In two earlier papers, the researchers concluded that corridors encourage the movement of plants and animals across the fragmented landscapes. They also found that bluebirds transfer more berry seeds in their droppings between connected habitats, suggesting that the corridors could help plants spread.
The latest research tackled a much broader question: Do corridors increase plant biodiversity overall? To get at the issue, researchers Ellen Damschen and Nick Haddad, of North Carolina State University, did a detailed census of evenly distributed plots in six sets of connected and unconnected patches. They started in summer 2000 and returned every year through 2005 except for 2004, when a fire burned the landscape.
The site was set up in 1999, when forest service loggers carved out the plots, and there was little difference among plot covers just one year later in 2000. But a different pattern became clear in ensuing years. Not only were there more plant species in connected plots than unconnected ones, there were more native species.
"They started with the same diversity and then diverged," Levey said. "Native species definitely benefited, and yet there was absolutely no evidence that exotic species benefited."
The difference arose because unconnected patches gradually lost native species, whereas the natives persisted in connected patches. Over the five years, the unconnected patches lost about 10 native species. Meanwhile, the corridors seemed to have no impact on the number of exotic or invasive species in the connected and unconnected patches.