Can Restoring Wolves Aid in Lynx Recovery? Ripple, Wirsing, Beschta & Buskirk (2011)

Abstract

Herein, we examine the hypothesis that relatively low densities of snowshoe hares (Lepus americanus) and the imperiled status of lynx (Lynx canadensis) may be partially due to an ecological cascade caused by the extirpation of gray wolves (Canis lupus) inmost of the conterminous United States decades ago. This hypothesis focuses on 2 plausible mechanisms, one involving ‘‘mesopredator release’’ of the coyote (C. latrans), which expanded its distribution and abundance continentally following the ecological extinction of wolves over the temperate portion of their geographic range. In the absence of wolves, coyotes may have affected lynx via increased predation on snowshoe hares, on which the lynx specializes, and/or by direct killing of lynx. The second mechanism involves increased browsing pressure by native and domestic ungulates following the declines in wolves. A recovery of long-absent wolf populations could potentially set off a chain of events triggering a long-term decrease in coyotes and ungulates, improved plant communities, and eventually an increase in hares and lynx. This prediction, and others that we make, are testable. Ecological implications for the lynx may be dependent upon whether wolves are allowed to achieve ecologically effective populations where they recolonize or are reintroduced in lynx habitat. We emphasize the importance of little-considered trophic and competitive interactions when attempting to recover an endangered carnivore such as the lynx.

Under the auspices of the Endangered Species Act of 1973, the Canada lynx (Lynx canadensis) was listed in 2000 as a threatened species across the conterminous United States (US; U.S. Fish and Wildlife Service 2000). [...] Lynx face numerous factors that could limit their distribution and abundance, including competition with other mid-sized carnivores and habitat alteration by stand-replacing fires, timber harvest, and insect outbreaks. [...] Even so, there is broad agreement that the fate of the lynx at lower latitudes is closely linked to the distribution and abundance of its obligate primary prey, the snowshoe hare (Lepus americanus) [...] That is not to say that widespread abundance of snowshoe hares is the only condition for lynx recovery; however, it is a necessary one. [...] 

[...] Snowshoe hare populations occupying the forests of northern Canada and Alaska typically exhibit dramatic stable limit cycles with periods of 9–11 yr. [...] The mechanism underlying the hare cycle across this region is debated vigorously, with 2 chanistic pathways contending for priority. The first, a tri-trophic-level interaction among quantity of winter browse, hare population density, and densities of hare predators involves time lagged density dependence. [...] The second, a hare–winter–browse hypothesis, invokes plant secondary chemical responses to herbivory as the chief factor eliciting the hare cycle. Decadal-scale climate fluctuation is invoked with both mechanisms to account for broad geographic synchrony. [...]

[...] Dampened dynamics in southern hare populations have been attributed to a combination of forest fragmentation and predation by prey-generalists. [...] By implication, forces that reduce forest fragmentation or suppress facultative hare predators could enhance the size of southern hare populations and perhaps promote cyclic dynamics. Increased hare abundance in this region can plausibly be expected to improve the viability of southern lynx populations.

The objective of this article is to briefly examine a hypothesis that chronically low densities of southern snowshoe hares and the imperiled status of lynx may be partially the result of an ecological cascade caused decades ago by the extirpation of the gray wolf (Canis lupus) over most of its conterminous US range. This hypothesis focuses on the subsequent ecological release of coyotes and of the ungulate prey of wolves and leads us to the prediction that wolf restoration could help facilitate lynx recovery in the conterminous US.

