Ch 19 · Food Webs · Ecology for All!

CHAPTER OVERVIEW 19: Food Webs Learning Objectives Differentiate among the components of food webs, the sources of energy to food webs, and the types of food webs Use quantifiable characteristics to describe and compare food webs Appreciate the key ecosystem role that apex predators play through trophic cascades 19.1: Introduction to and Components of Food Webs 19.2: Quantifying Food Webs 19.3: Trophic Cascades 19.4: Sources and Attributions Summary The food webs of all ecosystems have autotrophs at the base; these producers may fix carbon using energy from the sun (photoautotrophs) or from specific chemical compounds (chemoautotrophs). The energy produced at the base of the food web moves through several levels of heterotrophs, including detritivores, herbivores, omnivores, and carnivores, but some of this energy is lost during each transfer up the food web. When the apex predator or keystone predator of an ecosystem is removed, this can result in a trophic cascade that impacts the entire food web. Food webs are extremely complex, and cannot be easily depicted on paper, so the food webs we see are simplifications of reality; the simplest form is a food chain, which involved a linear sequence of interactions from primary producer to apex predator. Scientists quantify food webs by calculating transfer efficiency, food web length, and food web connectance. 19: Food Webs is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

19.1: Introduction to and Components of Food Webs 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.

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

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

: 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.

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.

Autotrophs 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

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

: 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

Not Your Average Food Web: Deep Sea 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 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 & 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 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 (credit: 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 & 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 al., 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 & 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). Coprophagy 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).

: 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 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.

: 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 (troph) 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.

: 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 & 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).

: 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 & Palomares, 2005)

where is the fractional trophic level of the prey j, and 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

: 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.

: 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 &

Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy (Odum & 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 & 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 & Barrett, 2005; Wang et al., 2009).

: 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.

19.1: Introduction to and Components of Food Webs is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

19.2: Quantifying Food Webs Ecologists collect data on trophic levels and food webs to statistically model and mathematically calculate parameters, such as those used in other kinds of network analysis (e.g., graph theory), to study emergent patterns and properties shared among ecosystems. There are different ecological dimensions that can be mapped to create more complicated food webs, including: species composition (type of species), richness (number of species), biomass (the dry weight of plants and animals), productivity (rates of conversion of energy and nutrients into growth), and stability (food webs over time). A food web diagram illustrating species composition shows how change in a single species can directly and indirectly influence many others. The sections below describe three metrics that ecologists use to describe and quantify food webs (Transfer Efficiency, Food Chain Length, Connectance). Transfer Efficiency The biomass of each trophic level decreases from the base of the chain to the top. This is because energy is lost to the environment with each transfer as entropy increases. About eighty to ninety percent of the energy is expended for the organism's life processes or is lost as heat or waste. Only about ten to twenty percent of the organism's energy is generally passed to the next organism (Spellman, 2008). The amount can be less than one percent in animals consuming less digestible plants, and it can be as high as forty percent in zooplankton consuming phytoplankton (Kent, 2000). Graphic representations of the biomass or productivity at each trophic level are called ecological pyramids or trophic pyramids. The proportion of energy that is transferred from one trophic level to the next is known as the transfer efficiency of a food web. The ten percent law of transfer of energy from one trophic level to the next can be attributed to Raymond Lindeman (1942) (Odum & Barrett, 2005). However, Lindeman did not call it a "law" and cited transfer efficiencies ranging from 0.1% to 37.5%. According to this law, during the transfer of organic food energy from one trophic level to the next higher level, only about ten percent of the transferred energy is stored as flesh. The remaining is lost during transfer, broken down in respiration, or lost to incomplete digestion by higher trophic level. When organisms are consumed, approximately 10% of the energy in the food is fixed into their flesh and is available for the next trophic level (carnivores or omnivores). When a carnivore or an omnivore in turn consumes that animal, only about 10% of energy is fixed in its flesh for the higher level. Again, it is important to remember that the 10% value represents an estimate and that, in reality, transfer efficiency varies highly among ecosystems, time periods, and trophic levels. Example: The Sun releases 10,000 J of energy, then plants take only 100 J of energy from sunlight (this is an exception to the 10% rule, since only 1% of energy is taken up by plants from sun); thereafter, a deer would take 10 J (10% of energy) from the plant. A wolf eating the deer would only take 1 J (10% of energy from deer). A human eating the wolf would take 0.1J (10% of energy from wolf), etc. The ten percent law provides a basic understanding on the cycling of food chains. Furthermore, the ten percent law shows the inefficiency of energy capture at each successive trophic level. The rational conclusion is that energy efficiency is best preserved by sourcing food as close to the initial energy source as possible.

