(CN) — The kelp forests along the coast of California have been under attack by purple sea urchins, who have spiked in numbers following the endangerment of their keystone predators, but sea otters in Monterey Bay have been keeping the urchins at bay and protecting what little kelp remains.
Researchers from UC Santa Cruz, the U.S. Geological Survey, and the Monterey Bay Aquarium have teamed up to investigate this ongoing battle in a new study published Monday March 8 in the journal Proceedings of the National Academy for Sciences.
Kelp forests are impressive marine ecosystems made up of large canopies of fast-growing kelp, some reaching over 100 feet tall. They are important biodiversity hotspots, able to provide food and shelter for thousands of species.
These ecosystems once thrived along the coast in cold, nutrient-rich waters with plenty of sun until an onslaught of environmental stressors destroyed 95% of kelp forests. A sudden and unfortunate boom in the purple sea urchin population, who are notorious for devouring kelp, kicked off a major decline of these ecosystems.
Some patches of kelp forest still exist in areas maintained by healthy predator and prey relationships, and among them is California’s Monterey Bay thanks to the incredibly helpful and adorable sea otters.
Sea otters are a keystone species, meaning they help to balance the ecosystems they inhabit, and they protect the kelp forests by eating the sea urchins that would otherwise wreak havoc. In Monterey, they are well supported to feast on the intruders, allowing fragments of kelp forests to survive.
"Here in Monterey Bay, we now have a patchy mosaic, with urchin barrens devoid of kelp directly adjacent to patches of kelp forest that seem pretty healthy," said lead author Joshua Smith, a UC Santa Cruz graduate student. "We wanted to know how did this sea urchin outbreak happen where there are so many otters, how did the otters respond, and what does that mean for the fate of kelp forests here on the Central Coast?"
The decline of kelp forests began in 2013 when a marine illness called sea star wasting syndrome dramatically diminished the sunflower sea star population, a keystone predator of the purple sea urchins. Around the same time, a dreadful marine heat wave called ‘The Blob’ moved in and El Niño storms became more common, both inhibiting kelp growth.
In turn, the sea urchins who would normally hide in rock crevices and feed on the nearby kelp, had less readily available food, and without their main predator around they began coming out of hiding to feast. As a result, the population grew uncontrollably as the urchins steadily ate their way through the kelp forests until nothing was left, and what remained was overgrazed seafloor that became known as urchin barrens.
"It happened so fast, before we knew it we had lost over 80 percent of the historic kelp forest cover in Northern California," Smith said. "We also had an urchin outbreak on the Central Coast, but not to the same extent as in the areas north of San Francisco."
The research team soon saw the sea otters spring to action, eating nearly three times their normal urchin intake, which subsequently helped their population grow from approximately 270 to 432 otters in Monterey Bay. Despite this, the urchin barrens persisted.
Smith and his colleagues then began a three-year-long underwater survey on the Monterey Peninsula to see how the sea otters were feeding and why the urchin barrens were not improving. They found that the otters were removing urchins from the living kelp but typically avoiding the urchin barrens.
"It's easy to see from shore where they are diving repeatedly and coming up with sea urchins," Smith said.
After collecting urchins from both locations, the researchers found a very clear difference between the two. Those collected from the kelp beds were higher in nutritional value with energy-rich gonads, and the urchins from the barrens were starved and provided no benefit.
"Some people call them zombie urchins," Smith said. "You open them up, and they're empty. So the otters are ignoring the urchin barrens and going after the nutritionally profitable urchins in the kelp forest."
The authors explained that the otters are actually maintaining the ecosystem by doing this. By defending the giant kelp, it increases the chances of regrowth and perhaps even revival of the adjacent barrens. They will need assistance, however, as they cannot cover enough ground to put a serious dent in the barrens.
Some further solutions would require more predator introduction, an urchin-killing disease, a large storm to displace the urchins, or human intervention, which has been underway in some regions of the Pacific coast.
"The difference in Southern California is that even though they lost the sea stars, they have other predators like the spiny lobster and California sheephead that are able to control urchin populations and allow the kelp forests to persist," said Mark Carr, professor and chair of ecology and evolutionary biology at UC Santa Cruz, commenting on the different challenges facing the different kelp forests along California’s coast.
While Monterey’s kelp forests are battling their urchin outbreak and the loss of kelp forests, some areas farther up north have seen much more damage in the absence of predators like the sea stars and the sea otters.
"It is possible that the presence of a healthy sea otter population in the north might have made those kelp forests more resilient, but it's hard to speculate," Carr said. "The role of a predator can be very different depending on where you are."
The authors have high hopes for the otter population in Monterey Bay. They are inspired by the kelp forest revival in the Aleutian Islands of Alaska, made possible by the sea otter population there.
"This study not only fine tunes our understanding of the role of sea otters in kelp forests, it also emphasizes the importance of animal behavior," Smith said. "So much of this is driven by behavior — the urchins shifting their behavior to active foraging, and the otters choosing to prey on healthy urchins in the kelp forest — and these behavioral interactions have implications for the overall fate of the ecosystem."
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