Once you flip to the urchin barren state…
Photograph by Oliver Dodd
From Yale Environment 360:
In eastern Tasmania, sea surface temperatures have increased at four times the average global rate, according to Johnson, who along with colleague Scott Ling has closely studied the region’s kelp forest losses. This dramatic environmental change began in the mid-20th century and accelerated in the early 1990s. Giant kelp — Macrocystis pyrifera — does best in an annual water temperature range of roughly 50 to 60 degrees Fahrenheit, according to Johnson. He says routine summertime spikes into the mid-60s pushed the kelp over the edge. First in Australia, and subsequently in Tasmania, the kelp forests vanished. The Australian government now lists giant kelp forests as an endangered ecological community.
As waters warmed, something else also happened. The long-spine sea urchin, which generally cannot tolerate temperatures lower than 53 degrees Fahrenheit, traveled southward as migrant larvae and established new territory in Tasmanian waters. Lobsters — which prey on urchins — had been heavily fished here for decades, and consequently few predators existed to control the invading urchins, whose numbers boomed.
Since the 1980s, long-spine urchins — Centrostephanus rodgersii — have essentially taken over the seafloor in southeastern Australia and northeastern Tasmania, forming vast urchin barrens. An urchin barren is a remarkable phenomenon of marine ecology in which the animals’ population grows to extraordinary densities, annihilating seafloor vegetation while forming a sort of system barrier against ecological change. Once established, urchin barrens tend to persist almost indefinitely.
“For all intents and purposes, once you flip to the urchin barren state, you have virtually no chance of recovery,” Johnson says.
In some places, like the southwestern coast of Hokkaido, in Japan, and the Aleutian Islands, urchin barrens have replaced kelp forests and have remained for decades.
This bodes poorly for eastern Tasmania, where expansive areas in the north have already been converted into barrens. Urchins have not yet overrun southeastern Tasmania. “But we’re seeing the problem moving south, and we’re getting more and more urchins,” says Johnson, who expects roughly half the Tasmanian coastline will transition into urchin barrens. “That’s what we have in New South Wales.”