: The relative energy in trophic levels in a Silver Springs, Florida, ecosystem is shown. Each trophic level has less energy available, and usually, but not always, supports a smaller mass of organisms at the next level.

Food Chain Length A common metric used to quantify food web trophic structure is food chain length. Food chain length is another way of describing food webs as a measure of the number of species encountered as energy or nutrients move from the plants to top predators (Post, 1993). In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web or the maximum trophic level of a species in that food web. The mean chain length of an entire web is the arithmetic average of the lengths of all chains in a food web (Pimm, 1979; Odum & Barrett, 2005). In a simple predator-prey example, a deer is one step removed from the plants it eats (chain length = 1) and a wolf that eats the deer is two steps removed from the plants (chain length = 2). Because energy is lost at each transfer from one trophic level to the next (see Transfer Efficiency), the maximum number of trophic levels is generally constrained to only four or five levels. The number of trophic levels is ultimately determined by the productivity at the base of the food web and by how efficiently this energy is transferred up the food chain. Greater productivity and higher efficiency can support a greater number of trophic levels.

Connectance Food webs are extremely complex. Complexity is a measure of an increasing number of permutations and it is also a metaphorical term that conveys the mental intractability or limits concerning unlimited algorithmic possibilities. In food web terminology, complexity is a product of the number of species and connectance (Neutel et al., 2002; Leveque, 2003; Proctor et al., 2005). Connectance is "the fraction of all possible links that are realized in a network" (Dunne et al., 2002). These concepts were derived and stimulated through the suggestion that complexity leads to stability in food webs, such as increasing the number of trophic levels in more species rich ecosystems. This hypothesis was challenged through mathematical models suggesting otherwise, but subsequent studies have shown that the premise holds in real systems (Neutel et al., 2002; Banasek-Richter et al., 2009). While the complexity of real food webs connections are difficult to decipher, ecologists have found mathematical models on networks an invaluable tool for gaining insight into the structure, stability, and laws of food web behaviors relative to observable outcomes. Quantitative formulas simplify the complexity of food web structure. The number of trophic links (tL), for example, is converted into a connectance value:

where, S(S-1)/2 is the maximum number of binary connections among S species (Paine 1988). "Connectance (C) is the fraction of all possible links that are realized (L/S2) and represents a standard measure of food web complexity..." (Williams et al., 2002).

: Food web and trophic level of the Chengjiang and Burgess Shale. S: number of species (nodes). L: number of

trophic links. C: connectance; L/S2. MaxTL: maximum trophic level of a species in the web. Paleoecological studies can

reconstruct fossil food-webs and trophic levels. Primary producers form the base (red spheres), predators at top (yellow spheres),

the lines represent feeding links. Original food-webs (left) are simplified (right) by aggregating groups feeding on common prey

into coarser grained trophic species (Dunne et al., 2008). Figure developed by Dunne et al. 2008 is licensed under CC-BY 4.0.

: Worked example of calculating connectance for a food web. Diagram by N. Gownaris and A. Wilson.

Techniques for Studying Food Webs Food web linkages, or the feeding relationships between species inhabiting a shared ecosystem, are an ecological lens through which ecosystem structure and function can be assessed, and thus are fundamental to informing sustainable resource management. Empirical feeding datasets have traditionally been painstakingly generated from stomach content analysis, direct observations and from biochemical trophic markers (stable isotopes, fatty acids, molecular tools). Each approach carries inherent biases and limitations, as well as advantages. Traditional Approaches to Studying Food Webs Traditional approaches to food web analysis include gut and scat content analysis. In these analyses, the diet of species is determined based on the remains of prey found in their stomachs or in their feces. These approaches are limited because they only allow researchers to examine the hard parts or otherwise undigested components of prey (e.g., otoliths, or ear bones, of fish and teeth of mice). Over the past two decades, stable isotope analysis has become an increasingly common approach to studying food webs. Even more recently, two new methods to studying food webs have emerged: the use of remotely operated

vehicles and the use of DNA in feces and gut contents. Below, we provide an example of a recent study using remotely operated vehicles to study food webs and diet in the deep sea.

