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Scientists have long realized that urbanization is likely a major contributor to the disappearance of amphibians, but a new study by Los Angeles-area biologists indicates that even minimal alterations in watersheds create problems for native stream amphibians. The study found that increasing numbers of coastal watersheds in southern California, even in such protected areas as national and State parks, are being altered in ways that make them unsuitable for frogs and salamanders native to this region.
Thirty-five Los Angeles-area streams were intensively surveyed in a 3-year study conducted by biologists from the National Park Service, Pepperdine University, the U.S. Geological Survey (USGS), and the Resource Conservation District of the Santa Monica Mountains. The results of their study have been published online and in the December issue of the scientific journal Conservation Biology (see URL http://www.blackwell-synergy.com/doi/abs/10.1111/j.1523-1739.2005.00295.x). The watersheds surveyed ranged from those in national or State parks with no development to those where as much as 37 percent of the watershed area is urbanized, including roads and commercial, industrial, and residential areas.
Previous studies in other parts of the country have indicated that when 15 to 20 percent of a watershed is developed or urbanized, stream organisms begin to suffer. This study, however, reveals that as little as 8-percent urbanization results in habitat changes that make streams unsuitable for native amphibians. Urban streams were commonly observed to have long uniform stretches of deeper water and to lack the variety of deep pools and shallow riffles needed to support native biodiversity. "The more urbanized streams had been transformed from babbling brooks to sand- and mud-filled trenches that were missing native species but full of nonnative invaders," said Seth Riley, a National Park Service wildlife ecologist and lead author of the study.
According to the study, the urban streams were missing such native amphibians as California newts and California treefrogs but were rife with introduced invasive species, including crayfish, bass, bluegill, and bullfrogs. Although southern California streams commonly dry up in the late summer and in dry years, the urban streams examined in the recent study continued flowing year-round, likely as a result of increased runoff and water inputs. The urban-related changes facilitate invasion by nonnative animals that compete with or prey upon native amphibians.
"We hope that these new results will provide valuable guidance for land managers and policymakers throughout southern California," said Lee Kats, a professor of biology at Pepperdine University and one of the study coauthors. "Amphibians represent a fragile group of animals in southern California and elsewhere, and this study suggests that even minimal urbanization can contribute to their disappearance." Ray Sauvajot, Chief of Planning, Science and Resource Management with the National Park Service's Santa Monica Mountains National Recreation Area, agreed: "Identifying factors that impact sensitive amphibians will help park managers maintain pristine stream conditions where they exist and restore or improve habitat conditions in areas affected by urbanization."
This study will help researchers and land managers further understand the causes of amphibian declines nationally, said Robert Fisher, a research biologist with the USGS and study coauthor. "We continue to discover that these declines are not driven by single 'smoking gun' causes but instead are the result of synergistic effects of multiple stressors in the environments inhabited by amphibians."
This work was funded in part by the USGS Amphibian Research and Monitoring Initiative (ARMI), initiated in 2000 to investigate the status and trends in the Nation's amphibians and study the causes of their declines (for more information, visit the Amphibian Research and Monitoring Initiative Web site).
in this issue:
Study Links Urbanization to Amphibian Decline
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