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Conservation efforts should focus on entire communities of plants and animals instead of just endangered species, say scientists at the University of Wisconsin-Madison, United States. Their research, published in this week's Nature, shows the importance of the order in which species go extinct — and the difficulty of predicting knock-on effects when they do.

Extinctions can lead to greater stability and resilience to additional losses within ecosystems or they can promote a cascade of further extinctions. Using computer models, the researchers compared ecosystem stability when species go extinct randomly to when they go extinct in order of their sensitivity to an environmental stress.

"We know that some species are more sensitive to environmental [stresses], and they often go extinct in order of their sensitivity," says lead author Anthony Ives.

When this happens, the less sensitive species left behind will be more suited to new conditions and less prone to extinction. Communities as a whole can therefore become more resistant to environmental stresses such as climate change or pollution.

But another factor — food-web interactions — complicates the picture. Because species eat and are eaten by each other species, and compete for access to resources, extinctions can have ramifications throughout an ecosystem. The loss of one species can, for example, allow others to increase in number.

"A species that seems insignificant now may become important later on once it's released from predation or competition," says co-author Bradley Cardinale.

This means that species that were formerly resilient to extinction can rapidly become sensitive to environmental degradation following extinction of other species. The researchers urge whole-system approaches to conservation as these food-web dynamics can change the order of extinctions, making predictions of future impacts for ecosystems difficult.

"We can't just go out and conserve one species," says Cardinale. "We have no idea what species may make the community resistant in the future, [so] we would be most prudent to conserve as many as we can right now."

Link to the full paper in Nature

Reference: Nature 429, 174 (2004)