: Traditional food web analysis has often entailed gut content analysis or fecal content analysis. Scientists use

the bits of prey that remain to calculate estimates of variables like percent occurrence for each prey type. The top photo (taken

by N. Gownaris, licenced CC-BY) shows a tilapia in the stomach of a Nile perch; very rarely is gut content analysis this

straightforward! The bottom photo (inside the owl pellet by Art Siegel and licensed under CC-BY-NC 2.0) shows prey remains

removed from an owl pellet, including teeth, claws, and bones of owl prey.

: An example of a food web study using stable isotopes of seven key fish species in Lake Turkana, Kenya. In

stable isotope studies, 13C provides information on the foraging habitat of each species, while 15N provides information on

the trophic level. Species foraging more offshore or more pelagically have a lower 13C signature and species feeding at lower

trophic levels have a lower 15N signature. In this example, H. forskalii is a pelagic fish (i.e., it feeds in the lake's open waters)

and has the highest trophic level among the species studied. Figure from Gownaris et al., 2015.

The Use of Remotely Operated Vehicles to Understand Understudied Food Webs Within the deep sea, Earth's largest ecosystem, the challenge of gathering empirical feeding data for food webs is particularly formidable due to logistical access and sampling constraints (Robison, 2004). Analyzing the contents of a consumer's stomach (gut or stomach content analysis) is the common and most directly quantitative way of inferring diet, and is an irreplaceable approach for determining the taxonomic identity of food web components. However, for deeper-dwelling fishes with internal gas spaces, stomach eversion can confound this approach (Drazen & Sutton, 2017). These analyses may also fail to quantify gelatinous prey that are readily digested and become quickly unrecognizable (Hyslop, 1980; Choy, 2013).

: A suite of six illustrative ROV frame grabs of pelagic predators and their prey included in Choy et al. (2017).

No scale bars for size are available from these sequences. From left to right, top to bottom: (a) Gonatus sp. (squid) feeding on a

bathylagid fish (Bathylagidae); (b) Periphylla periphylla, the helmet jellyfish, feeding on a gonatid squid (Gonatidae), with a

small narcomedusa (Aegina sp.) also captured; (c) an undescribed physonect siphonophore known as `the galaxy

siphonophore' feeding on a lanternfish of the family Myctophidae; (d) a narcomedusa, Solmissus, ingesting a salp chain

(Salpida); (e) the ctenophore Thalassocalyce inconstans, with a euphausiid (Euphausiacea) in its gut; and (f) the trachymedusa,

Halitrephes maasi, with a large, red mysid (Mysidae) in its gut. Figure from Choy et al., 2017.

Choy et al. (2017) used 27 years (1991-2016) of in situ feeding observations collected by remotely operated vehicles (ROVs) to quantitatively characterize the deep pelagic food web of central California within the California Current, complementing existing studies of diet and trophic interactions with a unique perspective. Seven hundred and forty-three independent feeding events were observed with ROVs from near-surface waters down to depths approaching 4000 m, involving an assemblage of 84 different predators and 82 different prey types, for a total of 242 unique feeding relationships. The greatest diversity of prey was consumed by narcomedusae, followed by physonect siphonophores, ctenophores and cephalopods. This study highlighted key interactions within the poorly understood `jelly web', showing the importance of medusae, ctenophores and siphonophores as key predators, whose ecological significance is comparable to large fish and squid species within the central California deep pelagic food web.

: An in situ perspective of the food web derived from ROV-based observations of feeding, as represented by

20 broad taxonomic groupings. The linkages between predator to prey are coloured according to predator group origin, and

loops indicate within-group feeding. The thickness of the lines or edges connecting food web components is scaled to the log

of the number of unique ROV feeding observations across the years 1991-2016 between the two groups of animals. The

different groups have eight colour-coded types according to main animal types as indicated by the legend and defined here:

red, cephalopods; orange, crustaceans; light green, fish; dark green, medusa; purple, siphonophores; blue, ctenophores and

grey, all other animals. In this plot, the vertical axis does not correspond to trophic level, because this metric is not readily

estimated for all members. Note that for the Sip and Med groups, there are overlapping sub-groups (calycophoran and

physonect siphonophores, and trachymedusa and scyphozoan medusa, respectively), which is attributed to varying levels of

taxonomic discrimination possible from in situ video observations. Figure from Choy et al., 2017.

