Coral reefs have appeared off Tasmania, and even though the corals are from species that grow in temperate waters, these are still hundreds of kilometers beyond their historic range. The causes are more complex than warming alone, but marine biologists studying these reefs have confirmed climate change is playing a big part.
Talk of coral reefs brings to mind the fabulous tropical corals. Other sorts of corals inhabit cooler waters, including both deep-water species and Plesiastrea versipora, which grows in more temperate waters, such as near Sydney, as well as in shaded overhangs in tropical waters.
Nevertheless, when P. versipora was found off the islands of the Kent Group, Tasmania, in 2004, it was quite a surprise. “Versipora is occasionally found further south but occurs more as isolated individuals,” Dr Scott Ling of the University of Tasmania told IFLScience. “Coral-dominated reef in Tasmania sounds crazy right! “
The effect is not unique to the Kent Group Islands. In Coral Reefs, Ling reports that across 607 temperate sites, corals are appearing where the waters are warming and where kelp is being heavily grazed by urchins or herbivorous fish. The Kent reefs are just the most extreme example.
The islands lie at 39º South, normally ridiculously far from the equator for shallow water reefs. However, the East Australian Current brings warm water from the tropics. In recent years, the current has accelerated, giving the area a double whammy, experiencing both the background level of global warming and the extra warmth the stronger current brings.
Ling told IFLScience: “Based on their size, [the corals] were probably established 20 years prior [to their discovery].” Initially, the corals appeared in deep waters without enough light for the kelp forests that until recently dominated shallower waters.
“We used to see juvenile fish of northern species settle on to Tasmanian reefs during summer,” Ling said to IFLScience. The winters, however, used to prove too cold for them. “Now due to coastal warming ... they survive to adulthood and breed locally,” forming an ecosystem with the corals.
Along with creating coral-friendly conditions, increased warming has allowed long-spined sea urchins to thrive in these waters. In moderate numbers, the urchins remove the kelp, allowing corals to take up the same space. When too numerous, however, the urchins harm corals as well. Ling said heavy harvesting of lobsters, the urchins' primary predators, has caused a boom in urchin numbers, which has denuded the kelp, making room for corals to grow at depths of 10-14 meters (33-46 feet).
Tropical coral reefs are among the world's most biologically rich environments, but the Tasmanian reefs currently host far fewer species. Meanwhile, Ling said, “we lose about 150 kelp-associated species” when reefs displace kelp forests.