A Mesopredator is a predator that occupies a mid-ranking trophic level in a food web.^[1] There is no standard definition of a mesopredator, but mesopredators are usually medium-sized carnivorous or omnivorous animals, such as raccoons, foxes, or coyotes.^[2][3] They are often defined by contrast from apex predators or prey in a particular food web.^[3][2][4] Mesopredators typically prey on smaller animals.^[2]

Mesopredators vary across different ecosystems. Sometimes, the same species is a mesopredator in one ecosystem and an apex predator in another ecosystem, depending on the composition of that ecosystem.^[3] When new species are introduced into an ecosystem, the role of the mesopredator often changes; this can also happen if species are removed.^[4]

The Mesopredator Release Effect

When populations of an apex predator decrease, populations of mesopredators in the area often increase due to decreased competition and conflict with the apex predator.^[2] This is known as the mesopredator release effect, which refers to the release of mesopredators from the trophic cascade.^[5] These mesopredator outbreaks can lead to declining prey populations, destabilize ecological communities, reduce biodiversity, and can even drive local extinctions.^[2][4]

Typically, mesopredators are in competition with apex predators for food and other resources.^[2] Apex predators reduce mesopredator populations and change mesopredator behaviors and habitat choices by preying on and intimidating mesopredators.^[6] When apex predator populations decline, mesopredators can access hunting and den areas once controlled by the apex predators, essentially assuming the role of an apex predator.^[2] However, mesopredators often occupy different ecological niches than the former apex predator and will have different effects on the structure and stability of the ecosystem.^[3][4]

Mesopredator outbreaks are becoming more common in fragmented habitats, which are areas where a species' preferred environment is broken up by obstacles.^[4] Fragmented habitats can be caused by geological or human activity, and particularly affect larger animals that roam and hunt across large territories, such as apex predators.^[7] Fragmented habitats can drive these species to leave and find more suitable habitats.^[4]

Additionally, in many fragmented habitats, apex predators have more encounters with humans, leaving them susceptible to harmful or deadly conflicts, sometimes resulting in eradication of the apex predator population entirely.^[4] Human development also promotes mesopredator outbreaks through increasing access to resources such as pet food, trash, and crops.^[4]

The mesopredator release effect is not entirely understood. Most research has been conducted on mammal species, with limited studies on non-mammal animal species.^[3] Additionally, it is not well understood how these dynamics may play out in ecosystems with many mesopredator and apex predator species.^[3]