19.2: Quantifying Food Webs is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

19.3: Trophic Cascades Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore). The trophic cascade is an ecological concept which has stimulated new research in many areas of ecology. For example, it can be important for understanding the knock-on effects of removing top predators from food webs, as humans have done in many places through hunting and fishing. A top-down cascade is a trophic cascade where the top consumer/predator controls the primary consumer population. In turn, the primary producer population thrives. The removal of the top predator can alter the food web dynamics. In this case, the primary consumers would overpopulate and exploit the primary producers. Eventually there would not be enough primary producers to sustain the consumer population. Top-down food web stability depends on competition and predation in the higher trophic levels. Invasive species can also alter this cascade by removing or becoming a top predator. This interaction may not always be negative. Studies have shown that certain invasive species have begun to shift cascades; and as a consequence, ecosystem degradation has been repaired (Kotta et al., 2018; Megrey & Werner). These trophic interactions shape patterns of biodiversity globally. Humans and climate change have affected these cascades drastically. One example can be seen with sea otters (Enhydra lutris) on the Pacific coast of the United States of America. Over time, human interactions caused a removal of sea otters. One of their main prey, the pacific purple sea urchin (Strongylocentrotus purpuratus) eventually began to overpopulate. The overpopulation caused increased predation of giant kelp (Macrocystis pyrifera). As a result, there was extreme deterioration of the kelp forests along the California coast (Estes & Palmisano, 1974; Szpak et al., 2013). This is why it is important for countries to regulate marine and terrestrial ecosystems (Zhang et al., 2018).

: Diagram representing the trophic cascade that occurs when sea otters are removed from coastal marine ecosystems

in California, ultimately driving a reduction in habitat due to the loss of Kelp. Developed by N. Gownaris using loopy. Create your

own using LOOPY: a tool for thinking in systems (ncase.me)

Predator-induced interactions could heavily influence the flux of atmospheric carbon if managed on a global scale. For example, a study was conducted to determine the cost of potential stored carbon in living kelp biomass in sea otter (Enhydra lutris) enhanced ecosystems. The study valued the potential storage between

05 million and $408 million dollars (US) on the European Carbon Exchange (2012) (Wilmers et al., 2012). One criticism of trophic cascades is that, in nature, communities rarely act as food chains with discrete trophic levels. Most communities, however, have complex food webs. In real food webs, consumers often feed at multiple trophic levels (omnivory),

organisms often change their diet as they grow larger, cannibalism occurs, and consumers are subsidized by inputs of resources from outside the local community, all of which blur the distinctions between trophic levels (Polis & Strong, 1996). Classic Examples of Trophic Cascades The earliest empirical demonstrations of trophic cascades came from marine and, especially, aquatic ecosystems. In addition to the sea otter example discussed above, two famous examples include: In North American lakes, piscivorous fish can dramatically reduce populations of zooplanktivorous fish; zooplanktivorous fish can dramatically alter freshwater zooplankton communities, and zooplankton grazing can in turn have large impacts on phytoplankton communities. Removal of piscivorous fish can change lake water from clear to green by allowing phytoplankton to flourish (Carpenter et al., 1985). In the Eel River, in Northern California, fish (steelhead and roach) consume fish larvae and predatory insects. These smaller predators prey on midge larvae, which feed on algae. Removal of the larger fish increases the abundance of algae (Power, 1990).

: Healthy Pacific kelp forests, like this one at San Clemente Island of California's Channel Islands, have been shown

to flourish when sea otters are present. When otters are absent, sea urchin populations can irrupt and severely degrade the kelp

forest ecosystem. This image by Aquaimages is licensed under CC BY-SA 2.5.

