Food Web on Land: Who Eats Who?

What is the Food Web? How does it begin? How many trophic levels does it have? What are primary producers? What are apex predators? How many different types of food webs are there? What is a food energy pyramid? How do toxins flow through a food web? How does climate change affect the food chain? We explain the answers to all these questions and more.
Rabbit Eating Grass
Rabbit (heterotroph) eating grass (autotroph). Photo by David Iliff. License: CC BY-SA 3.0

In this article we examine the terrestrial food web, its taxonomy and trophic levels, as well as its energy flow and biomass. The Food web is the main energy pathway of the biosphere and the biogeochemical cycles that support it. In addition, it plays a key role in climate change mitigation through the process of photosynthesis, which serves as a natural carbon capture and storage mechanism.

There are roughly 2.5 million species of plants and (non-insect) animals on Planet Earth, so it’s impossible to do justice to the complicated nature of the terrestrial food web without making gross oversimplifications. 1 Which is why, for example, most food pyramids and charts are no more than schematic diagrams.

What is the Food Web?

The term food web refers to the complex network of predator/prey relationships which exists in any given ecosystem or biome. In simple terms, it explains ‘who eats who’.

It used to be called the food chain, or food cycle, but as these minimalist concepts were unable to represent the dense thicket of connections between predators and prey, the term food web is used almost exclusively by researchers and scientists. In popular parlance, however, the terms food web and food chain are used interchangeably as they are, also, in this article.

Ecological Pyramid showing the four trophic levels of the terrestrial food web
Pyramid depicting the four main trophic levels of a food web. Underneath is the soil layer with its community of decomposers and detritivores. Photo: © Thompsma (CC BY-SA 3.0)

How Does the Food Web Start?

At its simplest, a food web traces the movement of energy from the sun (that is, sunlight) to all corners of the biosphere, through the bodies of living organisms like plants and animals. This energy, along with various nutrients, provides all the fuel needed by organisms to grow, develop, and reproduce. But how is sunlight converted into useable energy? Answer: by photosynthesis.

Photosynthesis is a biochemical process, performed only by plants, that converts solar energy into chemical energy, which is then stored in plant tissue in the form of complex carbohydrate. These carbohydrates are the starting point for the synthesis of fats and proteins.

Photosynthesis is the start of the food chain. Without it – and without the plants who produce it – there would be no food energy for any living thing, ourselves included. 2 Experts say that one of the reasons why the dinosaurs perished, after a large asteroid slammed into Mexico, was because sulfate aerosols blocked the sun for several years, chilling the planet and sharply reducing the rate of photosynthesis. 3

Plants are known as “Primary Producers”, because they are the ones who produce the energy that feeds everyone else. They are the feeding group (trophic species) at the bottom of every food chain. At the bottom of the marine food web, for instance, are microscopic algae plants known as phytoplankton (e.g. diatoms, cyanobacteria).

Terrestrial Food Web Taxonomy & Classification

This is the jargon section, but it’s really quite straightforward. Just remember, “troph” means “eater”. Scientists divide up the various trophic species and sub-species, as follows:

Autotrophs (Primary Producers)

Organisms that make their own energy are called autotrophs. There are two types. Those who use photosynthesis to make energy from sunlight are called photoautotrophs; and another small group, called chemoautotrophs, who use chemosynthesis to create energy from inorganic chemicals (such as hydrogen sulfide, sulfur, or iron) emitted from hydrothermal vents, methane seeps, and other geological features. 4

Heterotrophs (Consumers)

Organisms who are unable to create energy from sunlight or chemicals, and who must therefore get their energy from other plants or animals, are called heterotrophs. A plant survives by making its own energy from sunlight, but a rabbit can’t do this. Instead, he eats the plant. Then the rabbit is eaten by a weasel, who in turn is killed and eaten by a hawk. All three of these creatures are heterotrophs.

