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HABITAT PATCH OCCUPANCY BY THE TOADS (BUFO PUNCTATUS) IN A NATURALLY FRAGMENTED, DESERT LANDSCAPE
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).
Impact/Purpose:
The primary objectives of this research are to:
Develop methodologies so that landscape indicator values generated from different sensors on different dates (but in the same areas) are comparable; differences in metric values result from landscape changes and not differences in the sensors;
Quantify relationships between landscape metrics generated from wall-to-wall spatial data and (1) specific parameters related to water resource conditions in different environmental settings across the US, including but not limited to nutrients, sediment, and benthic communities, and (2) multi-species habitat suitability;
Develop and validate multivariate models based on quantification studies;
Develop GIS/model assessment protocols and tools to characterize risk of nutrient and sediment TMDL exceedence;
Complete an initial draft (potentially web based) of a national landscape condition assessment.
This research directly supports long-term goals established in ORDs multiyear plans related to GPRA Goal 2 (Water) and GPRA Goal 4 (Healthy Communities and Ecosystems), although funding for this task comes from Goal 4. Relative to the GRPA Goal 2 multiyear plan, this research is intended to "provide tools to assess and diagnose impairment in aquatic systems and the sources of associated stressors." Relative to the Goal 4 Multiyear Plan this research is intended to (1) provide states and tribes with an ability to assess the condition of waterbodies in a scientifically defensible and representative way, while allowing for aggregation and assessment of trends at multiple scales, (2) assist Federal, State and Local managers in diagnosing the probable cause and forecasting future conditions in a scientifically defensible manner to protect and restore ecosystems, and (3) provide Federal, State and Local managers with a scientifically defensible way to assess current and future ecological conditions, and probable causes of impairments, and a way to evaluate alternative future management scenarios.
Description:
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.