Trophic Ecology: _{Feeding relationships and energy transfer}

Food chains: Understanding basic feeding relationships

A “community” is a group of organisms living in the same area and interacting with each other. The oak tree, the worm, the robin, and the Cooper’s hawk are all members of a meadow community. How they interact is largely shaped by what they eat. As English ecologist Charles Elton put it, “Food is the burning question in animal society, and the whole structure… of the community is based on the food supply.” (1927).

Elton coined the term “food chain” to describe a linear sequence of feeding relationships in a community. Figure 1 shows just one of the many food chains you could encounter in a natural meadow or urban grassy patch. Decaying material in the soil is eaten by earthworms, which are eaten by ground-feeding birds, which are preyed on by hawks. Wherever different species co-exist, there are food chains. Elton recognized that these feeding relationships are largely what structure communities.

Elton was by no means the first to recognize the feeding relationships that shape community interactions. Forty years earlier, American entomologist Stephen Forbes published “The Lake as a Microcosm,” (Forbes, 1887). In his paper, Forbes described how the “animals of such a body of water are …closely related among themselves in all their interests.” And even before Forbes, Native American ways of life involved an understanding of the feeding relationships in the ecosystems they inhabited, a necessity for living sustainably with the organisms around them.

Many tribes were agriculturists, while others were more migratory and prolific hunters/fishers and gatherers.

Mark Ford, Chiricahua Apache and Tewa/Tiwa, 2021

Elton formalized the term “trophic relationships,” or feeding relationships (trophic = related to feeding, from the Greek word for food). Every organism can be assigned to a “trophic level,” or feeding level relative to other species of organism in its community.

For example, as shown in the meadow food chain pictured in Figure 1, plants form the base level of every community on dry land. On the next level above plants are herbivores like worms, which are eaten by carnivores. There can be multiple levels of carnivores. For instance, primary carnivores, such as blackbirds, eat herbivores, while secondary carnivores, such as hawks, eat the primary carnivores. The animals at the very top of a food chain are called top predators or “apex predators.”

Knowing this, why do you think photosynthesizers are at the base of the food chain?

To answer that question, consider what would happen if you removed plants from the food chain in Figure 1. Through photosynthesis, plants harvest energy from the sun to produce energy-rich organic molecules such as sugar, which then feed the other organisms in the chain (see our module Photosynthesis I: Harnessing the energy of the sun). No plants mean less food for worms. Fewer worms result in less food for blackbirds, meaning less food for hawks, and so on.

Since plants and other photosynthetic organisms don’t have to eat to gain energy, they are called “autotrophs” (or producers, since they produce their own food). Organisms that must eat other organisms to get energy are called “heterotrophs” (or consumers).

In the food chain pictured in Figure 2, which organisms are autotrophs and which are heterotrophs?

In terrestrial (land-based) ecosystems, plants are the primary producers at the base of the food chain. However, in aquatic (water-based) ecosystems, seaweed, algae, and other phytoplankton (microscopic photosynthesizers) are the autotrophs that produce the food consumers rely on. Figure 2 depicts a river food chain where green algae are autotrophs. Like plants, green algae use chlorophyll pigment to capture the sun’s energy and turn it into food. The herbivorous snails eat the green algae and then become prey for fish, which are ultimately prey for black bears.

There can be various layers and organisms in a food chain, but at the base are always the producers, like plants, algae, and other photosynthesizers. The energy they harvest from the sun is then passed up the food chain as chemical energy.

What limits the number of trophic levels?

The food chains we’ve examined so far each have 4-5 trophic levels. In nature, food chains tend to have no more than five trophic levels, encompassing the autotrophs, the herbivores, and the carnivores. Another category of feeders - the decomposers or “detritivores” - feed on dead matter, returning nutrients to the base of the food chain. These include scavenging vultures, hyenas, and other organisms that feed on decaying animal and plant matter. Fungi and bacteria absorb the remaining chemical energy that is inaccessible to other heterotrophs. Animal waste, animal bones, decaying plants, fallen trees, and more are all eventually recycled down to molecules, a key part of materials cycling through food chains.

