Heatwave transformed Australian marine life

Heatwaves aren't just a problem for humans. They can reshape marine ecosystems too. Such extreme weather events will become more common because of climate change. They can ravage land ecosystems, but until now little has been known about their effects in the seas.

Events last year in the sea off Australia's west coast suggest that the impact can be extreme and rapid. For more than ten weeks beginning in January, sea temperatures were between 2 ?C and 4 ?C warmer than usual along a 2000-kilometre stretch of coast ? the area's most extreme warming event since records began.

In November 2011, Daniel Smale at the University of Western Australia in Perth and colleagues surveyed the area, as they have done every year since 2006. The formerly pristine and stable ecosystem had completely changed.

"In less than a year, we can have ecological switches from one kind of habitat to another," Smale says.

The ecosystem had lost complexity. The kelp (Ecklonia radiate) that covered 80?per cent of the area, providing a range of habitats, had declined to cover just 50?per cent. Mats of algal "turf", which create fewer distinct niches, had moved in instead.

Smale will return to the area this November to see whether the changes are permanent ? he suspects that some will be.

This is not the first evidence that marine heatwaves can have a devastating impact. The 2003 heatwave that gripped Europe triggered a heatwave in the Mediterranean Sea. Temperatures rose by between 1 ?C and 3 ?C, and in places 80 per cent of sea fans died (Global Change Biology, DOI: 10.1111/j.1365-2486.2008.01823.x).

We don't yet know whether climate change triggered Australia's marine heatwave, but there is good evidence that it triggered Europe's 2003 heatwave. Models suggest that such events will become more common.

Working out what effect that will have on biodiversity is tricky. "It's a thing we all know is important, but it's very difficult to deal with," says Chris Thomas of the University of York, UK.

Thomas predicts that climate change will commit 15 to 37 per cent of species to extinction by 2050 (Nature, DOI: 10.1038/nature02121). He says the toll may be made worse by more frequent extreme weather events.

It's a concern that the International Union for Conservation of Nature (IUCN) shares. The IUCN Red List of Threatened Species factors in climate change, but a species that may seem stable as temperatures rise gradually might be hit much harder by an extreme event.

"If dramatic ecosystem changes happen, that may be something that takes us by surprise," says Rebecca Miller, programme officer at the IUCN red list unit.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate1627

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