September 2002

From North Carolina State University

Wildlife corridors shown effective

Animals, plants derive benefits

In one of the first large-scale studies of wildlife corridors - thin strips of habitat that connect isolated patches of habitat - Dr. Nick Haddad, assistant professor of zoology at North Carolina State University, and a team of researchers from across the country have found that a number of plant and animal species derive great benefits from corridors.

The findings suggest that corridors helped increase animal movement rates between patches in the study area. Further, plant pollination and seed dispersal rates, which are other important characteristics of healthy ecosystems, were also larger between patches connected by corridors than isolated patches of habitat.

An aerial photograph of one of the eight sites shows the connected patches as well as three isolated patches of habitat.

The study will be published in an online version of Proceedings of the National Academy of Sciences the week of Monday, Sept. 16. The print edition will be published Oct. 1.

"Our findings suggest that corridors can give plants and animals access to greater areas. Higher movement can help prevent loss of populations, and ultimately biodiversity," Haddad says. "This study looks at not just the effects of corridors on movement rates, but how these movement rates are mediated by interactions between plants and animals."

Common sense, and years of academic theory, say corridors are pathways that give plants and animals the opportunity to survive and thrive in habitats increasingly encroached upon by urban sprawl and other land-use changes. If animals are able to move more freely, the theory postulates, they're more likely to not only move to wider areas, but also increase their interactions with plants. Haddad and his colleagues set out to scientifically prove this theory in large areas.

"Until recently, there's been no scientific evidence that corridors promote movement," Haddad says. "For corridors to work, they have to increase the movement of plants and animals."

The researchers tested their corridors at the Savannah River Site National Environmental Research Park, a federally protected area on the South Carolina-Georgia border that is mostly dominated by pine tree forests. On the researchers' request, the U.S. Forest Service had its workers arrange eight similar sites; each site included five areas cleared of trees and groundcover. The central patch was connected to one other patch by a 150-meter-long, 25-meter-wide corridor, while three other patches were isolated from the central patch - and themselves - by forest.

The researchers then conducted separate tests to determine the effectiveness of corridors by examining the movement of two different species of butterflies, the movement of pollen from a holly plant, and seed dispersal from a holly plant species and a wax myrtle plant species.

After marking and then recapturing butterflies, the researchers found that butterflies moved between the connected patches - that is, the patches with corridors - at higher rates than between isolated patches.

To study pollen movement, the researchers planted male deciduous holly plants in the central patch and female holly plants in the connected and isolated patches. Since this area had no other holly pollen source, the only way for fruit to grow was via pollination from male holly in the central patch.

The researchers found more fruit in the connected patches than in unconnected patches.

"Butterflies are among the pollinators of this holly species," Haddad explains. "Butterflies move more frequently between connected patches, while those hollies that are pollinated by butterflies and other insects are more likely to be pollinated and bear fruit in connected patches than in unconnected patches."

Finally, the researchers tested seed dispersal by examining fecal samples from birds that ate seeds from the center patch that were marked with a fluorescent dye powder. The examination of bird droppings showed that the holly and wax myrtle seed was dispersed in higher rates to the connected patch than to isolated patches.

"Not only have we shown that corridors affect movement rates, but we've also shown how plants and animals interact in these landscapes," Haddad says.

To gauge whether more movement in connected patches might be caused by larger area, researchers used a control mechanism to study this effect. They added the area of the corridor itself to the isolated patches. According to Haddad, "The corridor was more beneficial above and beyond the benefits of just making a habitat area bigger."

Note to editors: An abstract of the Proceedings of the National Academy of Sciences paper follows.

"Corridors affect plants, animals, and their interactions in fragmented landscapes"

Authors: Douglas J Levey, Joshua J. Tewksbury and Patricia Townsend, University of Florida; Nick Haddad, Aimee Weldon, Jory Brinkerhoff and Ellen I. Damschen, North Carolina State University; Sarah Sargent, Alleghany College; John L. Orrock and Brent J. Danielson, Iowa State University

Published: Oct. 1, 2002, in Proceedings of the National Academy of Sciences

Abstract: Among the most popular strategies for maintaining populations of both plants and animals in fragmented landscapes is to connect isolated patches with thin strips of habitat, called corridors. Corridors are thought to increase the exchange of individuals between habitat patches, promoting genetic exchange and reducing population fluctuations. Empirical studies addressing the effects of corridors have either been small in scale or have ignored confounding effects of increased habitat area created by the presence of a corridor. These methodological difficulties, coupled with a paucity of studies examining the effects of corridors of plants and plant-animal interactions, have sparked debate over the purported value of corridors in conservation planning. We report results of a large-scale experiment that directly address this debate. In eight large-scale experimental landscapes that control for patch area and test alternative mechanisms of corridor function, we demonstrate that corridors not only increase the exchange of animals between patches, but also facilitate two key plant-animal interactions: pollination and seed dispersal. Our results show that the beneficial effects of corridors extend beyond the area they add, and suggest that increased plant and animal movement through corridors will have positive impacts on plant populations and community interactions in fragmented landscapes.

Additional Media Mick Kulikowski,
919-515-3470 or
mick_kulikowski@ncsu.edu











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