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

Jeffery R Werner; Elizabeth A Gillis; Rudy Boonstra; Charles J Krebs

doi:10.7717/peerj.2303

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

PeerJ. 2016 Aug 23:4:e2303. doi: 10.7717/peerj.2303. eCollection 2016.

Authors

Jeffery R Werner¹, Elizabeth A Gillis², Rudy Boonstra³, Charles J Krebs⁴

Affiliations

¹ Biodiversity Research Centre, Department of Zoology, University of British Columbia , Vancouver , British Columbia , Canada.
² Department of Resource Management and Protection, Vancouver Island University , Nanaimo , British Columbia , Canada.
³ Department of Biological Sciences, University of Toronto, Scarborough , Toronto , Ontario , Canada.
⁴ Department of Zoology, University of British Columbia , Vancouver , British Columbia , Canada.

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.

Keywords: Allee effect; Apparent competition; Extirpation; Functional response; Indirect effects; Numerical response; Population cycles; Predator-mediated extinction; Predator–prey interaction.

Grants and funding

Funding was provided from the Natural Science and Engineering Research Council of Canada, the Yukon Fish and Wildlife Enhancement Trust, the Northern Science Training Program of Environment Canada (administered by the University of British Columbia), The W. Garfield Weston Foundation Fellowship Program (a program of the Wildlife Conservation Society Canada funded by The W. Garfield Weston Foundation), the Northern Research Endowment fund (administered by Yukon College), a grant-in-aid from the Arctic Institute of North America and logistical support from Yukon Territorial government. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.