More recent examples of trophic cascades in marine ecosystems have been identified: An example of a cascade in a complex, open-ocean ecosystem occurred in the northwest Atlantic during the 1980s and 1990s. The removal of Atlantic cod (Gadus morhua) and other ground fishes by sustained overfishing resulted in increases in the abundance of the prey species for these ground fishes, particularly smaller forage fishes and invertebrates such as the northern snow crab (Chionoecetes opilio) and northern shrimp (Pandalus borealis). The increased abundance of these prey species altered the community of zooplankton that serve as food for smaller fishes and invertebrates as an indirect effect (Frank et al., 2005). A similar cascade, also involving the Atlantic cod, occurred in the Baltic Sea at the end of the 1980s. After a decline in Atlantic cod, the abundance of its main prey, the sprat (Sprattus sprattus), increased (Alheit et al. 2005) and the Baltic Sea ecosystem shifted from being dominated by cod into being dominated by sprat. The next level of trophic cascade was a decrease in the abundance of Pseudocalanus acuspes (Mollmann et al., 2008), a copepod which the sprat prey on. On Caribbean coral reefs, several species of angelfishes and parrotfishes eat species of sponges that lack chemical defenses. Removal of these sponge-eating fish species from reefs by fish-trapping and netting has resulted in a shift in the sponge community toward fast-growing sponge species that lack chemical defenses (Loh & Pawlik, 2014). These fast-growing sponge

species are superior competitors for space, and overgrow and smother reef-building corals to a greater extent on overfished reefs (Loh et al., 2015). The fact that the earliest documented trophic cascades all occurred in lakes and streams led a scientist to speculate that fundamental differences between aquatic and terrestrial food webs made trophic cascades primarily an aquatic phenomenon. Trophic cascades were restricted to communities with relatively low species diversity, in which a small number of species could have overwhelming influence and the food web could operate as a linear food chain. Additionally, well documented trophic cascades at that point in time all occurred in food chains with algae as the primary producer. Trophic cascades, Strong argued, may only occur in communities with fast-growing producers which lack defenses against herbivory. Subsequent research has documented trophic cascades in terrestrial ecosystems, including: A classic example of a terrestrial trophic cascade is the reintroduction of gray wolves (Canis lupus) to Yellowstone National Park, which reduced the number, and changed the behavior, of elk (Cervus canadensis). This in turn released several plant species from grazing pressure and subsequently led to the transformation of riparian ecosystems. This example of a trophic cascade is vividly shown and explained in the viral video "How Wolves Change Rivers" (Sustainable Human, 2014). In the coastal prairie of Northern California, yellow bush lupines are fed upon by a particularly destructive herbivore, the rootboring caterpillar of the ghost moth. Entomopathogenic nematodes kill the caterpillars, and can increase the survival and seed production of lupines (Strong et al., 1999; Preisser, 2003). In Costa Rican rain forest, a clerid beetle specializes in eating ants. The ant Pheidole bicornis has a mutualistic association with Piper plants: the ant lives on the Piper and removes caterpillars and other insect herbivores. The clerid beetle, by reducing the abundance of ants, increases the leaf area removed from Piper plants by insect herbivores (Letourneau & Dyer, 1998). Critics pointed out that published terrestrial trophic cascades generally involved smaller subsets of the food web (often only a single plant species). This was quite different from aquatic trophic cascades, in which the biomass of producers as a whole were reduced when predators were removed. Additionally, most terrestrial trophic cascades did not demonstrate reduced plant biomass when predators were removed, but only increased plant damage from herbivores (Polis et al., 2000). It was unclear if such damage would actually result in reduced plant biomass or abundance. In 2002 a meta-analysis found trophic cascades to be generally weaker in terrestrial ecosystems, meaning that changes in predator biomass resulted in smaller changes in plant biomass (Shurin et al., 2002). In contrast, a study published in 2009 demonstrated that multiple species of trees with highly varying autecologies are in fact heavily impacted by the loss of an apex predator (Beschta & Ripple, 2009). Another study, published in 2011, demonstrated that the loss of large terrestrial predators also significantly degrades the integrity of river and stream systems, impacting their morphology, hydrology, and associated biological communities (Beschta & Ripple, 2011).

An Ecosystem-Wide Trophic Cascade: The Wolves of Yellowstone National Park The critics' model is challenged by studies accumulating since the reintroduction of gray wolves (Canis lupus) to Yellowstone National Park. The gray wolf, after being extirpated in the 1920s and absent for 70 years, was reintroduced to the park in 1995 and 1996. Since then a three-tiered trophic cascade has been reestablished involving wolves, elk (Cervus elaphus), and woody browse species such as aspen (Populus tremuloides), cottonwoods (Populus spp.), and willows (Salix spp.). Mechanisms likely include actual wolf predation of elk, which reduces their numbers, and the threat of predation, which alters elk behavior and feeding habits, resulting in these plant species being released from intensive browsing pressure. Subsequently, their survival and recruitment rates have significantly increased in some places within Yellowstone's northern range. This effect is particularly noted among the range's riparian plant communities, with upland communities only recently beginning to show similar signs of recovery (Ripple & Beschta, 2012).