If you’re wondering why there aren’t more trophic levels in food chains, you’re not alone. Ecologists have developed several hypotheses about what limits the number of trophic levels to no more than five.

Hypothesis 1: Energy-Productivity

The amount of energy flowing to top trophic levels depends on primary production and the efficiency at which it is converted to production at each trophic level.

American ecologist Freya E. Rowland, 2015

The hypothesis suggests that food-chain length is limited by available energy. Food contains energy, and each trophic level consumes some of this energy to build, maintain, and perform all its activities. As a result, only a small portion of the energy is passed up to the next level. The amount of energy transferred to the next level is called the “trophic transfer efficiency” (TTE).

Chinese fisheries biologist Long-qi Sun attempted to calculate the TTE for Sanggou Bay (Sun et al., 2020). The bay is an ecosystem connected to the Yellow Sea containing a mix of wild marine species and aquaculture (or farmed) species such as kelp, scallops, and oysters. With the assistance of ecosystem modeling software, Sun and colleagues came up with a TTE of 10.76% from primary producers to first-level consumers . Studies of other habitats have given similar results, leading ecologists to conclude that in natural ecosystems, roughly 10% of the energy required at one trophic level is available for consumption by the next level.

But the laws of physics tell us that energy is conserved; it cannot disappear. So, where does the rest of the energy go?

Besides powering the growth, maintenance, and physical activity of the organisms at each level, energy is also lost as heat and decomposition. As animals metabolize, heat is lost as a byproduct. This metabolic heat is what animals use to keep their bodies warm. In addition, every organism consumes energy as it performs the basic work of life, from maintaining cells and tissues to running, playing, and hunting. Also, organisms die at each level of the Energy Pyramid (see Figure 3) before they’re eaten. Their energy is not lost altogether since every community includes detritivores that help recycle dead matter back into the ecosystem.

If the Energy Hypothesis is correct, “productivity”—or the amount of energy available as food at the base of the food chain—should positively correlate to the number of trophic levels.

More energy available at the base → more trophic levels can be supported

Laboratory studies conducted during the 1990s showed that food chain length correlated with productivity. Specifically, researchers manipulated the nutrients in a growth medium for aquatic bacteria and protists. They found that the longest food chains—three levels—persisted only at high nutrient conditions, which boosted the bacteria population (Kaunzinger and Morin, 1998). However, research in the field has so far failed to confirm energy transfer (TTE) between trophic levels as the factor limiting food chain length.

Hypothesis 2: Disturbance

In the 1970s, scientists proposed another hypothesis to explain food chain length called the Dynamic Constraints Hypothesis. The hypothesis holds that frequent disturbances to the ecosystem lead to shorter food chains (Pimm and Lawton, 1977). Recovery from disturbances takes time and begins at the bottom of a food chain before moving up. As a result, disturbances are often most disruptive to the highest trophic levels since they depend on all the lower levels being intact.

For example, in an ecosystem disturbed by frequent hurricanes or fires, each level will take time to recover. The highest level, the top predators, are likely to starve before the recovery has progressed enough to provide for them. On the other hand, more stable environments could allow longer food chains to persist.

However, research in the field has generated inconsistent results. Some early studies found that North American rivers with steadier flows had longer food chains (Sabo et al., 2010), but more recent studies yielded conflicting conclusions.

Australian evolutionary biologist Nicholas P. Moran compared the Great Artesian Basin, which has highly stable groundwater springs, to the Lake Eyre Basin, which has highly variable rivers influenced by seasonal water availability (Moran et al., 2022). Contrary to predictions of the Dynamic Constraints Hypothesis, the contrasting water bodies displayed similar food chain lengths. Instead, the study results suggested that ecosystem size was a better predictor, with the largest groundwater spring and the largest river having longer food chains.

Moran’s studies directly relate to a third hypothesis below.

Hypothesis 3: Ecosystem size

The Productive Space Hypothesis proposes that food chain length is determined by ecosystem size, as found by Moran (2022). It seems logical: Small habitats have fewer resources, which may be too limited to provide enough resources for top predators. Larger ecosystems have greater total availability of resources starting at the bottom, promoting longer food chains as the resources travel upward through the system.

