Diverse foraging opportunities drive the functional response of local and landscape-scale bear predation on Pacific salmon
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The relationship between prey abundance and predation is often examined in single habitat units or populations, but predators may occupy landscapes with diverse habitats and foraging opportunities. The vulnerability of prey within populations may depend on habitat features that hinder predation, and increased density of conspecifics in both the immediate vicinity and the broader landscape. We evaluated the relative effects of physical habitat, local, and neighborhood prey density on predation by brown bears on sockeye salmon in a suite of 27 streams using hierarchical Bayesian functional response models. Stream depth and width were inversely related to the maximum proportion of salmon killed, but not the asymptotic limit on total number killed. Interannual variation in predation was density dependent; the number of salmon killed increased with fish density in each stream towards an asymptote. Seven streams in two geographical groups with ≥23 years of data in common were then analyzed for neighborhood density effects. In most (12 of 18) cases predation in a stream was reduced by increasing salmon abundance in neighboring streams. The uncertainty in the estimates for these neighborhood effects may have resulted from interactions between salmon abundance and habitat that influenced foraging by bears, and from bear behavior (e.g., competitive exclusion) and abundance. Taken together, the results indicated that predator–prey interactions depend on density at multiple spatial scales, and on habitat features of the surrounding landscape. Explicit consideration of this context dependency should lead to improved understanding of the ecological impacts of predation across ecosystems and taxa.
KeywordsPredation Functional response Density Habitat Bears Salmon Hierarchical Bayes Ursus Oncorhynchus
We gratefully acknowledge the late Donald Rogers, who initiated bear predation work in the Wood River system and played the key role in maintaining the long-term research program on sockeye salmon in Bristol Bay. We also thank the many entities that have provided financial support for the program over the years, including the seafood industry, National Science Foundation’s BioComplexity and Coupled Natural and Human Systems programs, the Gordon and Betty Moore Foundation, and Alaska Department of Fish and Game. Field work has been conducted by very many people but we especially thank Daniel Schindler and co-workers for the surveys of tributaries of Lake Nerka and other upper lakes in the Wood River system, and Jackie Carter and Chris Boatright for Lake Aleknagik tributaries, database management, and overall program assistance. Finally, we thank the anonymous reviewers whose comments greatly improved the paper.
Author contribution statement
TPQ has been leading this study and collecting the data for over two decades. CJC assisted in data collection and did the Bayesian modeling. AJW assisted in data collection and conceptualization of the study. All authors contributed substantially to manuscript preparation.
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