Skip common site navigation and headers
United States Environmental Protection Agency
Exposure Research
Begin Hierarchical Links EPA Home > Research & Development > Exposure Research > Publications/Presentations > End Hierarchical Links

 

Habitat Patch Occupancy By the Toads (Bufo Punctatus) in a Naturally Fragmented, Desert Landscape

spacer
spacer
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 two predictions based on metapopulation theory, i.e., the incidence of habitat patch occupancy is (1) directly related to patch size and (2) inversely related to patch isolation. In a 20,000 km2 area of the eastern Mojave Desert, 128 potential habitat patches (primarily springs) were identified and surveyed for local environmental characteristics and presence/absence of B. punctatus. Patch size metrics reflected extent of water and riparian vegetation of several types. Patch isolation metrics were based on nearest neighbor distances, calculated both as Euclidian distance and distance via connecting drainage channels. B. punctatus was found at 72% of the sites, including 13 of 14 historic (pre-1970) sites. Patches were generally quite small, e.g., median linear extent of surface water was approximately 200 m (72 m2 median area). Median nearest neighbor distances among patches were 1.8 km Euclidian distance (range: 0.4 - 22.0 km) and 6.8 km via drainage channels (range: 0.5 - 64.9 km). Based on stepwise logistic regression, incidence of patch occupancy significantly increased with patch size, including both water- and vegetation-based metrics. Patch occupancy was also significantly related to elevation, latitude, and four metrics that were associated with rocky terrain, periodic scouring water flows, and ephemeral water. In contrast, incidence of patch occupancy was not significantly related to patch isolation. 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. However, application of the patchy population model is perplexing given the maximum known dispersal distances for B. punctatus (0.8 km) and Bufo spp. (2.6 km), which are well short of many of the observed nearest neighbor distances in this study.
spacer
Citation:Bradford, D. F., A. C. Neale, M. S. Nash, D. W. Sada, and J. R. Jaeger. Habitat Patch Occupancy By the Toads (Bufo Punctatus) in a Naturally Fragmented, Desert Landscape. ECOLOGY 84(4):1012-1023, (2003).
spacer
spacer
Contact: Chris Siebert - (702) 798-2234 or siebert.christopher@epa.gov
spacer
Division: Environmental Sciences Division
spacer
Branch: Landscape Ecology Branch
spacer
Product Type: Journal
spacer
Published: 05/06/2003
spacer
Related Entries:
spacer
Bullet Item Development of Landscape Indicators for Use in Regional Ecological Risk Assessments
spacer Relationship Reason:   A Project of the Product
spacer
spacer
spacer

 

ORD Home | Search EPA | Search NERL | Search EIMS | Contacts | Help

 
Begin Site Footer

EPA Home | Privacy and Security Notice | Contact Us

Last Updated on Monday, October 22, 2007
URL: http://cfpub.epa.gov