The interaction of intraspecific competition and habitat on individual diet specialization: a near range-wide examination of sea otters
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The quantification of individuality is a common research theme in the fields of population, community, and evolutionary ecology. The potential for individuality to arise is likely context-dependent, and the influence of habitat characteristics on its prevalence has received less attention than intraspecific competition. We examined individual diet specialization in 16 sea otter (Enhydra lutris) populations from southern California to the Aleutian Islands in Alaska. Because population histories, relative densities, and habitat characteristics vary widely among sites, we could examine the effects of intraspecific competition and habitat on the prevalence of individual diet specialization. Using observed diet data, we classified half of our sites as rocky substrate habitats and the other half containing a mixture of rocky and unconsolidated (soft) sediment substrates. We used stable isotope data to quantify population- and individual-level diet variation. Among rocky substrate sites, the slope [±standard error (SE)] of the positive significant relationship between the within-individual component (WIC) and total isotopic niche width (TINW) was shallow (0.23 ± 0.07) and negatively correlated with sea otter density. In contrast, the slope of the positive WIC/TINW relationship for populations inhabiting mixed substrate habitats was much higher (0.53 ± 0.14), suggesting a low degree of individuality, irrespective of intraspecific competition. Our results show that the potential for individuality to occur as a result of increasing intraspecific competition is context-dependent and that habitat characteristics, which ultimately influence prey diversity, relative abundance, and the range of skillsets required for efficient prey procurement, are important in determining when and where individual diet specialization occurs in nature.
KeywordsIndividual diet specialization Habitat Intraspecific competition Stable isotopes
We thank Luke Tyrrell, Kelli Blomberg, Ryan Jones, and Deborah Boro for the laboratory assistance and Anne Jakle for constructive reviews. We also thank the USGS Pacific Nearshore Project team members, especially Keith Miles and Liz Bowen (USGS), Heather Coletti (NPS), Tom Dean (Coastal Resource Associates), Brianna Wright (Fisheries and Oceans Canada), and USGS personnel Brenda Ballachey, Dan Monson, George Esslinger, Kim Kloecker, and Ben Weitzman for the logistical and intellectual support. Captured sea otters and carcasses from Kachemak Bay and Kuiu Island were under the authority of an MMPA permit (MA041309-5) issued to the U.S. Fish and Wildlife Service, Marine Mammals Management. Invertebrate prey were collected at Kodiak Island and Kuiu Island under Alaska Department of Fish and Game permits CF-10-03, CF-10-008, CF-11-039, and CF-11-049 issued to V.A. Gill. The Alaska Department of Fish and Game (Commercial Fisheries) in Homer provided invertebrate samples from Kachemak Bay. Thanks to Eric Munk (NMFS) for the collection of prey items at Kodiak Island, AK. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the U.S. Fish and Wildlife Service. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. government.
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