Are wolves really all that?
08/18/2012 by DeLene
Ever since the reintroduction of gray wolves to Yellowstone National Park, and by extension the Northern Rocky Mountain ecoregion, the role of apex predators in regulating trophic cascades has been an issue of great debate. Among the first to publish a correlation between a return of aspen and willow recruitment to stands where they’d been long absent, at the same time that wolves were reintroduced, were a pair of researchers from Oregon State University, Ripple and Beschta. They promulgated an idea dubbed the ecology of fear which postulated that the presence of wolves caused a behavioral shift in elk, leading them to graze less often in open riparian corridors where they were more likely to be attacked by wolves. Their warier behavior, and shift in browsing pressure, led to a rebound in the aspen and willow growth. It’s become a familiar, almost calcified narrative, and one that many wildlife proponents have embraced (myself included).
But in his newest paper, Mech reviews the literature both supporting and refuting wolves as the mechanism of a behaviorally-modulated trophic cascade in Yellowstone. He asserts that other factors may be at play in stimulating the willows and aspen to regrow, and that they at least deserve more serious discussion. Mech seems to feel that some conservation scientists have become so myopically focused on wolves as the mechanism of ecological change that we tend to view as positive that they are unwilling or unable to look beyond wolves for alternative or contributing factors.
I have to admit, if this paper had been written by someone other than Mech, I’d probably have not have paid as much attention to it. This is because I find myself wanting to believe the wolf-as-ecological-mediator narrative. I freely admit, I’m biased in this regard. But the fact that a wolf biologist as learned and experienced as Mech produced this definitely caught my eye.
In the first part of the paper, Mech divides the current literature on wolf-driven trophic cascades into five categories:
1.) Reduction of coyotes — hypothesized to trigger a mesopredator release, or an increase in the abundance of smaller predators that coyotes keep in check such as foxes, badgers and even raptors; though this has not happened in Yellowstone
2.) Benefits to scavengers — reports claim that wolves benefit species which scavenge upon carrion for a living (as a function of leaving more carcasses on the landscape) such as bears, ravens, eagles, foxes, coyotes and even beetles. This extra food source is theorized to then allow for greater reproduction capabilities in scavengers that benefit from the wolf-killed carcasses. But Mech points out that wolves typically “reduce the available biomass of individual prey carcasses by 75-100 percent,” although this figure has not been documented to also occur in Yellowstone. If that same animal had died of other causes, scavengers would have had access to all of it, not just less than 25 percent. Mech writes that whereas ungulate deaths not due to wolves tend to occur more in seasonal bunches, wolf-killed ungulates are distributed more uniformly throughout the year and across the landscape. So it’s not clear whether less biomass available more regularly is helping scavengers any more than seasonal “bursts” of more biomass available due to seasonal deaths such as starvation die-offs. (In the later case, some scavengers will cache food from these events so as to be able to access it over longer periods of time.) On this one, Mech concludes, the jury is out.
3.) Cascading effects of wolves — This is the issue that has garnered the most attention in recent years from both scientists, the media, and the general public. Trophic effects which cascade beyond one link in the food web have been attributed to wolves via two different mechanisms: wolves reducing the population numbers of their prey, and the presence of wolves altering the behavior of their prey (namely, elk). Elk numbers have declined since the reintroduction of wolves, and several researchers have documented a behavioral response by elk to the new presence of wolves.
4.) Effects of wolf predation — Given the above, it becomes a case of correlation or causation. Are wolves the cause of ecological changes in the park, or did they just happen to come along at a time when things were changing due to other factors? For example, is the elk decline a function of wolf predation, or has drought, harsh winters, hunting, and predation by mountain lions, grizzly and black bears, and coyotes also played a role? Mech notes that it is typically only when other factors such as adverse winters, or when prey populations become small and isolated, that wolf predation takes on enough weight to seriously reduce the numbers of prey in a given area.
5.) Indirect effects of wolves — Mech allows that despite what he considers the fuzziness of the case for cascading effects of wolves in Yellowstone, there are demonstrable cases of wolves having an indirect effect on species they interact with such as elk. He does not dispute that there are documented cases of elk modifying their behavior when wolves became present on the landscape again, as one would expect. This behavioral modification has been what many researchers have tried to link to the existence of a trophic cascade driven by wolves. It’s this last point that he appears to maintain reservations about, however.
In the second part of the paper he reviews the early literature on behaviorally-mediated trophic cascades in Yellowstone National Park with a keen eye to the research methods and data used. Scientists now disagree, he says, as to whether a behaviorally-mediated trophic cascade (driven by wolves) occurred in the park and, at the very least, whether the hypothesis questioning if one were occurring has been adequately tested.
To the point that wolves were preventing elk from overbrowsing willow and aspen near streams, Mech cites two studies indicating that willow and aspen consumption actually increased within the park in the presence of a large wolf population. A rising water table due to the renewed presence of beavers — which were released just outside of Yellowstone’s park boundaries on Forest Service land, but which found their way into the park — may offer an alternate explanation for why willow and aspen rebounded in some areas, Mech writes.
Regarding wolves being responsible for effecting a behavioral change on elk which then led to a trophic cascade, Mech offers several alternative explanations. First he writes that most of the literature in this area which compares willow and aspen recruitment measurements from pre-wolf and post-wolf time periods assumes that wolves were the cause of recruitment changes. But perhaps harsh winters reduced the elk population which then caused a decline in browsing, or perhaps the increasing grizzly population played a role in reducing the numbers of elk, or maybe the reduction of moose over the longterm since widespread fire two and a half decades ago had a hand. Or, maybe the fact that climate change has increased the growing season in Yellowstone by 27 days also had an influence.
All of these are very interesting factors to consider. I have to admit, I like the narrative of wolves causing a trophic cascade. It’s got all the swagger and power of an apex predator sitting atop an attractive good-news ecology story — what’s not to like about that? But I’m glad to have read this paper by Mech because it reminds me to be more skeptical and to always, above all, question conclusions.
It is still entirely possible that wolves are causing a trophic cascade within the park. If they are, then the scientists working to document it are going to have to tease apart the indirect and direct effects of wolves from the alternative factors listed above — not an easy job. But even if they are, what does that tell us about the wider Northern Rocky Mountain region, where wolves have dispersed to but which lacks the intact ecological communities of Yellowstone? It’s not likely that wolves could have the same effect in a heavily human modified landscape. Which leads back to the idea that if parks like Yellowstone are to retain their nautral and scientific value into the future, they must be connected to other parks, reserves and protected areas to give the species within them a fighting chance at retaining their ecological function for tomorrow.
L. David Mech (2012). Is science in danger of sanctifying the wolf? Biological Conservation, 150, 143-149 DOI: 10.1016/j.biocon.2012.03.003