This blog post on ‘Predation and herbivory’ is part of the BES ‘Key Concepts in Ecology’ series, designed to help ecologists in learning the key topics in ecology! Take a look at the full series for a list of key topics you might typically find in an ecology textbook, each providing a quick introduction to the topic, and a list of suggested papers for students to refer to.  

Predation in the broad sense occurs when an individual of one species consumes individuals of another. Predation can be a major selective force on prey species. Many adaptations in organisms such as warning coloration, camouflage, and toxicity, are shaped by predator-prey coevolution (Reimche et al. 2020). Historically, predation was considered to have little impact on prey populations, as predators were thought to kill only the sick, injured, or starving individuals (that were likely to die anyways), termed the ‘doomed surplus’. Although this can be the case in some systems, predators can have a major impact on prey population abundance (Krebs et al. 1986).  

The degree to which predators impact prey abundance is dependent on their numerical and functional response. The numerical response refers to the change in predator density relative to prey density, whereas the functional response represents the kill rate of a predator relative to prey density. Both the numerical and functional response can be influenced by habitat, climate, and interactions with other species (Mocq et al. 2021) but examining them is crucial for understanding the total impact of predators on prey populations. 

Predators can impact prey species through direct killing (consumptive effects), but also by influencing prey behaviour and physiology through what are termed non-consumptive effects. The mere presence of predators on the landscape can cause increased stress in prey animals (Sherif et al. 2009) or alter their foraging to minimize predation risk (Gavini et al. 2020). Non-consumptive effects can ultimately lead to decreased prey reproduction and survival, impacting population size beyond that attributed to consumption alone. Perhaps the most infamous example of non-consumptive effects occurred during the reintroduction of wolves (Canis lupus) to Yellowstone National Park. The newly established predator altered elk (Cervus canadensis) foraging patterns, which was thought to reduce reproduction and contributed to their population declines.  

The impacts of predators on prey abundance and behaviour can have cascading effects on the ecosystem, called trophic cascades: predators indirectly influence a third species, through their impact on their prey species (Lundgren et al. 2022). For example, the introduction of wolves has also influenced various other plants and animals in Yellowstone National Park through their reduction in elk abundance and changes to elk foraging behaviour (Ripple et al. 2013). 

Herbivory can be considered a form of predation according to the broad definition mentioned earlier. Herbivores consume plants or their seeds and fruits and is unique from other forms of predation because the plants themselves are typically eaten but not killed. Herbivory can also be beneficial for plants in some cases, such as pollination or seed dispersal (Samuel & Levey 2005). Although herbivores can limit plant abundance, the world is considered relatively green, implying herbivores are ​​prevented from depleting their food sources. This could be caused by predators controlling herbivores, holding them to densities too low to substantially limit plant abundance. Plants also have ​​chemical defenses, which negatively impact herbivores and reduce herbivory (Torregrossa & Dearing 2009). 

Introduction written by Michael Peers (Associate Editor, Functional Ecology). Reading list curated by the BES journal Editors. 

References and suggested reading   

Predators and herbivores limit abundance   

• Brandell, E.E. et al. (2022) Examination of the interaction between age-specific predation and chronic disease in the Greater Yellowstone Ecosystem. Journal of Animal Ecology, 91: 1373-1384.
• Sheriff, M.J. et al. (2009) The sensitive hare: sublethal effects of predator stress on reproduction in snowshoe hares. Journal of Animal Ecology, 78: 1249-1258.
• Krebs, J.C. et al. (1986) Population Biology of Snowshoe Hares. I. Demography of Food-Supplemented Populations in the Southern Yukon, 1976-84. Journal of Animal Ecology, 55: 963-982.
• Miller, J.R.B. et al. (2013) Fear on the move: predator hunting mode predicts variation in prey mortality and plasticity in prey spatial response. Journal of Animal Ecology, 83: 214-222.

Consumers and prey population fluctuations   

• Nisi, A.C. et al. (2021) Temporal scale of habitat selection for large carnivores: Balancing energetics, risk and finding prey. Journal of Animal Ecology, 91: 182-195.

Functional and numerical responses   

• Mocq, J. et al. (2021) Disentangling the nonlinear effects of habitat complexity on functional responses. Journal of Animal Ecology, 90: 1525-1537.

Predator-prey/Herbivore-plant coevolution   

• Reimche, J.S. et al. (2020) The geographic mosaic in parallel: Matching patterns of newt tetrodotoxin levels and snake resistance in multiple predator–prey pairs. Journal of Animal Ecology, 89: 1645-1657.
• Gavini, S.S. et al. (2019) Intraspecific variation in body size of bumblebee workers influences anti-predator behaviour. Journal of Animal Ecology, 89: 658-669.  
• Torregrossa & Dearing. (2009) Nutritional toxicology of mammals: regulated intake of plant secondary compounds. Functional Ecology, 23: 48-56.  
• Samuels & Levery. (2005) Effects of gut passage on seed germination: do experiments answer the questions they ask? Functional Ecology, 19: 365-368.

Trophic cascades 

• Lundgren et al. (2022) A novel trophic cascade between cougars and feral donkeys shapes desert wetlands. Journey of Animal Ecology, 91: 2348-2357.
• Ripple et al. (2013) Trophic cascades from wolves to grizzly bears in Yellowstone. Journal of Animal Ecology, 83: 223-233.