By Julianna Kettlewell BBC News Online science staff

Most current programmes focus on "charismatic" species like tigers

Anthony Ives and Bradley Cardinale modelled extinctions and found they could have unpredictable effects on the animals and plants left behind.

Remaining species could thrive because they had lost a competitor; and they could also struggle with future change.

The team says protection programmes must take a broad view of conservation.

Conflicting forces

Some researchers fear that, due to climate change and habitat loss, our planet is facing its sixth mass extinction.

Currently, most conservation programmes aim to boost the numbers of so called "charismatic" animals, such as tigers and gorillas.

There is nothing wrong with this approach - indeed history has shown it can be very effective. For example, the cirl bunting, a sparrow-sized farmland bird, was recently brought back from the brink of extinction in the UK.

But should we be looking at the problem of conservation in a slightly different way?

We can't just go out and conserve one species Bradley Cardinale, University of Wisconsin-Madison

No animal is an island: nearly every organism lives in a complex interacting web of other species.

One creature's extinction can affect the other members of its ecosystem. That creature may not be well known or cuddly, but its departure can have a wide-reaching influence, say scientists.

A pair of researchers from the University of Wisconsin-Madison, US, have been looking into what happens when a "community" is put under stress - by acid rain, say, or climate change - and species start to decline.

They found that, as individual species disappear, two conflicting forces begin to act on the ecosystem as a whole.

One of those forces makes the community more tolerant of present conditions, while the other force makes it less so.

Good and bad

When an environment is damaged, some species will be more sensitive than others - and will struggle and decline first. For their immediate competitors, this is - in the short term at least - good news: it means more resources to go round.

That is why, temporarily, the remaining species may seem tolerant of the environmental damage.

"They don't like the change, but they like their competitor - or predator - even less, so they do well," Professor Ives told BBC News Online.

"This is called a compensatory response. And whenever you have food webs, with strong interactions between species, these compensatory responses are likely."

However, observers should not suppose an ecosystem is doing fine, just because an extinction has provided an initial "boost".

"This idea of compensation works if you hate your competitors, because while they are decreasing that release from competition will be good for you," explained Professor Ives. "But after your competitors are extinct, you are exposed only to the negative impacts of the environment, so you too may begin to decrease."

In other words, if an animal does not have many competitors or predators left, more extinctions will not do it much good. At this point an ecosystem really hits trouble.

"As more and more species go extinct, the effect of compensation will get less and less," said Professor Ives.

Hard to predict

Although scientists know how ecosystems tend to react to negative forces - first they compensate, then they begin to collapse - it is much harder to predict the fate of individual species.

"You don't know down the road which species will compensate when another species declines," Professor Ives said. "It's hard enough to try to predict how species are interacting with each other as it is.

"To then ask, when species go extinct, how do those interactions change? It's almost impossible."

Unpredictability is the main reason, according to these researchers, why conservationists should operate on the scale of the ecosystem rather than the species.

"We can't just go out and conserve one species," said Bradley Cardinale, the other co-author, "because we have no idea what species may make the community resistant in the future; we would be prudent to conserve as many as we can."