Assessing the state of an ecosystem solely on the basis of short-term changes in the number of different species it contains can lead to false conclusions, a new study by an international team of researchers led by the ecologist Helmut Hillebrand shows. In order to assess ecosystems in a way that is meaningful for nature conservation, experts should instead focus on analysing the turnover of species within a system.

A growing number of species are under threat of extinction – in particular due to global environmental changes. Political instruments such as the International Convention on Biological Diversity or the EU's Marine Strategy Framework Directive aim to mitigate this biodiversity crisis. But assessing the state of an ecosystem is a tedious task. Thus, in practice, taking the number of species (species richness) as a simple metric seems an obvious approach, explains Prof. Dr. Helmut Hillebrand of the Helmholtz Institute for Functional Marine Biodiversity.

However, this metric has hidden weaknesses, as Hillebrand and his fellow researchers are pointing out in their new study. This is because “the metric doesn't fully reflect the changes in an ecosystem," Hillebrand says, who is also a researcher at the Institute for Chemistry and Biology of the Marine Environment. The researchers therefore analysed data from different ecosystems and applied a mathematical model to reveal how informative measurements of species richness are.

The results of the study show: Negative influences on an ecosystem do not automatically result in a reduction in species richness. Conversely, the number of species in a system does not automatically increase as soon as an ecosystem recovers from human impact. The reason for this: "Species richness is a result of the balance between the immigration and the extinction of species," Hillebrand explains. However, these two processes do not occur at the same speed, he adds. A few individuals of a species can quickly migrate to a local habitat and colonise it, but it may take several generations for a species to be eliminated by a new, more competitive species, or to die out as a result of changed conditions. "This means you can't reliably say, on the basis of short-term trends, whether more or fewer species will be left in an ecosystem over a long period of time," Hillebrand stresses, adding: "So species richness can be a false friend."

In their study, published online in the "Journal of Applied Ecology", the researchers therefore recommend to analyse environmental monitoring data more thoroughly: How many species are migrating to a system? How many are leaving it? And how many species are becoming more or less abundant within the system? As an example the scientists used this method to analyse long-term measurements from various ecosystems - such as data on drifting microalgae (phytoplankton) from the Dutch mud flats and the lakes of North America, as well as data from grassland ecosystems on six different continents.

"We can show that the species identities change partially or even completely – even if over time the actual number of species remains unchanged," Hillebrand says. "This is a major biodiversity change that is completely uncoupled from species richness." Taken to an extreme, the ecologist explains, that would mean that if in a forest all the tree species were replaced by just as many species of grass, the species richness would remain the same, but the forest would be gone.

For their analyses the researchers explicitly used data gathered by conservationists in environmental monitoring programmes. In this way the scientists want to ensure that their tool can be used with the available resources, which in practice are often limited. "We hope that in this way we can build a bridge between the basic research and nature conservation in practice," Hillebrand says.