Scientific American —
While scuba diving around the Aleutian Islands in 2014, marine ecologist Doug Rasher saw little sign of the curtains of lush green kelp forests he would have had to push through decades earlier. “It feels like a ghost town,” says Rasher, a researcher at the nonprofit Bigelow Laboratory for Ocean Sciences. The eeriness did not end there: during a boat ride, one of Rasher’s colleagues pointed to a cove where he had seen hundreds of sea otters splashing in the frigid water in the 1970s. Only a handful remained.
The two losses are connected. As sea otters declined (for reasons scientists are still trying to understand), their favorite prey—sea urchins—exploded in number. The voracious echinoderms not only mowed down the kelp but are also tearing apart and devouring the massive, slow-forming limestone reefs on which this seaweed grows, Rasher and his colleagues recently reported in Science. Rising ocean temperatures and acidification are compounding the damage.
Restoring otter populations could rein in the urchins and help protect the larger ecosystem, and ecologists are increasingly interested in applying this idea more broadly. “Our study … highlights the power of trophic cascades in nature and the potential for large predators to ameliorate some of the effects of climate change in the near term,” Rasher and his co-authors wrote. (“Trophic cascade” refers to the compounding effects of removing an organism from an ecosystem.) Many climate impact studies on species have not adequately acknowledged this kind of ecosystem complexity as a factor, according to Rasher and other scientists. Incorporating it would offer a clearer picture of what a warmer future holds in store. “I think we’re all coming to realize that there’s going to be a lot of synergies between species loss and climate change,” says Hillary Young, an ecologist at the University of California, Santa Barbara, who was not involved in the new study. “So what you would expect to happen with the loss of a species in the absence of climate change is not what’s going to happen with the loss of that species in the presence of climate change …. And this paper is the holy grail of showing that interaction.”