04/04/2018 - 12:15pm
Hubbs Hall 4500
I. Implementation of ecosystem-based fisheries management (EBFM) requires a clear conceptual and quantitative framework for assessing how different harvest options can modify benefits to ecosystem and human beneficiaries. We address this social-ecological need for Pacific salmon fisheries, which are economically valuable but intercept much of the annual pulse of nutrient subsidies that salmon provide to terrestrial and aquatic food webs. We used grizzly bears, vectors of salmon nutrients and animals with densities strongly coupled to salmon abundance, as surrogates for ‘‘salmon ecosystem’’ function. Combining salmon biomass and stock-recruitment data with stable isotope analysis, we assess potential tradeoffs between fishery yields and bear population densities for six sockeye salmon stocks in Bristol Bay, Alaska, and British Columbia, Canada.
II. But why do we care about bears? In temperate coastal ecosystems throughout much of the world, anadromous fish historically supported large omnivorous bear populations. In Alaska and British Columbia, where anadromous fish are still prolific, bears distribute salmon derived nutrients to plants, insects and vertebrate scavengers, yet little is understood about the how salmon, by supporting large bear populations, indirectly contribute to ecosystem processes. As large omnivores, bears may serve as keystone hubs that fulfill many roles in ecosystems. For example, it is now known that bears are effective top predators that can maintain ungulates at low abundance, but it is currently unknown whether bears rather than birds are the primary seed dispersers of the fleshy fruited plants that dominate the understory in coastal AK and BC, whether their seed-filled bear scats influence the dynamics of small mammal populations, and whether secondary seed dispersal from bear scats by small mammals improves seedling recruitment and plant community dynamics. We addressed these questions by (1) quantifying the number of seeds in brown and black bear scats, (2) by monitoring small mammal activity at bear scats, (3) by using cameras to monitor the rate of fruit consumption by different vertebrate seed dispersers and seed predators, and (4) by monitoring the response of small mammal populations to experimental bear scat addition. We discuss the results of this research and the implications for what has changed in the Pacific Northwest in the absence of a strong bear-salmon interaction.
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