Isolated Forest Patches Lose Species, Diversity

A new study reveals that decades of fragmentation of Wisconsin's forests have taken a largely unseen toll on the sustainability of these natural ecosystems. The long generation times of trees and other plants have masked many of the ecological changes already under way in the patches of forest that remain, says study co-author Don Waller, a professor in the Department of Botany and Nelson Institute for Environmental Studies at the University of Wisconsin-Madison. "Things may look healthy, but over time we see an erosion of biodiversity," he says. To better catalog the changes at work, he and colleagues looked beyond the trees to the forest understory — the shrubs, grasses, and herbs covering the forest floor — to witness how Wisconsin's forests are really faring. Their results, published online June 8 and appearing in an upcoming issue of the journal Conservation Biology, show that fragmentation is reducing the abundance and diversity of native plants in southern Wisconsin forests.

Though negative effects of fragmentation on biodiversity have gone largely unrecognized in the past, the impacts appear to be intensifying over time, the researchers say. Nearby cities and towns now strongly affect local woodlots, causing smaller plots in particular to lose species.

"When we isolated these forest patches 50 or 100 years ago, we were dooming species to extinction," says Waller. "It may not happen right away — and in that sense it's an 'extinction debt' — but it will accumulate over time."

They believe that isolation takes its biggest toll on forest plant diversity by cutting off routes for native plants to re-colonize areas where local populations may have disappeared. "Plant species might go locally extinct for lots of different reasons," including natural cycles of turnover, Waller says. "But typically the area will be re-colonized very soon by nearby populations of the same species. That's what does not happen once a habitat becomes isolated or that patch becomes smaller."

People are a really important part of the system, says David Rogers, an assistant professor of biological sciences at UW-Parkside who led the study while a UW-Madison graduate student. "We are having a greater influence over our local ecology, whether we want to or not. That puts the responsibility on people to take care of it, protect it and maintain it."