Thesis

Landscape pattern a determinate of coastal California Gnatcatcher (Polioptila californica californica) occupation

The coastal California Gnatcatcher (Polioptila californica californica) is a federally threatened species affected by the loss of coastal sage scrub (CSS) habitat due to development. CSS has been identified as the optimal habitat for California Gnatcatchers, but they do not occupy all areas of suitable CSS habitat within the broad outlines of their geographic distribution (Atwood 1993, Campbell et al 1998). The Santa Monica Mountains National Recreation Area (SMMNRA), located in Los Angeles and Ventura Co. California, is an example of an area that contains CSS within the gnatcatcher's range, but does not have gnatcatchers. To investigate the extent to which landscape patterns determine occupancy and the potential of the SMMNRA to support California gnatcatchers, landscape patterns of a study area within the SMMNRA were compared with those of landscapes that contain gnatcatcher habitat and occurrence and landscapes that contain gnatcatcher habitat but are lacking in gnatcatchers. First, a habitat suitability model was used to identify CSS habitat within the study area that meet gnatcatcher suitability criteria. Then a set of landscape metrics were applied to the study area to measure the spatial composition of gnatcatcher habitat and surrounding land cover. In the SMMNRA 29% of the total land area was found to be suitable habitat for the California gnatcatcher. The gnatcatcher occupied (GC) and gnatcatcher unoccupied (NGC) landscapes were different for thirteen landscape metrics. Similarities were observed between GC and NGC sites for the other metrics. Of the thirteen metrics where differences were observed between GC and NGC sites the STUDY site in the SMMNRA was more similar to NGC values. The landscape patterns of the STUDY area in the SMMNRA were dissimilar enough to GC sites to suggest that future occupancy of California Gnatcatcher in the SMMNRA is unlikely. Landscape patterns may explain why gnatcatchers are not found in the SMMNRA. However, other sites within the SMMNRA should be explored for suitability for gnatcatchers. Alternatively more GC and NGC sites should be added to the analysis to refine the results.

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