Mesocarnivore Response to Wolf Presence, Prey Availability, and Snow Pack

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Increased coyote abundance throughout western North America has been attributed to the extirpation of wolves and is considered by many to be an example of “mesocarnivore release.” Mesocarnivore release is defined as the expansion in range and/or abundance of a smaller predator following the reduction or removal of a larger predator. These effects can extend beyond the small carnivores to include their prey. These “ripple effects” throughout an ecosystem are known as trophic cascades. But determining if, how, and when these cascades occur is very complex.

In the summer of 2012, Dr. Laura Prugh of the University of Washington (then at University of Alaska Fairbanks), started a study on interactions between carnivore species in Denali and surrounding areas. One goal of the study was to determine which are the most important factors controlling mesocarnivore populations in the park, particularly those of coyotes. Is it suppression by wolves? Availability of prey? Habitat characteristics like snow? This information can be used to determine the strength of mesocarnivore release and resulting effects on the mesocarnivore community.

The goal of the first part of the study, conducted by graduate students Kelly Sivy and Casey Pozzanghera, was to assess how wolf activity, prey availability, and habitat (vegetation and snow characteristics) affect the dynamics of mesocarnivore release and the resulting cascading effects on the mesocarnivore community.

Who's Pushing Who?

As an example of how complex the idea of mesocarnivore release is, imagine the following: since wolves often kill coyotes, the presence of wolves might limit coyote abundance. However, wolves also support coyotes by providing food in the form of ungulate carrion, so the net effect of wolves on coyote populations is hard to determine.

In northern ecosystems, coyote populations fluctuate with changes in abundance of their primary prey (snowshoe hares and voles). Population cycles of these small mammals could influence the degree to which coyotes rely on wolf-supplied carrion, which would change the dynamics of the relationship between coyotes and wolves. Furthermore, competition between coyotes and other mesocarnivores is thought to be influenced by habitat characteristics, such as snow depth and density, which are changing in response to long-term changes in climate. These complex dynamics also occur between wolves and the other mesocarnivores like wolverines.

So, is the upshot that wolves are determining which mesocarnivore species are being successful? Or are bottom-up forces such as snow and prey more important?
two diagrams indicating relationships between apex predators and smaller predators, explained in the page text below
A general conceptual diagram of hypothesized suppression vs facilitation cascades among apex and nonapex predators. Signed (+/-) paths show the hypothesized net effect between two species. A: A suppression-driven cascade, whereby the suppression of a competitively dominant mesopredator by an apex predator indirectly benefits other meso- and small predators. B: A facilitation-driven cascade, whereby facilitation of mesopredators leads to indirect suppression of small mesopredators (Modified from Sivy et al 2017).

Research Techniques

For this study, researchers took advantage of the ability to compare the ecosystem in Denali, which has natural levels of wolves, with that of an ecologically similar area of land (the upper Susitna drainage off the Denali Highway) that experiences extensive hunting and control of wolves, which heavily reduces their populations. By comparing data sets between the two areas, they could gain insight into the effects of wolves on the smaller carnivores.

In both areas, scat was collected, which allowed researchers to assess the diets of carnivore species. Prey populations were estimated by counting fecal pellets of hares and by live-trapping voles and other small rodents. Repeated snow track surveys were conducted in the winter to assess the density and degree of spatial overlap of carnivore species and their prey, and gather data on the quality of the snowpack being used by the animals. The tracks were also compared to wolf activity data from GPS-collared wolves.

Results

When it comes to diet, researchers found that the diets of coyotes and red foxes were comprised primarily of ungulate carrion, voles, and snowshoe hares. Foxes ate considerably less carrion in the area with higher wolf populations, and there was somewhat less overlap in diet between foxes and coyotes in the area with more wolves. Overall, this leads researchers to believe that when more wolves are present, coyotes take advantage of the scavenging opportunities, leading to less competition between coyotes and foxes for small prey.

The results of the snow-tracking surveys, which tell us how carnivores move around the landscape relative to one another, were more complex. Snow and prey were both found to be influential, although some species were more responsive to these factors than others, particularly lynx. In the area with lower wolf densities, there were more tracks of all the mesocarnivores – which would suggest that their populations increased in response to lowered wolf numbers, and that wolves represent a threat they might choose to avoid. However, in both study areas, there was a positive correlation between wolf activity – as noted by both tracks and collar data – and the tracks of the mesocarnivores. Based on these results, researchers hypothesized that there may be a “fatal attraction” between wolves and smaller carnivores, that drives smaller carnivores to maintain a proximity to wolves in order to scavenge, but that ultimately leads to their demise and reductions in their populations.

To further investigate this hypothesis, researchers in phase two of this study examined photos of scavenging activity at carcasses, and the data from GPS-collared coyotes and wolves.

Last updated: July 15, 2019