Large and connected forests are better for harboring biodiversity than fragmented landscapes, according to research supported by Michigan State University.
Ecologists agree that habitat loss reduces biodiversity. But they don't agree whether it's better to focus on preserving many smaller, fragmented tracts of land or fewer larger and more continuous landscapes.
The study, published in Nature and conducted by researchers from Michigan State University, University of Michigan and the German Centre for Integrative Biodiversity Research among others, examined 4,006 species of vertebrates, invertebrates and plants sampled at 37 sites around the world to provide a global synthesis comparing biodiversity differences between continuous and fragmented landscapes.
"This paper resolves a half-century old debate about how to conserve biodiversity in natural areas, one started by scientific luminaries including E.O. Wilson and Jared Diamond," said coauthor Nick Haddad , MSU College of Natural Science professor of integrated biology in the Ecology, Evolution and Behavior Program. "Conservation of more habitat will increase biodiversity. But that is not sufficient. The land must be conserved in larger parcels that are interconnected."
The researchers found that on average, fragmented landscapes had 13.6% fewer species at the patch scale, and 12.1% fewer species at the landscape scale.
Additionally, the findings suggest that primarily generalist species—species that are good at surviving in various environments—live in the fragmented areas.
The scientists investigated what's called alpha, beta and gamma diversity at these sites. Alpha diversity refers to the number of species in a patch, while beta diversity refers to how species composition differs between two areas. Gamma diversity refers to biodiversity over a whole landscape.
Think of driving through Ohio's farm fields and encountering patches of forests between fields, said co-author Nate Sanders, a professor of Ecology and Evolutionary Biology at the University of Michigan. Each patch of forest might contain a handful of bird species (alpha diversity), but each patch of forest will have different species of birds compared to the previous patch (beta diversity). The biodiversity of the entire landscape containing the fragmented patches—or likewise a continuous forest—is the area's gamma diversity.
"The heart of the debate is that people who argue that fragmentation isn't so bad say that because you have isolated habitats, you have different species composition, which means at a large scale, it's good: if they are different, we can assume that the gamma diversity is going to be higher," said Thiago Gonçalves-Souza, a research scientist for the the U-M Department of Ecology and Evolutionary Biology and lead author of the paper. "They say the opposite for large tracts of land: because this is a continuous and homogeneous patch, the species composition is too similar."
But previous research didn't properly compare fragmented landscapes to large, continuous forests, Gonçalves-Souza said. For example, prior research may have looked at only one component of diversity, or may have compared a few continuous forests to dozens of fragmented patches.
"One reason that this has been such a long-standing and unresolved debate is that we simply have not had the appropriate data and statistical tools to systematically evaluate the question at both smaller and larger scales," said coauthor Jonathan Chase , a professor with the German Centre for Integrative Biodiversity Research.
Haddad, Gonçalves-Souza and colleagues instead constructed an analysis that corrected for differences in sampling across different landscapes. The group discovered that fragmentation decreased the number of species across all taxonomic groups, but that the increase in beta diversity in fragmented landscapes did not compensate for species diversity loss at the landscape level.
"These results will sharpen focus in conservation on preventing both habitat loss and habitat fragmentation," Haddad said.
The researchers hope the study can move the conservation community past the debate over continuous versus fragmented landscape and focus on restoration of forests. This study resulted from a global collaboration of ecologists.
By Morgan Sherburne, University of Michigan, with contributions by Bethany Mauger, Michigan State University
