Evol Ecol Res 11: 209-225 (2009)     Full PDF if your library subscribes.

Apparent predation risk: tests of habitat selection theory reveal unexpected effects of competition

Douglas W. Morris

Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada

Correspondence: D.W. Morris, Department of Biology, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada.
e-mail: douglas.morris@lakeheadu.ca


Questions: Does reducing density of one species increase habitat use by its competitor? If so, can the competitive effect mimic predation risk?

Hypotheses: A competing species should increase its use of secondary habitat as the density of its competitor in that habitat declines. And it should forage in safe sites more readily when its competitor is abundant than when it is sparse.

Organisms: Two co-existing species of northern voles (Myodes gapperi and Microtus pennsylvanicus) known to have distinct habitat preferences.

Field site: Two pairs of interconnected rodent-proof enclosures in field and forest habitat at the Lakehead University Habitron near Thunder Bay, Ontario, Canada.

Methods: I predicted density-dependent habitat use from first principles, then measured the density of Myodes in the two habitats as well as its quitting-harvest rate in artificial food patches. I tested the predictions by contrasting treatments where I reduced the density of Microtus in the presence of Myodes, versus controls where I reduced an equal density of Myodes existing alone.

Results: Myodes used its preferred forest habitat more at high Microtus density than at low Microtus density. But Microtus occupied both habitats at all densities. Myodes used safe foraging sites more intensely in the treatment where Microtus was present than in the control where it was absent.

Conclusions: Competition between these two vole species is reduced by density-dependent habitat selection. But Myodes also trades off food for safety to avoid competition with larger Microtus. Ecologists must first eliminate competition if they are to accurately estimate the effects of predation risk on species co-existence.

Keywords: apparent competition, density dependence, habitat, habitat selection, optimal foraging, predation risk, vole.

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