It’s sea otter awareness week, and after reading enough posts about how sea otters save kelp forests, we thought it time that we set the record straight.
Sea otters (Enhydra lutris) are commonly thought to be the keystone species in kelp forest ecosystems along the west cost of North America, from the Aleutian Islands to California. The well known paradigm follows the logic that urchins can eat and destroy entire kelp beds, leaving a low diversity/productivity urchin barren. Sea otters are veracious predators of urchins, and when otters are present in a kelp bed, they control urchin populations and prevent barren formation. While sea otters can definitely be effective keystone predators in some systems, the full story is much more complex.
The fur trade in the 1800 nearly caused the extinction of the sea otters. In 1911, otters gained government protection and their populations began to recover. A number of studies showed that the recolonization of sea otters in the Aleutian Islands were able to mitigate urchin barrens (1,2,3). The effect was quick and broad. These systems were clearly dysfunctional in the absence of otters. Subsequently, this idea of top down control became prevalent in scientific circles and an overarching theme applied to all kelp beds. Yet, in California, where otters had been absent for a hundred years, many kelp beds were thriving. Of the 224 kelp beds in southern California in the 1980s, only 10% were classified as urchin barrens (4). How was it that these beds could be doing just fine without otters, while the beds in the Aleutians clearly needed them?
It turns out, that kelp beds in the Aleutians are simple systems and contain few other urchin predators, while the California beds have a number of other species that can fulfill that functional role: the sun star (Pycnopodia helianthoides), sheephead (Semicossyphus pulcher), and the spiny lobster (Panulirus interruptus) (5,6). Urchin barrens do occur in southern California, but generally these are areas (1) subjected to over-fishing of these urchin-eating predators or (2) high levels of pollution, temperature, or other disturbances that directly kill the kelp. Southern California also experiences urchin disease and El Ninos, which can control urchins independent of otters. In fact, its been over one hundred years since an otter swam through the kelp fronds of the worlds largest kelp forest at Point Loma in San Diego, yet this forest remains the global example of a well functioning kelp system.
Food webs within kelp beds are complex and vary in space and time. What is clear, is that kelp forest resilience depends on biodiversity, and over-fishing/ hunting and other disturbances can tip the scales in favor of urchins. This is currently happening in various locations along the Monterey Peninsula, in plan view of a healthy otter population, though this situation probably wont last long. Otters are veracious predators of urchins and can stop or help mitigate urchin population explosions, especially when they are the only form of protection from urchin grazing. Such simple kelp systems, however, are restricted to certain geographic regions (e.g. the Aleutians), whereas other kelp beds (like in California) have a higher diversity of sea otter predators and other forms of urchin control, and don’t require otters for the kelp forest to function properly. But with impending climate change and human impacts on kelp systems, who knows what will happen and those charismatic furry little urchin predators may serve as an insurance policy if all hell breaks loose. Until then, at least they are fun to watch.
Estes and Palmisano 1974
Estes et al. 1978
Estes and Duggins 1995
Foster and Schiel 1988