Do trophic cascades affect the storage and flux of atmospheric carbon? An analysis of sea otters and kelp forests
We combine data collected from the past 40 years to estimate the indirect effects of sea otters (Enhydra lutris) on ecosystem carbon (C) production and storage across their North American range, from Vancouver Island to the western edge of Alaska’s Aleutian Islands. We find that sea otters, by suppressing sea urchin (Strongylocentrotus spp) populations, allow kelp (Order Laminariales) ecosystems to develop with a net primary productivity (NPP) of 313–900 grams C per square meter per year (g C m−2 yr−1) and biomass density of 101–180 grams C per square meter (g C m−2). In the absence of sea otters, these areas would have an NPP of 25–70 g C m−2 yr−1 and biomass density of 8–14 g C m−2. Over an ecosystem area of approximately 5.1 × 1010 m2, the effect of sea otter predation on living kelp biomass alone represents a 4.4-to 8.7-teragram increase in C storage. At 2012 prices (US$47 per ton of C), this stored C would be valued at US$205 million–$408 million on the European Carbon Exchange. Although questions remain concerning the pathways and compartments of kelp C flux and storage, sea otters undoubtedly have a strong influence on these elements of the C cycle. Predator-induced trophic cascades likely influence the rates of C flux and storage in many other species and ecosystems.
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