This Productive Space Hypothesis, like the Energy Hypothesis, presumes certain dynamics: The baseline resources are the plants or other food-producing autotrophs. And, in tandem with energy lost at each food chain level (see Figure 3), total biomass (the mass of living things) diminishes at each trophic level. Thus, the biomass of an ecosystem, sorted by trophic levels, should also have a pyramid shape (see Figure 4).

The Productive Space Hypothesis, then, predicts that more initial plant biomass should sustain more levels. Japanese biologist Gaku Takimoto (Takimoto, 2008) and colleagues found field evidence that ecosystem size determines food chain length. They examined trophic levels on islands ranging from 500 m2 to 300000 km2. Food chain length increased by about one trophic level from the smallest to the largest islands.

But, other field studies show no effect of ecosystem size. American community ecologist Hillary S. Young and colleagues (Young et al., 2013) found no effect of ecosystem size on food chain length in a set of coral islands in the central Pacific Ocean (the Palmyra Atoll). Clearly, the determinants of trophic levels are varied and complex.

Visualize complex feeding relationships with food webs

Most theoretical studies to date have assumed that communities are organized into neat trophic levels, in the simplest case represented as simple, linear unbranched food chains...However, food webs are more complex than this, because of omnivory, where species’ trophic roles in effect straddle multiple levels.

Gaku Takimoto, Japanese ecologist, 2012

None of the three hypotheses above adequately explains the length of food chains. The hypotheses were derived theoretically and make good sense, but studies of actual communities have not produced data to resolve the questions fully.

Why not? Reflect on that question while looking at Figure 5.

A “food web” shows the complex feeding relationships within a natural ecosystem that a simplified food chain fails to represent. For instance, in Figure 5, every brown arrow shows a feeding relationship. While a food chain demonstrates one direct set of feeding relationships, it lacks the complexity of trophic dynamics in natural ecosystems.

As Figure 5 shows, nearly all the animals feed on more than one type of food. By definition, an omnivore feeds at more than one level of the food web. Although there are food specialists in every community, most animals eat a mix of foods.

Within a food web, animals may also feed on other animals at their trophic level. For example, the boy in Figure 5 fishes for both green sunfish and largemouth bass, but the largemouth bass also eats the green sunfish. These feeding relationships within a trophic level are called “intraguild predation” (because a guild is a group of animals feeding at a similar trophic level) and give the visual an even more weblike form with horizontal connections.

Food web complexity

The complexity of trophic relationships in a food web, coupled with other variables like abiotic (nonliving) conditions, further complicates the question of what determines food chain length.

Canadian ecologist Tiffany A. Schriever studied nine Ontario ponds over two years to find any relationships between environmental variables (such as ecosystem size and the frequency of disturbance) and the length of food chains (Schriever, 2015). Schriever characterized the food webs based on the amphibian, invertebrate, and detritus communities and relative to physical-chemical data like dissolved oxygen, pH, temperature, and water depth. She found that multiple variables affect food web structure, with no single variable explaining food chain length in all cases. Instead, food chain length varied depending on habitat, such as small wetlands versus large ponds.

Scientists like Takimoto have come up with more sophisticated models to reflect the complexity of food webs (Takimoto et al., 2012). Takimoto and colleagues included intraguild predation (preying on competitors occupying a similar trophic level) in a food web model, which generated more complex hypotheses. Overall, their model predicts that if intraguild predation is weak, then higher productivity, large ecosystems, and lower disturbance rates support longer food chains. However, if intraguild predation is strong, then productivity and disturbance actually limit food chain length.

This work showed how different hypotheses might explain food webs under different conditions, getting us closer to a complete understanding of ecosystems.

Food web dynamics

The different hypotheses developed by various scholars of the community ecology have to be considered in an integrated fashion in order to clearly understand and explain the plausible mechanisms that influence the community structure and trophic level interactions in the terrestrial ecosystems.

Solomon Ayele Tadesse, Ethiopian ecologist, 2017

Food webs are dynamic because feeding relationships respond to many environmental changes, such as drought or the introduction of a new species.

