What is a Trophic Cascade?

Trophic levels or feeding levels of organisms and effects on organisms at other levels.

Peter Miles
Age of Awareness
5 min readJan 7, 2023

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What is a Trophic Cascade? Image by Kate McIntosh.

Trophic levels or feeding levels, also termed trophic relationships, have been used to describe species interactions within an ecosystem based on the source of nutrients.

A trophic cascade is when the top predator in an ecosystem is removed, and it causes a chain reaction of events that alters the entire ecosystem.

The wolves In Yellowstone are a well-known trophic cascade example, when the last wolves were killed by human hunters in the 1930s, the elk were able to increase in numbers and over-graze the grasses, they also remained more stationary in areas and over-grazed the young willows. Beavers are dependent on young sapling willows to build their dams and as a result, the removal of all the wolves caused a declining beaver population.

The beaver dams also held back the stream waters, increasing the water height and flow out into the stream surrounds, raising the water table for water storage and use by plants and animals.

Species within ecosystems are classified as producers or consumers.

Producers or autotrophs, make the nutrients they need from chemical compounds and energy from their surrounding environment.

Plants are producers and are able to use the sunlight shining on their leaves to combine carbon dioxide and water to make carbohydrates and release oxygen in the process of photosynthesis.

In oceans, phytoplankton is the main producer.

Cyanobacteria started using photosynthesis several million years ago and gradually increased the atmosphere’s oxygen content.

Consumers or heterotrophs obtain their nutrients from other organisms.

Primary consumers are herbivores, for example, cattle and eat the plants, that is, the producers, carnivores are secondary consumers, such as lions and eat the herbivores, and higher-level tertiary consumers such as tigers and killer whales, eat herbivores and other carnivores.

There are also decomposers of plant and animal remains mostly bacteria and fungi, and detritus feeders which feed on dead bodies, such as hyenas, vultures, and earthworms (Knox et al., 2014; Miller & Spoolman, 2016).

A trophic cascade is a chain reaction that starts with a change in the population of one species and ends with a change in the population of another species.

The 2016 paper by Ripple and others, titled ‘What is a trophic cascade?’ and published in the Trends in Ecology and Evolution Journal, offers a definition of the term to be used by stakeholders with an interest in trophic cascades (Ripple et al, 2016).

Trophic cascades are indirect species interactions that originate with predators and spread downward through food webs.”

A trophic cascade occurs when one species in an ecosystem affects another species further down the food chain. For example, if a predator eats an herbivore, that will influence the plants the herbivore was eating.

The term has been used a lot by scientists, managers, and the general public because it is relevant to many topics in ecology, ecosystem management, and conservation.

A trophic cascade is when predators affect entire communities by interacting with their prey.

The term was first used in 1980 by Professor RT Paine in his Tansley Lecture, but the concept itself is much older.

One of the first written descriptions of a trophic cascade was in 1859 The Origin of Species. In this book, Darwin described how domestic cats controlled populations of mice that were otherwise free to devour the honeycombs of ‘humble bees’, in turn affecting plant-pollinator mutualisms.

Aldo Leopold famously described a wolf-deer-shrub cascade that resulted in “every edible bush and seedling browsed, first to anemic desuetude, and then to death” as a result of the wolves being hunted to local extinction.

In these examples, the effects of changing top predator abundance that propagated down food chains, ultimately having strong effects on plants, were apparent.

The idea that plant abundance was often regulated by such top-down effects of predators was formalized in 1960 as the Green World Hypothesis.

This hypothesis was extended by Fretwell and Barach’s 1977 paper ‘The Regulation of Plant Communities by the Food Chains Exploiting Them’ recognizing that the influence of predators on plants will vary with food chain length.

Oksanen et al.’s Exploitation Ecosystems Hypothesis 1981 paper ‘Exploitation ecosystems in gradients of primary productivity’ added the degree of primary productivity.

However, this theoretical work was just the beginning of what would develop empirically with a continuous series of discoveries of trophic cascades in diverse ecosystems.

Trophic cascades are a series of impacts that start at the top of a food web and move down to the bottom.

They were first discovered in aquatic food webs, sea otters regulated urchin abundance and overconsumption of kelp and other macroalgae, but later researchers found them in terrestrial food webs as well.

Gray wolves are an example of a large terrestrial carnivore that can cause a trophic cascade.

Trophic cascades are not only influenced by reduced herbivore numbers but also herbivore foraging behaviour of moving to other grazing sites because of fear of predators allowing for plant growth and recovery.

When the top predators in an area are killed off, the mid-sized mesopredators increase in number and have a bigger impact on their prey.

Wolves can exclude coyotes, which release grazing pronghorn calves, similar to antelopes, from predation by the coyotes.

Australian small mammals are released from predation by introduced foxes if dingoes are present.

This phenomenon is now quite common and is termed mesopredator release.

Ripple and colleagues’ definition includes these indirect effects of mesopredator release as trophic cascades.

Ripple and colleagues do not consider bottom-up effects to be part of a trophic cascade, even if involving high trophic level species, and are the consequence of a trophic cascade, but are considered to be knock-on effects of the cascade.

Examples of knock-on effects are grey wolves to elk to berry-producing shrubs, a trophic cascade, to the consumption of berries by grizzly bears, a knock-on effect; top-down effect of sea otters to sea urchins to kelp, a trophic cascade, to the effects of kelp on fish, herbivorous crabs, and atmospheric carbon dioxide, all knock-on effect.

The term “trophic cascade” is used to describe when one animal in an ecosystem affects the population of another animal further down the food chain.

The concept of a trophic cascade is important in conservation and management planning because it helps us understand how different animals in an ecosystem rely on each other (Ripple et al, 2016).

References:

Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.

Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (19th Ed.) Canada.: Cengage Learning. Book.

Ripple, W. J., Estes, J. A., Schmitz, O. J., Constant, V., Kaylor, M. J., Lenz, A., … & Wolf, C. (2016). What is a trophic cascade?. Trends in ecology & evolution, 31(11), 842–849. What is a Trophic Cascade? (oregonstate.edu)

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Age of Awareness
Age of Awareness

Published in Age of Awareness

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Peter Miles
Peter Miles

Written by Peter Miles

45 years in Environmental Science, B.Env.Sc. in Wildlife & Conservation Biology. Writes on Animals, Plants, Soil & Climate Change. environmentalsciencepro.com

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