|Title||A multi-scale conservation assessment of plant communities in Southern California|
|Year of Publication||1995|
|University||University of California|
Landscape ecology and conservation biology are integrally tied with geographic analysis and an understanding of the spatial relationships of the biological and physical components of the natural world. This dissertation examines the Southern California ecoregion at different resolutions with the purpose of assessing the conservation status and needs of this area's natural vegetation communities. The first section presents, in summary form, a regional assessment of the conservation status of terrestrial plant communities in Southwestern California using the Gap Analysis method. The practical constraints imposed by the size of the study area (34,000 km2), combined with the coarse-scale overview provided by Gap coverages, dictate a relatively large minimum mapping unit (100 hectares) and fairly general classification (76 dominant species) for the vegetation GIS database employed in this study. Land ownership/management profiles provide a crude measure of major terrestrial communities at risk nw or in the near-term future. Results of an analysis of vegetation communities and land ownership/management profiles confirm current concern for some communities currently regarded as at risk and provide an indication of communities that could be ten or twenty years from now. Communities potentially at risk in the future include all grassland dominated communities, many of the oak woodland communities, and some of the chaparral communities found at lower elevations are included. The second main chapter examines the comparative utility of vegetation maps, prepared with different spatial resolutions. This is accomplished through comparison of the Gap data for western San Diego County with two other data sets that had mapped vegetation at approximately 100 times greater spatial resolution (mimimum mapping unit of roughly one hectare). Overlay of the three different representations shows patterns of disagreement that arise from many different sources. Despite these apparent disagreements the Gap data set captures over 90% of the larger stands of vegetation that it is intended to find. However, finer grain representation of landscape features do not necessarily nest directly inside the coarse-grain representation. The third main chapter describes a GIS model developed to provide a mapped-based characterization of potential long-term conservation value of coastal sage scrub habitat. The model developed is based on application of the Natural Communities Conservation Planning program Conservation Guidelines. The criteria presented in these guidelines are spatial in nature and were well-suited to the development of the GIS model. The larger patches of habitat in the study area were examined for how they are shaped and connected to one and other. Investigations into potential landscape linkages (using GIS modeling techniques) show some locations still have a considerable number of viable options for biologically sound linkages, other are already so fragmented that the best opportunities for landscape linkages are long, narrow, and fragmented. The spatial/geometric criteria used in the model (e.g. patch size, adjacency, linkage) provide important insights to regional reserve design efforts. Although they are general in nature, they are reasonably simple to apply and most conservation biologists agree they will provide robust results. This kind of model needs to be tested with random transect data on the distribution and abundance of the target species to determine relative value of higher potential habitats, as rated by the model, in relation to habitats actually supporting viable populations. Field testing of such a model was not accomplished as part of this dissertation and is recommended for future research. The major conclusions reached in this dissertation include 1) Gap Analysis is an acceptable and important component of ecological analysis over the domain of an ecoregion; although more finely resolved analysis can identify areas outside of those represented in Gap, these areas are in general not crucial to regional reserve design requirements, and 3) simple, spatially defined GIS models offer a means of prioritizing lands under consideration for a regional reserve system. Each of the chapters deals with, in some substative way, the relationship between the scale of investigation (i.e. what and how we measure features on the landscape) and what our subsequent analyses can tell us (i.e. interpretation of what we measure). It is important for us to review and understand the assumptions we must make when we use map (i.e. GIS) data to represent the multitude of landscape features.