For example, ecologists studying grassland food webs found that the biomass pyramids (see Figure 4) varied depending on the amount of rainfall (Chase et al., 2020). More rainfall meant more grasses, resulting in larger populations of herbivores eating them. When ecologists excluded large herbivores, by preventing cattle from grazing, for example, grasses flourished. This proved that herbivores control grasses. However, excluding the cattle made little difference when rainfall was low and water scarcity limited the grasses.

If food webs respond to abiotic conditions, such as rainfall, and their effects on producers and consumers, their stability likely also depends on these interactions.

Ecosystem stability

Trophic interactions underpin an extensive range of crucial ecosystem functions characterizing a large proportion of total ecosystem performance …and have been a central tenet of ecology since Elton’s pioneering work.

Andrew D. Barnes, New Zealand ecologist, 2021

It has become increasingly clear that food web trophic dynamics—including the flow of energy, nutrients, and biomass—support ecosystem function. The way ecosystems function determines the services they naturally provide to humans (see our Ecosystem Services module). In Figure 6, Barnes (2021) details eight ecosystem services resulting from trophic web interactions within a community.

Consider what would happen if a species disappeared from the community. For example, in Figure 6, how might the community change if there were no birds at the top of the web?

According to the diagram, the birds feed on several types of invertebrates (wasps, beetles, worms and aquatic insects). If the birds are lost, this could free those invertebrate populations from predation, causing them to grow (see our Population Biology module). Rising invertebrate populations would provide more food for other animals that rely on them, causing those populations to grow. But it might also reduce the population of the organisms that the invertebrates feed on. Because these trophic relationships structure the community, any disruption changes it—sometimes drastically—which can threaten the entire community.

Food webs are controlled both bottom-up (by primary productivity) and top-down (by predators). Any factors that alter one food web level can cause disturbances on other levels. Russian ecologist Anton M. Potapov and colleagues studied trophic dynamics in Indonesia’s tropical rainforests and compared them to nearby ecosystems that had been altered by the creation of rubber or palm oil plantations (Potapov et al., 2019). They found that the plantations had smaller populations of small soil invertebrates (such as beetles) but more large earthworms. Biomass and energy were concentrated in these large decomposers in the altered ecosystems, altering energy conversions at other trophic levels.

Consider Figure 7. What differences do you notice in decomposition, herbivory, and predation in the natural rainforests versus the altered ecosystems?

Potapov and colleagues compared the rainforest (F) to the altered ecosystems (J, R, and O) based on “energy flux” (movement of energy through the system). They noticed that the movement of energy from herbivory was lower in the plantations compared to the natural rainforest. When rainforest is converted into monoculture (single-crop) plantations, the trophic dynamics are disrupted. This results in less energy flow into higher trophic levels, such as the herbivores and carnivores that inhabit natural rainforests.

Humans in trophic systems

Humans are the most successful predators on Earth. When you add them to an ecosystem as top predators, the feeding relationships change substantially. For example, Figure 8 shows five trophic levels of the ocean food web and how the fishing industry affects 165 different marine species.

In Figure 8, how many marine species impacted by the fishing industry can you count?

The fishing industry impacts just about every marine species directly or indirectly. And other human activities also change trophic systems.

For example, Brazilian environmental scientist Maria A. L. Lima studied the effects of the 2011 dam construction on the Amazon’s Madeira River. The river boasts the highest fish species richness (number of species in an ecosystem) in the world (see our Biodiversity I module). Lima compared the trophic structure of a river ecosystem before and after the dam’s construction. Using known trophic relationships, she estimated the biomass and energy flows pre- and post-damming and found the following:

• More detritus after damming, supporting more decomposer organisms
• Less total biomass of fish after damming
• Lower energy transfer efficiency between trophic levels
• Shifts in which fishes were top predators

Feeding relationships give structure to communities and respond to changes in environmental conditions. As such, each chain of relationships is part of a dynamic system. Understanding the trophic system requires multiple perspectives on its energy and biomass fluxes.

Think back to the earthworms in Figure 1. They’re not just herbivores feeding on plant seedlings but also decomposers feeding on fallen leaves and other bits of dead or dying organic matter. If an earthworm can select both seedlings and detritus off the menu, you can envision how a linear food chain is just the beginning of the full story of this ecosystem.