UBC Theses and Dissertations
Effects of sea otters on nearshore ecosystem functions with implications for ecosystem services Singh, Gerald Gurinder
Sea otters are nearshore predators whose impacts have potential implications for the provision of ecosystem services on the west coast of Vancouver Island. Sea otter predation on herbivorous sea urchins can allow kelp beds to flourish. Increased kelp production can act as a food subsidy to mussels which can promote faster growth. Otters also depredate mussels, which can affect the habitat provision function of mussels and limit them to sizes that are vulnerable to other predators. In this thesis I describe two empirical studies that explore these possible effects of otters on ecosystem processes that have implications for ecosystem services. First I investigate the impact of greater kelp productivity on carbon flow and productivity by using stable isotope analysis on kelp, water samples, and mussels in regions where otters are absent and present. I observed that mussels do not consistently assimilate higher proportions of kelp-derived carbon and do not grow faster where otters are present and kelp more abundant. This finding may be explained partly because kelp does not seem to be limiting for mussel diets where otters are absent – high observed phytoplankton biomass may dilute the kelp-derived carbon assimilated in mussel tissue. The second study explores the impact of otters as predators of mussels by sampling mussel bed characteristics in regions along a gradient of time since otters established. Mussel bed characteristics vary predictably between regions: e.g., depth and biomass are lower in regions of comparably higher otter influence. Aggregate community biomass is also lower where otters are present, and differences in dominant species may drive differences in community structure between regions. By restricting mussels to smaller sizes, otters may also subject a greater proportion of mussel growth to predation by seastars, potentially facilitating a greater proportion of energy flow through marine food webs. Otter may increase secondary productivity only where primary productivity is limiting, and they seem to constrain the habitat provisioning services of mussels. This study’s quantitative characterization of otter impacts on an ecosystem engineer (mussels), and the intertidal habitat they provide, complements existing studies of otter impacts on subtidal ecosystem processes that affect ecosystem services.
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