They’re purple, spiky and voracious, and just off the West Coast, there are more of them than you can count.
Purple sea urchins have exploded in recent years off California, covering the ocean floor in what divers describe as a “purple carpet.” And they devour kelp: the once-lush forests of seaweed that hugged the coastline are disappearing. Since 2014, 95 percent of the kelp have vanished across a large part of Northern California, most of it bull kelp.
Kelp forests provide a crucial ecosystem for a broad range of other marine life and animals, so their demise threatens the ecology across the entire stretch of California coast. The kelp’s abrupt decline is being driven by warming waters, and it’s a case of how climate change is helping push already-stressed ecosystems over the edge.
Urchins are a normal part of the kelp forest, but a double whammy of ecological change has caused a population boom. Even now, with little food left, the urchins are still able to survive.
“They’re kind of like zombies,” says Morgan Murphy-Cannella, kelp restoration coordinator with Reef Check California, a group of citizen scientist divers. “They can last for a long time without eating, and they can just live. They’re a very bizarre animal.”
To give kelp forests a chance against the urchin hoards, scientists and divers are trying out new strategies to carve out refuges. The hope is by removing the urchins, or even catching them for dinner tables, small pockets of kelp might begin to come back.
Across much of Northern California, purple urchins have little kelp left to eat, but still can survive.
Steve Lonhart/NOAA MBNMS
Underwater forests
California’s kelp forests are not unlike redwood forests on land. The towering seaweed reaches 30 to 60 feet tall.
At its peak, kelp grows up to two feet per day in the West Coast’s cold, rough waters, which are rich in nutrients. But in 2014, the ocean off California began warming.
A mass of warm water in the Pacific Ocean, known as “the Blob,” began expanding, raising temperatures high above normal. It was also low in nutrients, stressing the kelp, and it lasted for several years.
At the same time, a widespread disease was shifting the dynamics in the ecosystem. Purple sea urchins are usually kept in check by their main predator on California’s North Coast: the sunflower sea star. The large sea star (also known as a starfish) has more than 20 arms and spans several feet across.
In 2013, sea star wasting disease spread across the West Coast, decimating all kinds of sea star populations. Sunflower sea stars began disappearing and are now listed as critically endangered on the IUCN Red List of Endangered Species.
Sunflower sea stars, the purple urchin’s main predator in Northern California, have disappeared due to sea star wasting disease. In the image on the right, taken in 2015, you can see Sunflower sea star eating a purple urchin.
Steve Lonhart/NOAA MBNMS
With their predator largely gone, purple urchins boomed. While they usually hide in crevices, they became increasingly bold as competition for food increased.
“Sometimes we see dozens of them crawling up the stem of the kelp and taking it down from there,” says Meredith McPherson, a graduate student at UC Santa Cruz.