The conceptual model

During the 1800s and early 1900s, gray wolves were extirpated throughout much of the conterminous US, and ungulate and coyote irruptions often followed the loss of wolves. […] With wolves present, we hypothesize that coyotes would be maintained at low densities, resulting in little competition—either exploitative or interference—between coyotes and lynx. […]

Figure 1. Conceptual diagram showing how the (a) presence or (b) absence of wolves may affect lynx across multiple trophic levels. We hypothesize (a) that in the presence of wolves, this apex predator maintains coyote populations at low densities, thus minimizing the potential influence of coyotes on hares. However, (b) in the absence of wolves, coyote densities increase, causing exploitative competition with lynx through high predation pressure on hares. Furthermore, in the absence of wolves, wild ungulates may not only provide a prey and carrion subsidy to coyotes but can also increase herbivory levels upon forest understory plants that satisfy important habitat needs for hares. Changes in ungulate herbivory and interference competition are not shown on the above diagrams. Note:Wide arrows denote strong effects; thin arrows, weak effects; large ellipses denote high densities; small ellipses, low densities; concentric lines in ellipses, variable and/or
cycling densities.

[…] In the absence of wolves, coyote densities and distributions generally expanded in the US—into the Midwest, to the northeast as far as Newfoundland, and as far northwest as Alaska. And, because coyotes are known to be effective predators of hares, increased coyote populations can cause exploitative competition with lynx via higher predation pressure on hares.  Further, interference competition between coyotes and lynx could limit densities of the latter, since examples of coyotes killing lynx have been reported. […] Also, in the absence of wolves, population densities of wild cervids typically increase, creating prey and winter-killed carrion subsidies to coyotes.

[…]In northern latitudes (e.g., Yukon) where wolves are present and where hares are the main prey for coyotes, populations of coyotes generally occur at relatively low densities, as well as cycle up and down in concert with hares and lynx. This is in contrast to wolf-free southern latitudes (e.g., the conterminous US) where coyote densities are commonly an order of magnitude higher than those in the north. Coyote diets are also more varied in the south.

[…]The hypothesized negative effect of wolf removal on snowshoe hares and lynx that we describe here could be exacerbated by climate change. In the high-elevation regions of the conterminous US where lynx still occur, deep winter snow provides these boreal specialists with a competitive advantage over coyotes, which have a higher foot-load. […]Thus, if climate change results in warmer and less severe winters, declining snow pack could allow coyotes (released by the absence of wolves) to exploit hares at higher elevations. Accordingly, additional studies, with and without wolves, that elucidate the winter sympatry among southern coyotes, lynx, and hares at high elevations are required to test for this scenario. Results of this research could be important for lynx conservation efforts in the conterminous US.

Conclusions

If the removal of wolves initiated trophic and competitive adjustments resulting in the decline of hare and lynx populations in southern latitudes, would the restoration of this apex predator help their recovery? Based on the above considerations, we hypothesize that the answer is yes, but we are unsure as to what extent and how long it might take. A recovery of long-absent wolf populations could potentially set off a chain of events triggering a long-term decrease in coyotes and ungulates, recovery of previously degraded native plant communities, and eventually an increase in hares and possibly other leporids as well. Furthermore, we think the answer is at least partially dependent upon whether wolves are allowed to achieve ecologically effective populations where they recolonize or are reintroduced. […] We have proposed mechanisms that involve food of hares (via ungulate densities), predation on hares (coyote populations released from topdown control by wolves and food subsidized by ungulates and humans), and behavioral–physiological effects on hares via temporally sustained predation and harassment by facultative hare predators. We propose exacerbation of these effects by an abiotic factor: altered patterns of snow depth and hardness in the face of climate change. All of these hypotheses are testable, at least indirectly.

Where wolf restoration is the objective, we believe that it is especially important to consider the ecological roles of these top predators in the ecosystem, rather than focusing solely on their demography. Accordingly, wolf recovery criteria in regions where hares and lynx occur can and should include measures of coyote densities, to index predation on hares, and the recruitment of woody browse species, which provide food and cover for hares.

Can Restoring Wolves Aid in Lynx Recovery? Ripple, Wirsing, Beschta & Buskirk (2011) Wildlife Society Bulletin 9999:1–5; 2011http://bit.ly/R6KrAC (Publicado en internet por: http://uwacadweb.uwyo.edu/ )