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1 December 2013 Safety from predators or competitors? Interference competition leads to apparent predation risk
William D. Halliday, Douglas W. Morris
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Prey often react to predation risk by foraging preferentially in the safety of cover rather than in more risky open patches. Yet this pattern of patch use also can be caused by dominant interspecific competitors. We develop a simple theory of this form of apparent predation risk that describes the patch use of an optimal forager confronted with dominant individuals. The theory predicts that subordinate animals should increase their use of safe foraging patches as the density of nearby dominants increases. We tested the theory with meadow voles (Microtus pennsylvanicus) and southern red-backed voles (Myodes gapperi). We used dyadic encounters to confirm that meadow voles are dominant over red-backed voles. We then evaluated their respective foraging patterns in pairs of covered and open patches in 4 adjacent subgrids in an old-field enclosure. Subordinate red-backed voles foraged indifferently between covered and open patches when few meadow voles were present. Red-backed voles increased their use of both patches as the number of nearby meadow voles increased. Giving-up densities were lowest, and harvesting efficiency highest, in covered patches when the number of nearby meadow voles was high. These results document competition between the 2 species and suggest that vigilance toward dominant meadow voles magnifies the risk experienced by red-backed voles in open patches. Investigators assessing foraging behavior between “safe” and “risky” patches might misinterpret the competitive effect as predation risk unless they 1st account for competition among foraging individuals.

William D. Halliday and Douglas W. Morris "Safety from predators or competitors? Interference competition leads to apparent predation risk," Journal of Mammalogy 94(6), 1380-1392, (1 December 2013).
Received: 10 December 2012; Accepted: 1 June 2013; Published: 1 December 2013

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