CONSERVATIONISTS need to rethink their approach to biodiversity according to a study by Scottish researchers, which revealed that the number of species present in individual ecosystems remained the same over time.

Scientists at St Andrews University have contradicted expectations that the global biodiversity crisis would be mimicked at a small scale, for example, by individual lakes or forests. In fact, they found that while existing species in a particular area might be pushed out and replaced, the overall number of species in the habitat remained roughly the same.

Dr Maria Dornelas, of the university's Centre for Biological Diversity and Scottish Oceans Institute, said: "Contrary to expectations, we did not observe consistent loss of species through time - indeed we found as many surveys with a systematic loss as well as gain in the number of species recorded through time. This is surprising given current concerns of a biodiversity crisis and abnormally high extinction rates."

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The four-year study examined 100 communities and 35,000 species spanning from trees to starfish. It took in 6 million observations about animal and plant life in terrestrial, freshwater and marine habitats from the poles to the equator.

However, the researchers stress that while the results surprised them, they do not detract from the threat many of the world's species are under at a global level from loss of habitat, climate change and the spread of human settlements.

While at the small-scale individual ecosystems appear to remain equally diverse even as they change, when compared against one another the various separate ecosystems are becoming increasingly similar to one another as biodiversity declines overall.

This is reflected in wildlife with the decline of Scotland's native red squirrel in the face of competition from the more aggressive North American grey squirrel. At the same time, birds such as starlings have been able to proliferate across North America and north-west Europe because they are suited to a wide range of environments.

Professor Anne Magurran, also of the institute, said: "What we found was that there's more churning in the composition of the community than you would expect by chance. One likely explanation is that the species coming in are more suited to the habitats.

"So some species are becoming very common, they are found almost everywhere - the starling being one example.

"Other species which are more specialist in their habitat requirements are becoming less common. This is called homogenisation. So habitats, communities and ecosystems are becoming more similar to one another, even though the number of species living there do not change dramatically."

The study, published in the journal Science, was carried out in collaboration with US ecologists Nick Gotelli, of the University of Vermont, and Brian McGill, of the University of Maine. The research was funded by the Euro-pean Research Council project BioTIME.

Prof Gotelli, along with the other report authors, was keen to emphasise that the "findings do not negate the fact that many of the world's species and habitats are under grave threat". He added: "What we do suggest is that scientists and policymakers should expand the focus of conservation science and planning to cover biodiversity change as well as loss."

Prof McGill said: "Conservation scientists will need to shift from just talking about how many species are found in a place to talking about which species are found in a place."