What Are Urchin Barrens? How Sea Urchins Destroy Kelp

What does an urchin barren look like?

A healthy kelp forest is a tall, crowded habitat. Kelp stems rise toward the surface, smaller seaweeds grow underneath, and fish, crabs, abalone and many other animals shelter among them.

An urchin barren looks almost empty by comparison. The rock is covered with sea urchins, often dozens per square meter, sitting out in the open. Between them, the rock is coated in crustose coralline algae, a hard, pink, chalky crust that urchins scrape but cannot easily kill. Kelp stumps and holdfasts may remain for a while before they too are eaten.

The boundary between the two states can be surprisingly sharp. Divers sometimes swim from a lush kelp forest straight onto a barren in a few meters, with a front of urchins chewing along the edge.

How do urchin barrens form?

In a balanced kelp forest, urchins mostly hide in crevices and eat drift kelp, the broken pieces of seaweed that roll along the bottom. Predators keep their numbers down and make them cautious. On the Pacific coast of North America, the main urchin predators are the sea otter, the sunflower sea star, the California sheephead and spiny lobsters.

Barrens usually begin when something tips this balance:

• Predators disappear. Hunting, overfishing or disease removes the animals that eat urchins.
• Food runs short. Warm water, storms or poor nutrients thin the kelp, so less drift reaches the urchins.
• Urchins boom. A run of good years for young urchins floods a reef with new grazers.

When drift kelp becomes scarce, hungry urchins change their behavior. They leave their shelters and move out across the reef in dense groups, called grazing fronts, chewing through stems and holdfasts until whole kelp plants break loose. The purple sea urchin is famous for this, using the five teeth of its jaw, described in What is Aristotle’s lantern?

Why urchin barrens are hard to reverse

Barrens persist partly because the urchins in them are extraordinarily hard to starve. Starving urchins can survive for years by slowing their metabolism, scraping films of algae and coralline crust off the rock and absorbing their own gonads. Purple urchins can even shrink. Divers call these hungry survivors “zombie urchins”.

Zombie urchins are a poor meal, so predators may ignore them. A study of sea otters around the Monterey Peninsula, California, published in 2021, found that otters chose healthy, well-fed urchins in the remaining kelp patches rather than starving urchins in the barrens. That helped protect the patches that were left, but did not clear the barrens.

A global analysis led by Scott Ling, published in 2015 and covering 13 reef systems in 11 regions, found a striking pattern. Kelp collapses once urchins pass a certain density, but to get the kelp back, urchin biomass must fall to roughly one tenth of that level. Ecologists call such systems “alternative stable states”: a reef can stay as kelp forest or as barren under the same conditions, and flipping it back takes a big push.

Northern California’s kelp collapse

The most dramatic recent example came from the coast of Sonoma and Mendocino counties, where sea otters have been missing since the fur trade. Several blows landed in a row:

By the end, the bull kelp canopy had fallen by more than 90 percent along more than 350 km (220 mi) of coast. Bull kelp is an annual seaweed that regrows from spores each year, so urchins grazing the young plants could keep the forest from returning. The commercial red sea urchin fishery collapsed too, because red urchins starved alongside the abalone. Purple urchin numbers also surged on some reefs in Oregon.

The sunflower sea star, which declined by about 90 percent across its range, is now listed as Critically Endangered on the IUCN Red List. Its story is told in What is sea star wasting disease?

Urchin barrens around the world

Barrens are not unique to California. They occur on temperate rocky reefs in both hemispheres:

• Aleutian Islands, Alaska: barrens spread after sea otters crashed in the 1990s, one of the classic cases described in Why are sea otters a keystone species?
• Nova Scotia, Canada: green sea urchins turned large areas of kelp bed into barrens in the 1970s. In the early 1980s a disease outbreak caused by an amoeba killed many urchins, and kelp grew back within a few years.
• Norway: green urchins created barrens along much of the northern coast from the 1970s. Kelp has since recovered in parts of mid-Norway as urchin numbers fell, a change linked to warmer water and to predators such as crabs.
• Tasmania, Australia: the long-spined sea urchin spread south as the warm East Australian Current strengthened, forming barrens on parts of the east coast where fishing had reduced the large rock lobsters that eat urchins.
• New Zealand: kina urchins form barrens on reefs in the northeast. In the Leigh marine reserve, protected since the 1970s, recovering snapper and rock lobsters reduced kina numbers and kelp forests expanded.
• Mediterranean Sea: where fishing removes sea breams and other urchin-eating fish, sea urchins such as Paracentrotus lividus and Arbacia lixula create barrens, and well-protected marine reserves tend to keep more seaweed.

Can urchin barrens be restored?

Kelp can come back, sometimes quickly, once urchin pressure drops. Restoration efforts usually combine several approaches:

• Bringing back predators: protecting fish and lobsters in marine reserves, breeding sunflower sea stars in captivity, and studying whether sea otters could return to northern California and Oregon.
• Removing urchins: trials in Mendocino County and Monterey Bay, where divers removed most urchins from selected reefs, have seen kelp grow back on the cleared rock. Clearing whole coastlines this way is not practical.
• Urchin ranching: companies collect zombie urchins from barrens and feed them in tanks for a couple of months until their gonads fill, then sell them as uni.
• Helping kelp: scientists are testing ways to plant young kelp and to find strains that cope better with warm water.

Nature sometimes resets the system too. Disease outbreaks among urchins, cold, nutrient-rich years and strong recruitment of predators have all turned barrens back into forest. The lesson from every region is the same: kelp forests stay healthy when urchins have enough enemies.