Abstract
Throughout much of North America’s boreal forest, the cyclical fluctuations of snowshoe hare populations (Lepus americanus) may cause other herbivores to become entrained in similar cycles. Alternating apparent competition via prey switching followed by positive indirect effects are the mechanisms behind this interaction. Our purpose is to document a change in the role of indirect interactions between sympatric populations of hares and arctic ground squirrels (Urocitellus parryii plesius), and to emphasize the influence of predation for controlling ground squirrel numbers. We used mark-recapture to estimate the population densities of both species over a 25-year period that covered two snowshoe hare cycles. We analysed the strength of association between snowshoe hare and ground squirrel numbers, and the changes to the seasonal and annual population growth rates of ground squirrels over time. A hyperbolic curve best describes the per capita rate of increase of ground squirrels relative to their population size, with a single stable equilibrium and a lower critical threshold below which populations drift to extinction. The crossing of this unstable boundary resulted in the subsequent uncoupling of ground squirrel and hare populations following the decline phase of their cycles in 1998. The implications are that this sustained Type II predator response led to the local extinction of ground squirrels. When few individuals are left in a colony, arctic ground squirrels may also have exhibited an Allee effect caused by the disruption of social signalling of approaching predators.
Highlights
The comprehensive role that snowshoe hares (Lepus americanus) play in the food web dynamics of North America’s boreal forest epitomizes the notion of ‘‘foundation species’’ who, by virtue of their abundance and influence on other species, help to define an entire ecological community (Sinclair & Krebs, 2001)
In the case where such negative influences are not reciprocated between both prey species, ‘asymmetric’ apparent competition can lead to a variety of outcomes that depend on the intrinsic nature of the predator–prey relationship (Sinclair & Pech, 1996; DeCesare et al, 2010)
The influence of such positive indirect effects is generally fleeting, this effect may recur when population densities of the primary prey show cycles that result in repeated satiation of the shared predators and reduced predation on the secondary prey species (Abrams, Holt & Roth, 1998)
Summary
The comprehensive role that snowshoe hares (Lepus americanus) play in the food web dynamics of North America’s boreal forest epitomizes the notion of ‘‘foundation species’’ (sensu Dayton, 1972) who, by virtue of their abundance and influence on other species, help to define an entire ecological community (Sinclair & Krebs, 2001). The influence of such positive indirect effects is generally fleeting, this effect may recur when population densities of the primary prey show cycles that result in repeated satiation of the shared predators and reduced predation on the secondary prey species (Abrams, Holt & Roth, 1998). The alternating influence of apparent competition during the decline phase of the cycle, followed by positive indirect effects leading to temporary escape from predator regulation during the increase phase, are sufficient conditions for synchronous cycling of primary and secondary prey (Abrams, Holt & Roth, 1998; Norrdahl & Korpimäki, 2000)
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