Mesopredators gone wild

Red fox, Vuples vulpes.

Are we headed toward a world full of foxes, skunks and raccoons — but empty of lions, tigers and bears? Maybe. It’s a fact that many of the planet’s large carnivores are in dire straits. Where I live in the eastern U.S., we no longer have cougars or eastern wolves, top predators that used to range across the East several hundred years ago. Cougars are now geographically restricted to just the southern tip of Florida, where about 100 Florida panthers live in marginal habitat. And eastern red wolves are now confined to a tiny speck of land in North Carolina, where about 100 live in a managed population. Both species are listed under the federal Endangered Species Act. In their absence, entire ecosytems have changed.

Ecologists have long struggled to quantify what happens to an ecosystem when the top predators are lost. They’ve been equally stumped by proving causality that because top predators are lost or diminished, species lower down the trophic levels of the food web respond in a predictive fashion. Some biologists assert that losing apex predators sets in motion a phenomenon known as a “trophic cascade,” whereby effects ripple through the hierarchical levels of an ecosystem in a domino fashion. The tricky part is that because ecosystems have such a complex quantity of variables, it is nearly impossible to prove causality once you get two or three steps removed from a direct action.

First, consider that ecosystems are composed of living things, species, which have deeply interconnected relationships with each other. And the species are not only dependent upon each other, they are also dependent upon non-living, abiotic, things like water, minerals, nutrient cycling, sunlight and the earth’s magnetic field. The interlinked relationship between the living and non-living things are nearly infinite, and they grow in complexity depending upon the scale of the ecosystem being studied. For example, is the system a small pond, about 50-feet in diameter, or is it an ecoregion that encompasses 7,000 square miles and multiple ecosystems?  Couple the scaling problem with the fact that it is nearly impossible to draw a firm line around any ecosystem in order to define what variables are in the system of study and which are out, and you wind up with a situation where it is extremely difficult for scientists to provide experimental data that identify causative factors that drive trophic cascades.

The best they can do is provide case studies of the phenomenon in action whereby altering the balance of species in a structured, hierarchical food web ultimately upends the system and results in expansion of the species making up the trophic level just below the apex predators — which can sometimes result in the decimation of the apex predator’s prey base, if that base is then exploited by the intermediate predators (also called mesopredators). This particular cause-and-effect phenomenon is summed up as the mesopredator release hypothesis. One of the biggest weaknesses of this hypothesis, as I hinted at earlier, is the difficulty in establishing causality coupled with the lack of experimental data to support it.

But as with so many things in ecology and natural history, there are oodles and oodles of observational case studies to support the idea. Perhaps the greatest natural experiment to come around recently has been the reintroduction of gray wolves to Yellowstone National Park. Because these apex predators were absent for 80 or more years, the trophic interactions of the ecosystmes in YNP changed in their absence. The wolves’ prey, elk, abounded. And the elk’s food base, young cottonwood shoots, dwindled in some areas. In fact, no new cottonwood stands grew in the Lamar Valley and at least one study attributed this to over-browsing by elk. {1} But after wolves reintroduced to the park, the elk changed their feeding behavior due to the threat of predation by wolves — they stopped feeding freely in open meadows and valleys and began spending more time in forested areas. This allowed cottonwood and aspen stands a respite from browsing, and new growth occurred at levels not seen in the entire past century. {2} These are anecdotal examples of how interactions between feeding guilds can result in changes in the balance of an ecosystem. But hard core scientists want harder data. They want experimental data that weeds out all the correlations and gets to causative factors behind these relationships. (Whose to say climate change is not to blame for these ecosystem changes, for example?)

Raccoon

In my own neck of the woods, the nearly complete removal of red wolves and cougars from the east coast provides an interesting case study where — in their absence — coyotes have moved in and racoons and foxes have both expanded their ranges and distribution density, according to a new paper published in BioScience, The Rise of the Mesopredator.  I read this paper with great interest because it promised to deliver a continental overview of how apex predator losses have influenced the range expansions and/or the distribution density of mesopredators.  The paper describes how apex predators spanning from sharks to tigers and from bears to wolves have lost a tremendous amount of ground in the past 100 years or more, for reasons ranging from declining populations to range contraction. The study lays out a lot of interesting anecdotal evidence supporting this hypothesis, but if you are looking for hard facts then it falls short of offering experimental evidence to prove or disprove whether this sort of release occurs. It almost makes up for this lack with the sheer number of case studies though. (I will address the mechanisms of mesopredator release in a later post, today I’m only going to focus on the range issues.)

