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Posted May 2009

A study of sea squirts (ascidians) shows that increasing water temperatures could favour invasive forms by helping them outcompete native populations of these marine invertebrates.

In work conducted from 1991 to 2001, John Stachowicz, from the University of California, and colleagues compared the colonisation of tiles by native and introduced sea squirts at a study site in southern New England (US) where mean annual ocean surface temperatures had increased over the previous quarter-century.

They found that in warmer years the introduced sea squirts (Ascidiella aspersa, Botrylloides violaceous and Diplosoma listerianum) colonised habitat ahead of the local populations. This is a considerable advantage as community composition is determined by the order in which different species colonise.

The introduced species did best when winter ocean temperatures were high. In the year after the coldest winter, native species recruited at five times the rate of introduced species, but in the year after the warmest winter, the recruitment of introduced species was twice that of the natives. The difference between the mean temperatures of these two extremes was just 3°C, showing that small changes can have dramatic impacts.

In laboratory experiments, the two introduced sea squirts grew faster than a native species when temperatures neared the summer maximums.

The researchers concluded that warming is likely to see increased dominance and new invasions by exotic species from warmer areas but cautioned that ecological impacts of climate change may be difficult to predict because they may occur when temperatures are most extreme (rather than in response to changes in the mean) or when species pass through critical life-history stages.

References
Stachowicz JJ, Terwin JR, Whitlatch RB, Osman RW. 2002. Linking climate change and biological invasions: Ocean warming facilitates nonindigenous species invasions. Proceedings of the National Academy of Sciences 99: 15497-15500.

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