Too Many Mesopredators?

Too Many Mesopredators?

Illustration by Adelaide Tyrol

While controversy rages over the reinstated killing of wolves in western states, these predators may be making a quiet comeback in the northeast, slipping across the border from Canada. And the notion that cougars could be present in these parts continues to make news, particularly with the recent road kill of a cougar in southern Connecticut. Authorities determined that this animal had migrated all the way from South Dakota.

Top predators in New Hampshire and Vermont? While some applaud their potential return, others dread it. The fact is, though, that we are fully accustomed these days to a landscape devoid of these two large carnivores.

Why did we humans eliminate wolves and cougars from the Northeast in the first place? Food, primarily. We didn’t want them competing with us for food – domestic livestock and game animals like deer. And we feared becoming a big carnivore’s lunch. Once these top predators were gone, their preferred prey animals soon multiplied. But so did the populations of other species that we may not connect with top predators: carnivores of medium size – the so-called mesopredators.

That’s right – large carnivores kill carnivores of the next size down. Since these kills are generally not eaten, this behavior seems less about acquiring food and more about reducing competition. When bigger carnivores are present, mesopredators spend more time hiding and less time hunting, which serves as a further check on their population.

When large carnivores are extirpated, the subsequent explosion of medium-sized predators is known by biologists as “mesopredator release”. In New Hampshire and Vermont, the mesopredators with the most impact are coyotes, raccoons, foxes, and skunks.

Apart from sheer size, there are other characteristics that distinguish top predators from mesopredators. Top predators characteristically occur at low population density and range widely over large habitat areas. They are slow to reproduce, and their diet is restricted mainly to eating other animals, though wolves sometimes eat berries. They are feared by humans.

Mesopredators, on the other hand, are less feared. They thrive in fragmented habitat and tolerate human presence. What’s more, they are prolific breeders and can exist at high densities. Their dietary habits are opportunistic, meaning they have a flexible, omnivorous diet that allows them to prosper in all sorts of situations. Human habitation offers them a wealth of food choices, from scavenging from trash to stealing pet food, snacking on bird seed and eating crops. In fact, mesopredator outbreaks don’t only occur after top carnivore extirpation, they can result when habitat fragmentation and human habitation pushed the top predators to the periphery.

Mesopredators prey on eggs and young of songbirds and on turtles, snakes, lizards, amphibians, rodents, rabbits, and insects. Not surprisingly, mesopredator release leads to reduction of these animal species and can be an important factor in songbird decline. Some turtle species, especially wood turtles, are suffering from reproductive failure across Vermont and New Hampshire because mesopredators are eating their eggs. Steve Parren of the Vermont Non-Game Natural Heritage Program finds that trapping raccoons in the vicinity of turtle nesting beaches is necessary in order for turtle eggs to hatch.

Mesopredators don’t have to be native species; the prime non-native example being house cats. These human-associated mesopredators are recreational hunters, maintained in numbers far above the carrying capacity of the local ecosystem by food subsidies from their owners. They kill songbirds and other prey even when populations of prey are so low that they could not support wild predators.

Sometimes mesopredators can keep one another in check when one species acts like a top predator. In one study of habitat fragments surrounding housing developments in California, songbirds were more prevalent in areas where coyotes also lived because coyotes were keeping housecat numbers under control.

In northern New England, mesopredators are not generally viewed as a threat to livestock or crops. Their biggest threat to human health occurs during cyclical rabies outbreaks. From an ecosystem perspective, the biggest impact these animals have is on their prey species. By eliminating the top dog and top cat, and thereby releasing the mesopredators, we’ve ended up putting pressure on the animals farther down the food chain. Restoring the wolf and cougar would relieve this pressure, but only by redirecting some of it back towards us.

Li Shen is an adjunct professor at the Dartmouth Medical School and the chair of the Thetford, Vermont, Conservation Commission.


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