While autotrophs are producers, heterotrophs are consumers. In the example, the rabbit is a primary consumer, the weasel is a secondary consumer and the hawk, a tertiary consumer.

Mixotrophs

To complicate matters somewhat, some organisms – known as mixotrophs – survive on a mixture of food sources. The best-known mixotrophs are carnivorous plants, who obtain their energy from sunlight, but also get extra nutrition by eating insects and other arthropods. Other organisms combine autotrophy with heterotrophy, or a mixture of lithotrophy and organotrophy, or osmotrophy, phagotrophy and myzocytosis.

Schematic illustration of the terrestrial food web
Schematic diagram of a food web depicting the relationship between autotrophs and heterotrophs. The latter includes, herbivores and carnivores, as well as decomposers who maintain the cyclic movement of mineral nutrients. Image: © Thompsma (CC BY-SA 3.0)

Layout of a Food Web

Starting with the primary producers (plants), a food web shows how energy flows from them to primary consumers (usually herbivores), and from them to secondary and tertiary consumers, who occupy higher trophic levels. 5 The feeding pathways between different trophic species – like those between heterotrophic rabbits and autotrophic grasses – are known as “trophic links”. Incidentally, the term “trophic species” is used to describe groups of organisms that have the same predators and prey in a food web.

Here is a short summary of how energy is passed up the food chain as various species prey on those below them and, in turn are devoured by those above them:

• Level 1. Primary Producers
At the bottom of every food chain are the self-feeding autotrophs (land plants or ocean phytoplankton) who create energy from abiotic sources (sunlight/chemicals). They are known as “primary producers”.

Autotrophs use photosynthesis to make organic matter out of inorganic substances like carbon dioxide (CO2), using sunlight as an energy source. Only plants have the ability to photosynthesize.

• Level 2. Primary Consumers
Next come the heterotrophs, the herbivores (e.g. rabbits) who prey on the autotrophs. These herbivores are known as “primary consumers”.

• Level 3. Secondary Consumers
Next, come the carnivorous heterotrophs, (e.g. foxes) who prey on the herbivores. These carnivores are known as “secondary consumers”.

• Level 4. Tertiary Consumers
Next, come the larger carnivores, (e.g. larger land mammals or reptiles) who prey on the smaller or less aggressive carnivores.

• Level 5. Apex Predators
Next, come the apex predators, (such as lions, tigers, crocodiles, eagles) who have no natural predators while in good health.

• Levels 1-6. Saprotrophs: Detritivores and Decomposers
At any level, a variety of organisms, known as detritivores and decomposers, exist to rescue and recycle nutrients in the waste and remains of animals and plants. Examples of detritivores include: certain birds of prey, such as vultures, that consume the remains of dead animals, as well as dung beetles and numerous flies and other insects, that eat animal feces (coprophagia).

Decomposers like fungi, bacteria and protists perform a similar recycling function to detritivores, except they don’t consume and then digest dead matter internally. Instead, they release enzymes onto the decaying matter which breaks it down into its nutritional components, in a process known as lysis. They then absorb some of the nutrient-rich fluid (lysate) externally, via chemical and biological processes. The remainder is released into the soil for use by autotrophs.

Different Types of Food Web

There are as many different food webs as there are biomes or ecosystems. There are land-based or ocean-based food webs, which sub-divide according to biome (e.g. the rainforest food web), or ecological community (e.g. the pond food web, the soil food web, or the ocean floor food web), and so on.

In a pond, for example, tiny algae (autotroph) might be consumed by a tadpole, which itself is then devoured by a dragonfly larva, which in turn is preyed on by a fish, which is snapped up by a heron.

There are also food webs that depict feeding relationships at microbial level, and on a small-scale. For example, pea plants (autotrophs) are eaten by aphids, who are devoured by beetles, who in turn are consumed by spiders, who are preyed on by blackbirds, sparrows, crows, wrens and blue tits.

