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.

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 (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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