Habitat fragmentation remains one of the biggest current threats to biodiversity. One major concern associated with habitat fragmentation is edge effects, i.e. the ways that individual movement and behavior can be altered near the boundaries of fragmented patches. Although habitat fragmentation and edge effects are highly studied, there is as yet no one predictable global pattern of how species respond to fragmentation.
One possible explanation for this disparity lies in the extinction filter hypothesis, which suggests that species evolutionary histories shape their ability to respond to new stressors. For example, it would predict that those species that have historically been exposed to high-disturbance environments are better adapted to modern disturbances such as fragmentation, either because they evolved to tolerate disturbance better or because those species who were poorly adapted to disturbance went extinct.
A new study tested the theory that past exposure to disturbance shapes current response to fragmentation for species in forest ecosystems. Using data from BIOFRAG, over 35 regions across the globe were sampled within 73 data sets, resulting in information such as edge influence across the region and edge sensitivity of over 4,000 species. This data was combined with global disturbance data, such as forest fire severity, historical glacial impact, the presence of tropical storms, and historical anthropogenic forest loss.
The results showed strong support across all species for the extinction filter hypothesis, meaning that the geographic variability in how species responded to fragmentation (specifically, their sensitivity to forest edge) was well-explained by the historical disturbance regime of the region where they lived. Regions with historically severe disturbance had 79% lower odds of supporting species that were restricted to the forest core and avoided edges. In low-disturbance regions, 51% of species tended to avoid edges, compared to only 18% of species in high-disturbance regions. In addition, the edge sensitivity of forest core species tended to be higher in low-disturbance regions than high-disturbance regions.
This difference between high- and low-disturbance regions leads to a latitudinal gradient in fragmentation sensitivity. Across the globe, high-latitude areas historically experienced more disturbance, leading them to support more resilient species. In contrast, low-latitude areas have a six-fold increase in species’ sensitivity to fragmentation. It’s worth noting that the forests in low-latitude regions are mostly tropical, where the rate of habitat fragmentation is expected to accelerate in the future.
These results indicate that conservation efforts to combat fragmentation should not be standardized, but rather tailored according to species’ sensitivity to edges. In high-latitude regions, where species have less sensitivity, efforts could focus more on preserving mature forests and less on spatial configuration. In contrast, low-latitude regions where species are less resilient need a more concentrated effort on reducing fragmentation and its associated edges to reduce the risk of extinction.
Betts, M.G., et al. 2019. Extinction filters mediate the global effects of habitat fragmentation on animals. Science 366(6470): 1236-1239.
Balmford, A. 1996. Extinction filters and current resilience: the significance of past selection pressures for conservation biology. Trends in Ecology & Evolution 11(5): 193-196.