In addition, a particular biome (e.g. Arctic food web) may have different seasonal varieties, reflecting significant changes in climate and landscape, or even the presence or absence of migrating animals, birds and fish.

Food Web on Land, Ecological Pyramid
Ecological pyramid showing how much chemical energy (initially obtained from the sun) is transferred to each successive trophic level in an ecosystem. The green arrows depict the loss of heat energy via metabolic processes. Also, it shows nutrients being returned to the soil by decomposers, where it is taken up once again by plants. There are several different types of ecological pyramid. In addition to the energy pyramid, shown above, there are pyramids depicting numbers of organisms, the amount of biomass at each trophic level, and the amount of energy present at each trophic level. Image: © Swiggity.Swag.YOLO.Bro (CC BY-SA 4.0)

Food Webs and Energy Flow

Food webs depict the flow of chemical energy through the complex network of living organisms. All organisms, from plants all the way up to apex predators, store this energy in their tissues (that is, as biomass – either plant phytomass or animal zoomass). However, the amount of energy which is passed up the food chain decreases at every trophic level.

In fact, on average – according to Lindeman’s Ten Percent Law – no more than 10 percent of the energy is transferred up to the next level – the remaining 90 percent is used up in respiration, foraging and hunting, and other activities, or is lost to the surroundings as heat. In other words, only around one tenth of the energy goes into making body tissue, fat or muscle, in the next trophic level. 6

For example, for every 100,000 kilocalories (kcal) stored in rainforest vegetation, no more than 10,000 kcal will reach primary consumers, while only 1,000 kcal will reach secondary consumers and a mere 100 kcal will reach the tertiary level. Any apex predators above this will share 10 kcal between them.

The amount of energy that is transferred between trophic levels is known as the “trophic level transfer efficiency” (TLTE). As a rule, a food chain can sustain no more than four to five energy transfers before all the energy is used up.

Not all animals are equally efficient at incorporating the energy from its food into biomass, which can then serve as fuel for the next trophic level – a metric known as “net consumer productivity”. Cold-blooded animals (ectotherms), like reptiles, typically expend less energy on respiration and heat, than warm-blooded animals (endotherms), like mammals. As a result, endotherms are obliged to eat more often than ectotherms in order to acquire the energy they need for survival.

The relative inefficiency of energy use by warm-blooded livestock has negative implications for the meat industry, which diverts large amounts of crops to feed cattle. Because a significant proportion of the energy obtained from animal feed is lost, there is a growing lobby pressing for the consumption of non-meat and non-dairy foods.

Flow of Toxins in Food Web

Just as energy flows through an ecosystem’s food web, other substances do the same. Toxic substances, for instance, move much more efficiently through the food chain than energy does, and the effects can be devastating. In fact, as we go higher up the food chain, the concentration of harmful chemicals rises higher and higher, an effect known as biomagnification. This is not to be confused with bioaccumulation which is the simple build-up of a substance in an animal, rather than the increasing build-up in animals higher up the chain.

White Rumped Vultures
White-rumped vultures. The birds’ numbers plummeted through the 1990s in Nepal. For years scientists couldn’t work out why they were dying in such large numbers. Then, in 2004 researchers discovered the cause: a commonly-used pain reliever called diclofenac that farmers give their cows when the animals stop producing milk. While harmless in cows, it causes kidney failure in vultures. Since then Nepal has banned the use of the drug and vulture populations are starting to grow again. Photo: Kuveskar/CC BY-SA 4.0

Apex predators like birds of prey are especially vulnerable to the effects of toxic chemicals. Eagles are vulnerable to mercury poisoning from power plant emissions 7, while the Egyptian Vulture and the White-backed African Vulture have both been badly affected by toxic pesticides, including Furadan and a group of substances known as HPAs (polycyclic aromatic hydrocarbons).

