Sea Otters to the Rescue
SEA URCHINS ARE DECIMATING CALIFORNIA’S KELP FORESTS. BUT SEA OTTERS HAVE STEPPED IN.
The brutal consequences of climate change in California — the recordbreaking heatwaves, droughts, and wildfires — have been well documented. But the climate crisis has also taken a terrible, if more hidden, toll on California’s marine ecosystems.
Marine heatwaves in 2014 and 2015 stressed kelp forests, which need colder water to thrive. They also fueled sea-star-wasting disease, which began in 2013 and ultimately resulted in the largest marine extinction event in modern history. Untold billions of sea stars, or starfish, from at least 20 species disappeared from the entire Pacific coast, from Alaska down to Mexico. Enormous sunflower sea stars, now functionally extinct in California, were a major predator for sea urchins; in their absence, the urchin populations exploded.
No longer restrained by predation, urchins, a voracious herbivore, moved out of their former rock crevice habitats and laid waste to kelp forests already weakened by high ocean temperatures. Tragically, Northern California has now lost 95 percent of its once-verdant kelp forests. Where these richly biodiverse marine ecosystems once thrived, little more now remains than a seafloor carpeted with spiky urchins, dubbed “urchin barrens.”
The precipitous decline of kelp forests not only dealt a devastating blow to marine biodiversity, but it has also had dire economic consequences for the coastal communities that formerly depended on fish, abalone, and other marine resources the forests generated. Kelp forests, which can be up to 20 times more efficient at sequestering carbon than land forests, are also a very important ally in the fight against global warming.
However, the situation is complex; while nearly all California coastal systems have been disrupted by warming waters and massive sea star die-offs, not all kelp forests have declined equally. Central California’s kelp ecosystems have fared somewhat better than those in Northern California. A new study led by Joshua Smith, an ecologist at the University of California, Santa Cruz, examines the role robust sea otter populations — a keystone species and a natural predator for urchins — play in preserving existing kelp forests in the Monterey Bay.
The research project spanned three years, during which intensive underwater surveys produced some very interesting data about sea otter behavior. Sea otters have very high metabolisms that keep them warm in icy waters and which also make them ravenous consumers of shellfish, urchin, and fish — sea otters can eat 25 percent of their body weight in food in a day. At the observed sites, as the urchin population grew, the otter’s diet shifted to become more heavily reliant on urchins. In fact, the otters were eating three times as many urchins as they had been before 2014. As a result, they provided crucial protection from urchins to the remnant kelp forests.
The fact that otters were able to contribute to the health of the patchwork of surviving kelp forests by keeping urchin populations in check provides Smith with hope for the forests’ continued survival. “The important thing is that the urchins and the patches of barrens are ultimately reduced,” he says. “The sea otters have maintained patches of kelp forest that can then help replenish the barren areas to enhance the recovery of forests.”