One of the primary goals of utilizing corridors in conservation reserve design is to prevent species extinctions and promote their colonization among habitat patches. Yet, determining whether this theoretical prediction holds true for communities of species in “real” landscapes and over long time scales has remained challenging.
Most landscape studies of habitat loss and fragmentation occur on landscapes where changes in habitat area, isolation, and structure (e.g., edge effects) occur simultaneously, making it impossible to isolate their independent effects. Experimental tests are needed to separate the role of each of these impacts, but doing so at large spatial scales for entire ecosystems requires resources, coordination, and cooperation of many partners.
For the past twenty years, we have had the opportunity to measure the impacts of connectivity on plant community diversity using a large-scale, whole-ecosystem experiment to test for the impacts of habitat connectivity while controlling for area and edge effects. This has been possible because of a strong, long-term collaboration with the U.S. Forest Service and the U.S. Department of Energy at the Savannah River Site in South Carolina to create and maintain controlled and replicated experimental landscapes that test if and how corridors work.
At the Savannah River Site, eight experimental landscapes each contain a center 100x100m (1 ha) patch of open longleaf pine habitat that is either connected or unconnected to a peripheral 100x100m patch by a 150m long by 25m wide corridor. Unconnected patches are either “Rectangular”, adding the area of the corridor onto a 1ha patch so that they are the same area as the 1ha patch plus the corridor. “Winged” patches are also the same area and have the same amount of “edge habitat” as a connected patch. By comparing connected patches to unconnected Rectangular and Winged patches, it is possible to disentangle the role of connectivity per se from area and patch shape (respectively). These landscapes were created in the winter of 1999-2000 and each year since then, we have recorded each of the plant species that we have found in each patch.
This fall, we reported in the journal Science how corridors have affected colonization and extinction rates for 239 species over 18 years. Connecting habitat increased the number of plant species over time. Like compound interest in a bank, the number of species has increased at a constant rate each year (~5%), resulting in many more species in habitat patches connected by a corridor than in those that are isolated. After almost two decades, there are 24 more species in habitats connected by a corridor than in unconnected patches.
We were very surprised by the continual positive impact of corridors over the entire 18 years. We expected that the benefits might plateau, or even decrease, but we did not expect them to continue to accrue. After nearly 20 years, the positive impact of corridors on biodiversity has not slowed down.
Our results come from a threatened North American biodiversity hotspot, longleaf pine savanna, that is home to numerous threatened and endangered species such as gopher tortoises and red-cockaded woodpeckers. Grassland and savanna ecosystems like this one are experiencing unprecedented rates of species declines, thought to be largely due to habitat loss and fragmentation, altered disturbance regimes, and climate change.
Conservation investments have tradeoffs, and our study suggests that the time to start connecting habitat is now, as it will pay off down the road (like investing money in the bank with an annual, positive interest rate). Our results indicate that restoring connectivity through conservation corridors can prevent species losses and facilitate community recovery.
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