Effects of habitat features on size-biased predation on salmon by bears
Predators can drive trait divergence among populations of prey by imposing differential selection on prey traits. Habitat characteristics can mediate predator selectivity by providing refuge for prey. We quantified the effects of stream characteristics on biases in the sizes of spawning salmon caught by bears (Ursus arctos and U. americanus) on the central coast of British Columbia, Canada by measuring size-biased predation on spawning chum (Oncorhynchus keta) and pink (O. gorbuscha) salmon in 12 streams with varying habitat characteristics. We tested the hypotheses that bears would catch larger than average salmon (size-biased predation) and that this bias toward larger fish would be higher in streams that provide less protection to spawning salmon from predation (e.g., less pools, wood, undercut banks). We then we tested for how such size biases in turn translate into differences among populations in the sizes of the fish. Bears caught larger-than-average salmon as the spawning season progressed and as predicted, this was most pronounced in streams with fewer refugia for the fish (i.e., wood and undercut banks). Salmon were marginally smaller in streams with more pronounced size-biased predation but this predictor was less reliable than physical characteristics of streams, with larger fish in wider, deeper streams. These results support the hypothesis that selective forces imposed by predators can be mediated by habitat characteristics, with potential consequences for physical traits of prey.
KeywordsEco-evolutionary dynamics Predation Predator–prey interactions Stream characteristics Size-selective predation
We are grateful to the Heiltsuk First Nation for their support of our research. We thank Aaron Gaffney, Howard Humchitt, Jane Pendray, Ben Rabinovich, Michelle Segal, Mary Vasey, Marlene Wagner, Kirsten Wilcox and Ross Wilcox for field assistance and for making this such an enjoyable project. We also thank Isabelle Côté, Larry Dill and the Earth to Ocean Research Group at Simon Fraser University for their valuable project input. We thank Andy Cooper, Will Atlas, Joel Harding and Sean Godwin for their guidance in statistical analyses, and Lindsay Davidson for editing the manuscript. Finally, we thank Tom Quinn, Brian Shuter and an anonymous reviewer for suggestions that greatly improved this paper.
Author contribution statement
JDR obtained funding and supervised the project. LCA and JDR designed the study. LCA did the fieldwork and analyzed the data. LCA and JDR wrote the manuscript.
Compliance with ethical standards
All applicable institutional and/or national guidelines for the care and use of animals were followed.
Conflict of interest
The authors declare that they have no conflict of interest.
This research was funded by Simon Fraser University, the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Tom Buell BC Leadership Chair endowment funded by the Pacific Salmon Foundation and the BC Leading Edge Endowment Fund. Additional support was provided to LCA by the W. Garfield Weston Foundation.
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