You can hide but you can’t run: apparent competition, predator responses and the decline of Arctic ground squirrels in boreal forests of the southwest Yukon

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.

Hares and Small Rodent Cycles: a 45-year Perspective on Predator-prey Dynamics in the Yukon Boreal Forest

“…Given the apparent greening of the arctic, will the average densities of all these herbivores increase over time? At least in the case of the arctic ground squirrel, the opposite seems to have happened (Werner et al 2016).…”

Impact of rewilding, species introductions and climate change on the structure and function of the Yukon boreal forest ecosystem

“…Figure 5 shows the parallel rise and fall of arctic ground squirrel populations and snowshoe hares, a result of prey switching as hares became scarce. In 2000, ground squirrels collapsed to very low numbers and became locked in a predator pit (Werner et al 2015(Werner et al , 2016. They have become functionally extinct in the boreal forest, while alpine populations still thrive.…”

Experimental repatriation of snowshoe hares along a southern range boundary reveals historical community interactions

“…Across shorter time intervals, or when the numerical response of the predator is constrained, functional responses of predators drive prey dynamics to apparent mutualism, in which prey abundances are positively correlated (Holt & Lawton, 1994). Prey switching by predators within the framework of enemy‐mediated indirect interactions can stimulate synchronous population fluctuations of prey communities and even multiyear population cycling (Abrams et al, 1998; Werner et al, 2016). Cyclic dynamics have been theoretically linked to apparent mutualism and competition (Abrams et al, 1998), although the resilience of these cycles under changing enemy‐mediated indirect interactions are unknown (Holt & Bonsall, 2017).…”