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Local Scale Factors Determine Habitat Patch Occupancy By Red-Spotted Toads in a Naturally Fragmented Desert Landscape

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Abstract:Amphibians are often thought to have a metapopulation structure, which may render them vulnerable to habitat fragmentation. The red-spotted toad (Bufo punctatus) in the southwestern USA and Mexico commonly inhabits wetlands that have become much smaller and fewer since the late Pleistocene. This study tests (1) the prediction based on metapopulation theory that habitat patch occupancy is inversely related to patch isolation, and (2) a potentially competing hypothesis that patch occupancy is influenced by local environmental conditions. In a 20,000 km2 area of the eastern Mojave Desert, 128 potential habitat patches (primarily springs) were surveyed for presence/absence of toads and for local environmental characteristics at two spatial scales (breeding pool and habitat patch). Patch isolation metrics, representing the region scale, were based on nearest-neighbor distances calculated both as Euclidian distance and distance via connecting drainage channels. Based on stepwise multiple logistic regression, incidence of patch occupancy was significantly inversely related to isolation, as predicted, when only patch isolation and other region-scale metrics were included as independent variables. When metrics for the two local scales were also included, however, the incidence of patch occupancy was no longer significantly related to patch isolation but was significantly related to patch size, elevation, and four metrics that were associated with rocky terrain, periodic scouring water flows, and ephemeral water. These findings are consistent with a "patchy population" model, rather than the classical equilibrium metapopulation model, implying frequent dispersal among patches and virtually no local extinctions. The influence of local-scale environmental characteristics on patch occupancy demonstrates the importance of including patch quality metrics in tests of predictions for patch occupancy based on metapopulation theory .
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Citation:Bradford, D. F. Local Scale Factors Determine Habitat Patch Occupancy By Red-Spotted Toads in a Naturally Fragmented Desert Landscape. Presented at The Wildlife Society Annual Meeting, Calgary, CN, September 18-22, 2004.
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Contact: Chris Siebert - (702) 798-2234 or siebert.christopher@epa.gov
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Division: Environmental Sciences Division
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Branch: Landscape Ecology Branch
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Product Type: Abstrct/Oral
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Presented: 09/18/2004
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Related Entries:
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Bullet Item Development of Landscape Indicators for Use in Regional Ecological Risk Assessments
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Last Updated on Monday, October 22, 2007
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