drop organisms within an Antarctic food web template. Once students have placed cards, they can check their answers and will be given opportunities to fix mistakes. With each concept, students will answer a few questions to help make the research more concrete.

READ IT!

This station will provide students with a one page reading about food webs. In the reading students will discover what the term ecology means and methods of ecological interdependence. Students will also learn from the reading that the many relationships that occur in an ecosystem that allows organisms to thrive an survive. There are 4 follow-up questions that the students will answer to show reading comprehension of the subject.

ASSESS IT!

WRITE IT!

Students who can answer open-ended questions about the lab truly understand the concepts that are being taught.  At this station, the students will be answering three questions like describing the impact of removing an organism from a food web, describe the flow of energy in a marine food web, and explain the reason why humans are dependent on a healthy ecosystem.

ILLUSTRATE IT!

Your visual students will love this station.  Students will be creating a sample food web from an ecosystem they would find at a nearby park. Students will include the Sun, at least 7 organisms and arrows depicting the flow of energy.

ORGANIZE IT!

The organize it station allows your students to place organisms on a food web template. The marine food web contains 9 cards that students will place in the correct order showing the correct flow of energy.

Estimated Class Time for the Exploration: 1-2, 45 minute class periods

EXPLANATION

The explanation activities will become much more engaging for the class once they have completed the exploration station lab.  During the explanation piece, the teacher will be clearing up any misconceptions about food webs with an interactive PowerPoint, anchor charts, and interactive notebook activities. The food webs lesson includes a PowerPoint with activities scattered throughout to keep the students engaged.

The students will also be interacting with their journals using INB templates for food webs.  Each INB activity is designed to help students compartmentalize information for a greater understanding of the concept.  The food webs INB template will diagram the flow of energy that takes place in a terrestrial, freshwater, and marine ecosystem. Estimated Class Time for the Exploration: 2-3, 45 minute class periods

ELABORATION

The elaboration section of the 5E method of instruction is intended to give students choice on how they can prove mastery of the concept.  When students are given choice the ‘buy-in’ is much greater than when the teacher tells them the project they will have to create.  Each of the food web projects will allow students to show their understanding of the flow of energy transfer from organism to organism.

Estimated Class Time for the Elaboration: 2-3, 45 minute class periods (can also be used as an at-home project)

The final piece of the 5E model is to evaluate student comprehension.  Included in every 5E lesson is a homework assignment, assessment, and modified assessment.  Research has shown that homework needs to be meaningful and applicable to real-world activities in order to be effective.  When possible, I like to give open-ended assessments to truly gauge the student’s comprehension.

Estimated Class Time for the Elaboration: 1, 45 minute class period

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