In an unprecedented environmental collapse, Northern California’s iconic kelp forests have almost entirely disappeared. Starting between three and four years ago, the region’s population of purple sea urchins – which eat marine vegetation – exploded after a disease wiped out their main predators, sea stars. The spiny bulb-shaped urchins have since mowed down virtually all the bull kelp between San Francisco and the Oregon border and, with few predators afoot, taken over the sea floor.

“There are patches of remaining kelp, but they’re tiny,” said Cynthia Catton, an environmental scientist with the California Department of Fish and Wildlife who has been closely studying the crisis. “You can count the number of individual kelp plants.”

Now urchins – a type of echinoderm related to sand dollars, sea stars and sea cucumbers – have become the unlikely lords of the beleaguered ecosystem, covering the ocean bottom in places as they scour rocks clean and devour any scrap of algae that drifts by. Abalone, prized sea snails that also eat kelp, are meanwhile starving in droves, and eventually, rockfishes, which use kelp canopies as nursery habitat, may begin to vanish from the environment, warns Catton.

One key problem is that the kelp cannot grow back, since the carpet of urchins effectively acts like a herd of goats in a dusty pen, completely subduing any growth of vegetation. Another challenge is that urchins can live for years without substantial food and can even, Catton says, absorb nutrients directly from the water, making starvation unlikely to cull their numbers anytime soon.

Urchin barrens, as scientists refer to such ravaged environments, have often developed briefly at small scales in kelp ecosystems globally. However, Catton says she has never seen this type of ecological change at such a region-wide scale – at least not in North America – and she says no one knows when the ecosystem may recover.

“Once an environment shifts into an urchin barren state, it tends to stay that way,” Catton said. “Everything is pointing toward this being a long-term problem.”

The crisis began in 2013, when a viral outbreak swept the West Coast’s waters, affecting at least 19 species of sea star and almost annihilating several of them. The sunflower sea star – a large, bulbous species with numerous small arms – is a particularly voracious eater of urchins.

“We used to see five or six [sunflower sea stars] at each of our sites,” said Jan Freiwald, a Santa Cruz-based diver and the California director of Reef Check, a program that monitors the health of sea floor ecosystems throughout much of the world.

In 2016, he says, Reef Check’s statewide surveys counted two individuals.

With sea stars mostly removed from the ecosystem, urchin numbers went through the roof in 2014 and 2015.

“We once counted maybe 10 purple urchins per dive transect,” Freiwald said. “Now, we see thousands.” Put another way, densities have increased from less than one urchin per square meter to anywhere from a dozen to three dozen, he says.

The second blow of an ecological double whammy struck in 2015: Seasonal wind and ocean current patterns in the northeast Pacific changed direction, weakened or shut down. Much of the ocean’s surface, stagnating in the sun, warmed by several degrees as upwelling of cold, nutrient-rich bottom water ceased. Bull kelp, which dominates the kelp ecosystem of Northern California and thrives in frigid water, responded negatively to the change in environmental conditions. Entire kelp beds wilted and died. Large winter storms also uprooted many kelp plants.

Urchins, with free run of their environment, have prevented recovery, and today, much of the coastline from the Oregon border to San Francisco is kelp-less, or, at best, studded with decaying stumps.

“I was on a dive at Ocean Cove [in Sonoma County] in September and there were stalks all over the bottom, like a fire had swept through a forest,” said Keith Rootsaert, a diver who conducts surveys for Reef Check. “There was no algae, and the urchins were absolutely everywhere.”

Freiwald says urchin barrens are beginning to form at the southern end of Monterey Bay, near Carmel.

“But we haven’t seen it yet at Big Sur,” he said, referring to the coastline farther south.

That stretch of coast is the core habitat of threatened California sea otters that depend on kelp for shelter and food. Scientists at the United States Geological Survey have attributed a decline in the number of sea otters surveyed due in part to a lack of kelp. “The lower mainland count this year could be due to poorer counting conditions and very sparse kelp canopies, which likely influenced sea otter distribution,” Tim Tinker, a research ecologist who leads the USGS California sea otter research program, said in a statement released on September 29.

With almost nothing to eat, red abalone – phenomenally abundant in most locations north of Marin County until just several years ago – will likely be depleted before the urchin barrens revert to kelp forest, Catton says. Hundreds of thousands, if not millions, of the large sea snails are starving, their edible flesh withering within their shells, which are steadily accumulating like bones on the bottom. Catton says her department is seriously considering shutting down the state’s recreational abalone fishery.

The commercial sea urchin fishery has also been shaken by the kelp forest collapse. Commercial sea urchin divers harvest mostly red urchins – a larger cousin to the now-pestilent purples. But because of the diminished food supply, the creatures’ golden reproductive organs, called uni in sushi bars, have withered under the stress of hunger.

“They’re empty of gonad, which makes them not worth harvesting,” Catton said.

Urchin barrens can persist for decades, as they have in southeast Australia, where an invasive sea urchin species has devastated native kelp forests. Freiwald says California’s urchin barrens have stabilized and will persist until a drastic disturbance – perhaps an urchin-killing disease – affects the region. Though there are reports of predatory sea stars beginning to recover in some locations, sea star wasting disease continues to affect West Coast waters, and purple sea urchins seem poised to rule the sea floor for years.

“We’ve crossed a tipping point with kelp and urchins,” Catton sad. “Now, we’re losing the coralline algae, that pink crust you see on the rocks. The urchins are scouring it away.”

Eventually, impacts may ascend into the water column.

“The next species in line will be rockfishes,” Catton said. “They don’t eat kelp, but they use it for nursery habitat.”