They found that tree plantations had substantially less bird population diversity than did native forests and exurban areas. In some cases, exurban areas had more diversity than did the native forests.

"These findings suggest that urban sprawl is not all bad for wildlife," Haskell says. "This turns conventional wisdom about wildlife conservation on its head."

For years scientists have been concerned with the loss of biodiversity resulting from worldwide deforestation. Governments and private organizations have implemented conservation programs that discourage sprawl and promote tree plantations to replace deforested areas.

"Scientists had assumed that tree plantations were preferable to exurban areas for wildlife conservation," Haskell says. "This study firmly refutes this assumption, and has important implications for government policies, many of which subsidize plantations and penalize sprawl in the name of wildlife conservation."

For estimates of forest cover, the U.S. government classifies forest converted to tree plantation as "no loss of forest", and classifies wooded areas where houses have been built as "loss of forest."

"Yet our data show that plantations have much lower levels of biodiversity than do native forests and that exurban areas can retain much of the biodiversity of native forests," the researchers write. "Therefore, current methods of accounting for forests give potentially misleading results for biodiversity analyses."

Haskell says that tree plantations have nearly doubled in acreage in the U.S. over the last 15 years to nearly 45 million acres, due in large part to government policies encouraging such land use.

The researchers believe that extensive chemical and mechanical land clearing techniques used to prepare land for tree plantation, along with the fact that most plantations contain only a single type of tree, result in poor nesting habitats for many types of birds. Exurban areas on the other hand have a mix of forest, ornamental shrubbery, lawns and other structures that provide diverse nesting opportunities for a wide variety of bird species.

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