The most interesting part to me was a table depicting the range contraction of apex carnivores combined with the range expansion of mesopredators. For example, the authors state that jaguars have lost 75.9 percent of their historic habitat (more on that in this post), grizzlies have lost 55.4 percent, gray wolves have lost 42.3 percent and cougars have lost 36.6 percent — while coyotes have increased their range by 40.2 percent, racoons have increased theirs by 18 percent, and red foxes have increased theirs by 12 percent. Perhaps the biggest absolute loser of all was the black-footed ferret, which they say has lost 99.9 percent of its habitat. Only in a few cases do the authors attempt to link specific apex predators with specific mesopredators, but the global picture they paint is insightful for the alarming trend of range contraction of apex carnivores and corresponding range expansion for mesopredators. In fact, they calculate that 60 percent of the mesopredators they studied expanded their ranges, while all of the apex predators’ ranges contracted in the past 200 years (time scale of their study, corresponding to major land use changes and spread of humans throughout North America since the early 1800s). Here’s a great illustration of this, from Figure 3 in Prugh et al.’s paper:

Mesopredator Release Range Map

What this shows is that in map a, the historic apex predator richness was quite dense and at its most “rich” in the yellow, orange and red areas (which correspond to “richness” ranks of 3, 4 and 5 accordingly). But in map b, which represents the present today — note the vast sea of gray in the mid-West to the East! This is where apex predators such as gray wolves, eastern wolves and cougars have been completely pushed out of their ranges and wiped out regionally. Look closely back and forth between map a and map b: This is what the loss of an apex predator looks like, a vast sea of colorless nothing.

Now consider maps d and e, these make the same comparison of past to present, and interestingly the maps provide evidence for the loss of mesopredator richness, particularly in the mid-West and West. But map c and f are really the ones that tell the story, because these tell us where the greatest change has taken place in the species richness, and how the greatest losses of apex predators (oranges through to the reds) roughly correspond to the greatest increases in mesopredator richness (blues and purples). These ranges don’t necessarily match up exactly, but nor would we expect  them to. As discussed earlier, the sheer complexity of relationships between species and the abiotic factors means that there would almost never be a one-to-one factor of cause and effect.

Lastly, consider the last set of comparative maps and focus on map i which plots the corresponding addition or loss of both apex predators and mesopredators regionally. The dark green patches are where apex predators have been lost, and mesopredators have expanded. The entire southeast is a rich dark green, as is most of Mexico and a swath of Canadian provinces above the Great Lakes. Even more disturbing is the vast sea of khaki green  where both apex predators and mesopordators have lost ground and numbers. And it is only in the far north where it is too cold and icey and inhospitable for people to live in any real density or wide-distribution that could affect animals that both types of predators have flourished. I see these islands of red as very sad reminders of the controlling effect that people have on all animals — large and small.

In my mind, these maps are the best evidence I’ve yet come across describing the relationships between plummeting apex predators and sky-rocketing mesopredators. Perhaps we’ve been trying to go about investigating their relationships all wrong — looking for one-on-one species correlations to get at causation. Perhaps, instead, the global continental scale must be examined for regional patterns. If nature is fractal, then perhaps seeing the smallest bits of these relationship pattern at the community or population scale tell us very little, but seeing the broader pattern repeated again and again at larger scales, and evidenced at the regional continental scale, gets to the core of the issue.

NOTES:

I originally wrote this post in October 2009; but in switching over to using ResearchBlogging.org I needed to re-post it for their system to recognize it. Thanks for your patience.

{1} Beschta, R. 2005. Reduced cottonwood recruitment following extirpation of wolves in Yellowstone’s northern range. Ecology. 86: 391-403.
{2} Beschta, R. 2003. Cottonwoods, elk and wolves in the Lamar Valley of Yellowstone National Park. Ecological Applications. 13: 1295–1309.
{3} Prugh, L., Stoner, C., Epps, C., Bean, W., Ripple, W., Laliberte, A., & Brashares, J. (2009). The Rise of the Mesopredator BioScience, 59 (9), 779-791 DOI: 10.1525/bio.2009.59.9.9