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Taking a Bite Out of Climate Change: Predators and Carbon Storage

Everyone knows that sea otters are adorable, and larger numbers of people are learning that they play a key role in maintaining ecosystem diversity by preventing sea urchin populations from turning into kelp-forest mowing swarms that leave “urchin barrens” in their wake. We know this from a long and geographically diverse set of observations of semi-natural experiments where sea otters have been removed and then returned (for example in California, where they were hunted nearly to extinction and then repatriated the coast from isolated remnant populations in Big Sur) or just removed (through Alaskan killer whales switching to snacking on sea otters when their preferred marine mammal prey declined). Where otters return or remain, grazing urchins are kept to a minimum and kelp grows into forests that support a rich array of marine life and produce a wallop of primary productivity. What we are now learning is that these cute keystone species also have a beneficial role in global climate change. A new paper by Christopher Wilmers of UC Santa Cruz (where you can gaze down at kelp forests from the redwood lined coastal terrace) with Jim Estes (who did some of the seminal work on otter-kelp –urchin interactions) and co-authors, uses observations of kelp forest biomass across their geographic range—in locations with and without sea otters—to calculate the total contribution otters make to kelp based carbon storage. The paper, published in the October 2012 issue of Frontiers in Ecology and the Environment (Front Ecol Environ 2012: 10(8): 409-415, doi: 10.1890/1101176), concludes that the effect of sea otters amounts to up to a 8.7 teragram increase in C storage. On today’s (likely weak) Carbon market that kind of storage could be traded at a value of $400 million. Add that to sales of otter t-shirts and stuffed toys at places like the Monterey Bay Aquarium and you’d have to place sea otters pretty high on the ecosystem services list of nature’s top economic performers in a living role. The authors looked at both the carbon density in standing kelp forests and the carbon flux—or movement of carbon through the kelp forest as it lives and breathes and dies--to understand the relationship between kelp biomass and carbon storage. Then they looked at biomass in sample reefs with and without otters using both detailed field sampling on a small number of reefs and estimates based on sampling efforts at over 100 kelp forests with and without otters. They put these numbers together to estimate the total potential Carbon storage effects of otters. The range of their estimates suggests that if restored throughout their historical range, otters would indirectly contribute to 5.6-11% of the carbon storage of the atmospheric carbon for the atmosphere above their range. So, in the end, this is a significant proportion of the local atmosphere, but relatively insignificant for the atmosphere as a whole. It also relies on the assumption that complete restoration of otter populations would linearly scale up from the effects of otters on kelp forest dynamics as they now exist in their relatively small numbers. The authors do note that shifts in predators even higher in the food chain, like killer whales, will disrupt the beneficial effects of otters, and I imagine if otters were restored along their entire native range, we’d see a lot more killer whales coming to snack on “otter pops”. Nonetheless, Wilmers and his colleagues acknowledge that this is just an opening shot in making a case for the role of top predators in ultimately aiding carbon storage. As a matter of science, it’s a nice example of the kind of trend we document in our book Observation and Ecology where scientists are reaching out far beyond their small-scale experimental plots and laboratories to connect what they know to once widely-disparate fields of societal interest.