February 17, 2013 | 5
As I’ve mentioned before, my research in Kenya is focused primarily on the effects of rainfall on mesopredator populations, and how these effects may differ in places from which apex carnivores have largely been extirpated. This begs a critical question: what is a mesopredator, anyway?
The answer is not as simple as one might think. The term “mesopredator” has often been used to describe carnivores of small or intermediate body size, such as foxes or coyotes, in contrast to large “apex” carnivores such as bears and lions. Although this component of the definition is important, it is not always ideal.
Size-based classifications are relative terms. A particular species may be “intermediate” in size in one community and yet the largest in another community, if apex carnivores have been extirpated or were never present there at all. Defining mesopredators only by size means that community composition will determining how a species is classified, even if its behaviors and ecological traits are not functional for those of a larger predator.
For example, due to the nearly complete removal of wolves and grizzly bears from most of the United States, and the common persecution of black bears in non-protected areas, the coyote is the largest carnivore found across most populated areas of the country. The coyote, however, has a very different social structure, reproductive rate, diet breadth, and home range size than the wolf. Even though the coyote is now the “top dog” in terms of size, it is not the ecological equivalent of the historical apex predator.
Along the same lines, even the modern wolf may not be the functional equivalent of a more distant apex predator, the dire wolf. Thus, a more ecologically- and functionally-based definition is needed in order to fully understand the relative roles that large carnivores and mesopredators play in their ecological communities.
Many mammals considered to be “mesopredators” are fairly omnivorous, and this can be an additional (although not an exclusive) criterion. Nature tends to defy our attempts to cram organisms into neatly delineated boxes, and definitions are rarely absolute. Numerous species in the taxonomic order Carnivora actually have broad diets that include many food sources beyond just meat. For example, a 500-pound grizzly bear may have a more omnivorous diet than a 500-gram weasel, and a panda bear—in the same carnivoran family as the grizzly, Ursidae—consumes no meat at all. What is the true carnivore here?
Strictly speaking, all mammals in the order Carnivora are “carnivorans,” while only those with primarily meat-based diets can correctly be called “carnivores.” This is one reason the term “mesopredator” is often preferred over “mesocarnivore” for omnivorous carnivorans of small- to intermediate body size. In other words, mesopredators are not “little-large carnivores”: they don’t simply play the same roles at smaller scales. The dietary criterion for defining a mesopredator isn’t perfect, but it can help clarify some cases by taking into account the difference between taxonomic and ecological “carnivores,” especially in context of food webs and community dynamics.
One root of the misconception that mesopredators can simply “become” apex predators after declines of larger carnivores is that the majority of mesopredator release studies have been done in North America and Europe. These regions are occupied by limited numbers of carnivoran species, and there are often relatively modest differences in body size between apex carnivores and mesopredators.
While in some cases a coyote may not be a poor ecological approximation for a wolf (although never a perfect substitute), the situation is much different for a lion and a mongoose in Kenya, or a jaguar and a coati in Ecuador. In contrast to North American and European systems, there has been relatively little research done on mesopredator release in ecosystems such as the savannas of East Africa, where up to ten large carnivores and literally dozens of mesopredators may coexist. More complex communities have much more intricate interaction networks, producing range of ecological ramifications: broader or more evenly filled community niche space, variations in diet breadth for each species, or increased functional redundancy. Future work on species-rich tropical assemblages should yield new and interesting insights into these dynamics.
There is an additional issue challenging our attempts to define the term “mesopredator”: taxonomic bias. Although the earliest descriptions of mesopredator release effects involved coral reef invertebrates (Paine 1969) and arthropod-lizard interactions (Pacala and Roughgarden 1984), the vast majority of mesopredator release studies have exclusively looked at mammalian predators. In the few studies that have shed those taxonomic blinders, significant dietary overlap (Farias and Jaksíc 2007), predation pressures (Roemer et al. 2002), and indirect effects (Mezquida et al. 2006) have been observed among mammalian and avian predators. A significant amount of the mesopredator literature, however, still disregards the fact that raptors and other vertebrate predator populations exist and compete wit mammalian mesopredators, leaving out a key “plot line” in the story of food web interactions.
In a nutshell, the answer to “what is a mesopredator” is being refined and made more scientific as time goes on. In some cases size is clearly important, such as for many of the smaller cats. Even given the same degree of carnivory, a small bobcat could not inherit the ecological role of a puma by consuming the same assortment of large prey. Diet breadth, however, is indeed another important dimension in many communities. I plan to account for some often-neglected aspects of mesopredator ecology in my own research by including avian predator counts in my population surveys, as well as by quantifying diet breadth (degree of omnivory) with stable isotope analysis, rather than relying on the broad, qualitative dietary classifications that are sometimes applied to omnivores.
You may be wondering what drew me to mesopredators in the first place. First, these species tend to be extremely understudied in tropical countries, and in many places we still lack baseline data on their diversity, abundance, social structure, and foraging behavior. Mesopredators can’t exactly match lions and leopards for majesty in nature documentaries, and they tend to carry reputations as “pests.
This means that mesopredators don’t garner as much public interest—which, sadly, sometimes translates to a dearth of available research funds. As such, even some mesopredators of conservation concern haven’t received the same attention as “sexier” species such as pandas and cheetahs, in either the media or in scientific spheres. There are many discoveries waiting to be made about these animals, and the exploratory aspect of mesopredator ecology is especially intriguing for me.
Second, mesopredators are often highly interlinked within their food webs. They are often omnivorous, consuming a broader array of prey items than do larger predators. In addition, mesopredators they also serve dual roles as both consumers of smaller animals and potential prey of larger carnivores.
This puts a mesopredator a sort of “hub” position within its food web. Simplified, three-level conceptualizations of food chains overlook this dynamic. Fortunately, more complex complicated food web models have long been used by ecologists and are now gaining traction in the public awareness.
For example, the lion’s diet consists largely a handful of ungulate species, while the white-tailed mongoose utilizes dozens of diverse food sources. This is an important difference for several reasons. If apex predators experience severe declines and mesopredator populations expand as a result—a key phenomenon for my research known as “mesopredator release”—the mesopredators cannot simply take over the role of the extirpated apex species, even if they are now the largest predators in the community. Their diets, social structures, and habitat needs will be quite different, and it can’t be assumed that they will simply keep the ecological seat warm for apex predators until their populations can be restored.
Finally, mesopredators’ relatively small sizes and broad food options allow them to achieve higher population densities than their larger, more strictly carnivorous counterparts. All of these factors together mean that changes in mesopredator population dynamics can ripple widely throughout an entire ecological community. It has been estimated that over 95% of the world’s ecosystems have experienced declines in large carnivore populations (Ritchie & Johnson 2009), and it essential to gain a better understanding of how this will affect mesopredators and the dozens of species with which they interact.
My mesopredator trapping efforts should be kicking into high gear this week, and I hope to report back to you soon with news about how that goes!
Images: Wolf, by Cephas at Wikimedia commons. Other images copyright by author.
Previously in this series:
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