Analysis spanning several centuries suggests that biodiversity differentiates and homogenizes to a comparable extent

According to a new study published in Scientists progress.

Led by researchers from the German Center for Integrative Biodiversity Research (iDiv) and Martin Luther University Halle-Wittenberg (MLU), the researchers analyzed 527 datasets collected from ecosystems such as grasslands, shrubs and coral reefs 500 years ago.

This analysis is the first of its kind to provide a comprehensive assessment of how local and regional changes in biodiversity combine across landscapes over centuries.

“We wanted to examine the idea that biotic homogenization is a defining feature of the biodiversity crisis,” says first author Dr Shane Blowes of iDiv and MLU.

“By combining a classic measure of scale-dependent biodiversity change with unprecedented data collection, we found that it is just as likely that communities will become more different over time in response to ongoing human impacts on the environment.”

No change means more than you think

The analysis integrated measures of change in the number of different species (species richness) present in a single site with changes occurring across multiple sites combined within a landscape or geographic region. This allowed the researchers to determine whether species composition became more similar or different from site to site.

They found no change in spatial variation in species composition, which was the most common result. This is despite many cases of homogenization and differentiation between communities over time. In fact, net trends in biodiversity change at all scales were so small that they were often indistinguishable from zero.

The data also showed a weak trend toward homogenization in larger regions over several hundred years due to increases in the number of widespread species in communities; however, this trend was offset by community differentiation at smaller scales. These results were consistent across many life forms, from fish and birds to plants and mammals.

Studies of the evolution of biodiversity paint a complex picture, and researchers are often divided on how to characterize species’ responses to human-induced changes. However, the concept of homogenization remains a pervasive expectation in the scientific community and the general public.

“Our analysis reveals the complexity of the problem,” explains Professor Jonathan Chase, lead author of the study and professor at iDiv and MLU. “This doesn’t mean that serious changes aren’t happening in the world, it means that we need to move beyond the belief that homogenization is the primary way that biodiversity evolves.”

Models for multi-scale conservation

The researchers also present scenarios in their study that characterize changes in species occupancy: homogenization is largely driven by increases in average occupancy, while differentiation is driven by decreases in average occupancy.

The scenarios clearly show that even small changes in occupancy, due to fragmentation of the landscape by roads or artificial fences, the introduction of plants and animals into areas beyond their historical boundaries and to climate change, which can push species to shift their range, can tilt a community towards homogenization or differentiation.

The bright red Louisiana crayfish (Procambarus clarkii), native to northern Mexico and the southeastern United States, is an example of an introduced species that thrives in German freshwaters, thereby promoting homogenization.

“We are increasingly seeing that many threatened species require multi-site or landscape-scale protection to be effective,” adds Blowes.

“A simple framework can help biodiversity management and conservation evolve toward the multi-scale approach needed to fully understand biodiversity changes.”