Examples of this phenomenon include: A 2-3 fold increase in deciduous woody vegetation cover, mostly of willow, in the Soda Butte Creek area between 1995 and 1999 (Groshong, 2004). Heights of the tallest willows in the Gallatin River valley increasing from 75 cm to 200 cm between 1998 and 2002 (Ripple & Beschta, 2004). Heights of the tallest willows in the Blacktail Creek area increased from less than 50 cm to more than 250 cm between 1997 and 2003. Additionally, canopy cover over streams increased significantly, from only 5% to a range of 14-73% (Beschta & Ripple, 2007). In the northern range, tall deciduous woody vegetation cover increased by 170% between 1991 and 2006 (Baril, 2009). In the Lamar and Soda Butte Valleys the number of young cottonwood trees that had been successfully recruited went from 0 to 156 between 2001 and 2010 (Ripple & Beschta, 2012).

:This diagram illustrates trophic cascade caused by removal of the top predator. When the top predator is removed the

population of deer is able to grow unchecked and this causes over-consumption of the primary producers. This image by Zirguezi is

Trophic cascades also impact the biodiversity of ecosystems, and when examined from that perspective wolves appear to be having multiple, positive cascading impacts on the biodiversity of Yellowstone National Park. These impacts include: Scavengers, such as ravens (Corvus corax), bald eagles (Haliaeetus leucocephalus), and even grizzly bears (Ursus arctos horribilis), are likely subsidized by the carcasses of wolf kills (Wilmers et al., 2003). In the northern range, the relative abundance of six out of seven native songbirds which utilize willow was found to be greater in areas of willow recovery as opposed to those where willows remained suppressed (Baril, 2009). Bison (Bison bison) numbers in the northern range have been steadily increasing as elk numbers have declined, presumably due to a decrease in interspecific competition between the two species (Painter & Ripple, 2012). Importantly, the number of beaver (Castor canadensis) colonies in the park has increased from one in 1996 to twelve in 2009. The recovery is likely due to the increase in willow availability, as they have been feeding almost exclusively on it. As keystone species, the resurgence of beaver is a critical event for the region. The presence of beavers has been shown to positively impact streambank erosion, sediment retention, water tables, nutrient cycling, and both the diversity and abundance of plant and animal life among riparian communities (Ripple & Beschta, 2012). There are a number of other examples of trophic cascades involving large terrestrial mammals, including: In both Zion National Park and Yosemite National Park, the increase in human visitation during the first half of the 20th century was found to correspond to the decline of native cougar (Puma concolor) populations in at least part of their range. Soon after, native populations of mule deer (Odocoileus hemionus) erupted, subjecting resident communities of cottonwoods (Populus fremontii) in Zion and California black oak (Quercus kelloggii) in Yosemite to intensified browsing. This halted successful recruitment of these species except in refugia inaccessible to the deer. In Zion the suppression of cottonwoods increased stream erosion and decreased the diversity and abundance of amphibians, reptiles, butterflies, and wildflowers. In parts of the park where cougars were still common these negative impacts were not expressed and riparian communities were significantly healthier (Ripple & Beschta, 2006; Ripple & Beschta, 2008). In sub-Saharan Africa, the decline of lion (Panthera leo) and leopard (Panthera pardus) populations has led to a rising population of olive baboon (Papio anubis). This case of mesopredator release negatively impacted already declining ungulate populations and is one of the reasons for increased conflict between baboons and humans, as the primates raid crops and spread intestinal parasites (Prugh et al., 2009; Estes et al., 2011). In the Australian states of New South Wales and South Australia, the presence or absence of dingoes (Canis lupus dingo) was found to be inversely related to the abundance of invasive red foxes (Vulpes vulpes). In other words, the foxes were most common where the dingoes were least common. Subsequently, populations of an endangered prey species, the dusky hopping mouse (Notomys fuscus) were also less abundant where dingoes were absent due to the foxes, which consume the mice, no longer being held in check by the top predator (Letnic & Dworjanyn, 2011). 19.3: Trophic Cascades is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

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