In addition, the Oriental White-Backed Vulture and the Indian Vulture have suffered a 97 percent drop in numbers across the Indian sub-continent due to ingestion of the veterinary drug diclofenac, an anti-inflammatory drug given to working animals to extend their working life. 8

Vultures are not the prettiest of birds, but they perform a hugely important role in the prevention of disease, reduction of pollution and the suppression of other undesirable scavengers. Unfortunately, but not unexpectedly, with the collapse in vulture populations, rats and feral dogs have multiplied, leading to numerous health problems. This is because a vulture’s metabolism erases all pathogens, whereas dogs and rats simply become carriers of disease. Worse still, the now 18-million strong population of wild dogs is beginning to attract the attention of leopards, with added risks for local inhabitants.

Impact of Climate Change on the Food Web

The main impact of climate change on the food web, is the disruption of the normal predator/prey relationships, leading to serious impacts on biodiversity and habitat. This can happen either because of direct, or indirect effects.

Direct effects include physiological impacts from rising temperatures such as heatwaves and drought, or from wildfires and similar extreme events. Climate-related sea level rise also has a direct impact. All this can result in the death, or ill-health, or reduced body-size, of certain species, with knock-on effects across the food web.

Indirect effects arise through things like phenology shifts, where certain life-cycle events such as migration, reproduction and flowering are changed due to a rise in temperature. For example, survival rates are greatly reduced when migrating animals or birds arrive at a stop-over location, or destination, before or after food sources are present. Normal plant pollination timetables are easily disrupted by changes in the routines of pollinators, such as bees and butterflies. See also: 7 Effects of Climate Change on Plants.

Indirect effects of climate change also includes harmful impacts on animals’ ranges, water supply and habitats. This often leads to species migrating to a new habitat with added stress both for themselves and the local species. It also impacts on the predators of these species who stay behind and now have less to eat. All this inevitably leads to loss of biodiversity and problems for the ecosystem.

By contrast, as global warming extends tropical heat north and south of the equator, it enables mosquitoes and other similar disease carriers and parasites to extend their ranges, both in terms of altitude and latitude, and find lots more species on which to prey. See also: Effects of Climate Change On Animals.

Overall, studies show that indirect effects of climate change tend to be more damaging to animals and the food web than direct effects. For example, research shows that some species may be severely impacted despite being well inside their thermal niche. 9 See also: 10 Birds Threatened by Climate Change.

As the recent Australian bushfires showed, the direct effects of climate change can be lethal to billions of living creatures. However, the cascade of indirect effects on animals and plants – and their habitats – caused by climate, can be even worse. The IPCC estimates that 20-30 percent of the plant and animal species assessed to date are at risk of extinction by the end of this century, if global temperature projections play out as forecast. 10

References

  1. “How did IPBES Global Assessment on Biodiversity and Ecosystem Services Estimate 1 Million Species at Risk?” []
  2. Food Chains and Food Webs.[]
  3. “Baby, it’s cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous.” Julia Brugger, Georg Feulner, Stefan Petri. Geophysical Research Letters, 2016 []
  4. “Autotroph.” []
  5. “Food Web: Concept and Applications”. []
  6. “Ecological Pyramid.” []
  7. “Using an apex predator for large-scale monitoring of trace element contamination: Associations with environmental, anthropogenic and dietary proxies.” Alexander Badry, et al. Science of The Total Environment. Volume 676, 1 August 2019, Pages 746-755. []
  8. “Identification of a Novel Mycoplasma Species from an Oriental White-Backed Vulture (Gyps bengalensis)“. Oaks, J. L. et al; (2004). Journal of Clinical Microbiology. 42 (12): 5909–5912. []
  9. Food-web dynamics under climate change.” Lai Zhang et al. Royal Society. []
  10. IPCC. Fifth Assessment Report (2014). Settele, J., et al. “Terrestrial and Inland Water Systems.” In: “Climate Change 2014: Impacts, Adaptation and Vulnerability. Part A: Global and Sectoral Aspects.” Contribution of Working Group II to the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. []
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