Sea otters are the textbook example of a keystone predator: by eating sea urchins, they keep grazing pressure off kelp and give it room to grow back. But a 2025 study led by researchers at the University of Colorado Boulder, alongside the USGS and the University of British Columbia, found that “textbook” doesn’t mean “predictable everywhere.”
The team compared 30 years of data from two places where otters were reintroduced around the same era: Vancouver Island, British Columbia, in the 1970s, and San Nicolas Island, off Southern California, in the 1980s, modeling how otters, urchins and kelp interacted over time at each site. Despite similar starting conditions, kelp recovered far faster off Vancouver Island than at San Nicolas Island.
The difference wasn’t really about the otters. It came down to how many other species were competing for the same ecological role. California’s kelp forest ecosystem is more complex, with a wider cast of species that can partly buffer urchin populations even without otters, which, counterintuitively, diluted how much difference the otters’ return actually made. In British Columbia’s simpler system, otters had a much more direct, outsized effect. The researchers say the finding argues against assuming a keystone species will act the same way in every ecosystem, and they’re now applying the same modeling approach to try to predict how fast kelp might recover at other sites where otter populations are expanding or being reintroduced.
Further reading: Langendorf, R.E. et al., “Dynamic and context-dependent keystone species effects in kelp forests,” PNAS (2025).