Ecosystems follow two opposite trends

Red American swamp crayfish

The American red swamp crayfish (Procambarus clarkii), native to northern Mexico and the southeast of the USA, is an example of a species that also thrives in German fresh waters and thus promotes the homogenization of ecosystems. © M Murphy

More and more similar or more and more different? Biologists have discovered that both are true for biological diversity in ecosystems. Many communities become more and more similar over time due to local and regional changes, but almost as many develop in different directions. These two tendencies – biotic homogenization and differentiation – are roughly balanced. This must also be taken into account when assessing global biodiversity.

Nature is in crisis, more and more species are becoming extinct, global biodiversity is dwindling, according to science. But studies on the change in biodiversity paint a more complex picture upon closer inspection. When it comes to individual ecosystems, researchers often disagree about how the species living there will react to human-caused changes. “It is assumed that a key feature of the biodiversity crisis is that species communities are becoming increasingly similar,” says Shane Blowes from the German Center for Integrative Biodiversity Research (iDiv). According to this theory, the same species always prevail in different locations, while different, more specialized creatures disappear. This phenomenon is called biotic homogenization. But how common is this development actually?

Biodiversity theory put to the test

Blowes' team has now tested this hypothesis and, for the first time, comprehensively assessed how the species richness of various ecosystems has developed over long periods of time and how individual changes in the species communities have interacted. To do this, the biologists analyzed 525 long-term data sets that were collected in various ecosystems such as savannas, meadows or coral reefs. Some of the data is up to 500 years old. “We combined a classic measure of scale-dependent changes in biodiversity with a large amount of data,” explains Blowes. The scientists determined in detail how the numbers of different species changed, both at a single site and at multiple sites within a landscape or geographic region. In this way, the researchers were able to determine whether the species composition at the individual locations had become more similar or more different.

The result: “We found that communities become more similar over time as often as they become more different in response to human influences,” says Blowes. Although the number of species increased or decreased at many locations, the spatial variation in species composition did not change in most cases. The net trends were often close to zero, as the researchers found. Over a long period of several hundred years and in larger regions, the data actually showed a weak trend towards homogenization - mainly because widespread species increased in the communities. However, this trend was offset at the local level by the differentiation of communities. This applied to all living creatures, from fish to birds to plants and mammals, as Blowes and his colleagues report.

Complex development of biodiversity requires regular reassessment

“Our analysis shows how complex the topic is,” says senior author Jonathan Chase from iDiv. “Our results do not mean that there are no serious changes due to human impact on the environment, but we must abandon the assumption that biodiversity changes primarily in the form of homogenization.” Homogenization happens, for example, when species spread and occur in more and more places. The disappearance of common species in some, but not all, places, on the other hand, leads to differentiation.

According to the biologists, whether a local species community tilts towards homogenization or differentiation depends on various factors. The deciding factor can be, for example, roads or fences that fragment the landscape, but also climate change, which forces species to migrate. The introduction of alien plants and animals can also significantly change an ecosystem. One example is the red swamp crayfish, which is native to northern Mexico and the southeastern United States. Its spread into German waters is driving homogenization. Whether a species or an ecosystem needs to be better protected can therefore change over time and must be regularly reassessed in the context of other habitats, the researchers conclude. “We are increasingly finding that many endangered species need to be protected at multiple sites or at a landscape level,” adds Blowes.

Source: Shane Blowes (German Center for Integrative Biodiversity Research Halle-Jena-Leipzig) et al., Science Advances, doi: 10.1126/sciadv.adj9395

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