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Influence of Spatial Distribution of Habitat on the Distribution and Genetic Interchange Among Amphibian Populations in a Naturally Fragmented Landscape, the Mojave Desert
RESEARCH SUMMARY
Principal Investigators: David Bradford1, Anne Neale1, Maliha Nash1, Jef Jaeger2, Brett Riddle2, Donald Sada and Randy Jennings4
- U.S. EPA, Office of Research and Development, Environmental Sciences
Division, Landscape Ecology Branch, Las Vegas, NV
- University of Nevada Las Vegas, Department of Biological Sciences,
Las Vegas, NV
- Desert Research Institute, Reno, NV
- Western New Mexico University, Department of Natural Sciences, Silver City, NM
Amphibians are common denizens of aquatic habitats throughout the arid West, and changes in amphibian populations or communities may reflect ecosystem changes in either the aquatic or terrestrial environment. However, like many organisms, amphibians are often thought to have a metapopulation structure (i.e., groups of individuals inhabiting a system of habitat patches connected by migration), with large fluctuations in numbers and presence/absence of the species at individual sites. Consequently, monitoring metapopulations, rather than individual populations, may provide greater sensitivity in assessing the long-term prospects for a species’ persistence in an area. Monitoring metapopulations may present simplifications as well as challenges. For example, the determination of a species’ presence/absence (patch occupancy), rather than population size, may be adequate to detect changes in status of a metapopulation in an area, but knowledge will be needed for relationships between such factors as patch isolation, size, and quality relative to patch occupancy.
We are investigating such relationships for a common toad (red-spotted toad, Bufo punctatus) inhabiting small wetland patches in the Mojave Desert that are highly dispersed due to climatic drying. One hundred fifty-four potential habitat patches (primarily springs in rocky areas) have been identified and surveyed for habitat characteristics and occupancy by the species. Metrics for patch isolation are based on inter-site distances via networks of connecting drainage channels, and metrics for patch size are based on extent of water and riparian vegetation. Our results support the prediction from metapopulation theory that patch occupancy is directly related to patch size, but analyses do not support the prediction that patch occupancy is related to patch isolation.
We are also investigating genetic similarities among populations of the red-spotted toad and the relict leopard frog (Rana onca), a species once thought to be extinct. Results of these studies will provide an understanding of the landscape attributes (e.g., distance along connecting drainage channels or distance between habitat patches within a mountain range) that allow dispersal and consequently genetic interchange among populations.
Jaeger, J.R., B.R. Riddle, R.D. Jennings, and D.F. Bradford. 2001. Rediscovering Rana onca: Evidence for phylogenetically distinct leopard frogs from the border region of Nevada, Utah, and Arizona. Copeia 2001: 339-354.
Bradford, D.F., A.C. Neale, M.S. Nash, D.W. Sada, and J.R. Jaeger. 2003. Habitat patch occupancy by toads (Bufo punctatus) in a naturally fragmented desert landscape. Ecology 84: 1012-1023.
Bradford, D.F., J.R. Jaeger, and R.D. Jennings. 2004. Population status and distribution of a decimated amphibian, the relict leopard frog (Rana onca). Southwestern Naturalist 49: 218-228.
Bradford, D.F. 2004. Amphibian declines and environmental change in the
eastern Mojave Desert. In: Sada, D.W., and S.E. Sharpe (eds.).
Conference Proceedings, Spring-fed Wetlands: Important Scientific and
Cultural Resources of the Intermountain Region, May 7-9, 2003, Las Vegas,
NV. DHS publication no. 41210. Desert
Research Institute (http://www.wetlands.dri.edu) 
.
Bradford, D.F., R.D. Jennings, and J.R. Jaeger. Rana onca Cope 1875, relict leopard frog. Pp. 000-000 in M.J. Lannoo (ed.), Declining Amphibians: A United States' Response to the Global Problem, University of California Press, Berkeley. In Press.
Bradford, D.F., J.R. Jaeger, and S.A. Shanahan. Distributional Changes and Population Status of Amphibians in the Eastern Mojave Desert. Western North American Naturalist. In press.
Jaeger, J.R., B.R. Riddle, and D.F. Bradford. Cryptic Neogene vicariance and Quaternary dispersal of the red-spotted toad (Bufo punctatus): insights on the evolution of North American warm desert biotas. Submitted to Molecular Ecology.