Assessing Biodiversity

Assessing Biodiverstiy

Given the need to assess biodiversity at a wide range of scales, from regional build-out to site specific habitat restoration, three models of biodiversity are incorporated in the research strategy: landscape ecological pattern, single species potential habitat models, and species richness.

Landscape Ecological Pattern

The landscape ecological pattern model builds from the ongoing work of Richard Forman and Michel Godron, as presented in their 1986 book, Landscape Ecology, and elaborated in Forman's 1995 book, Land Mosaics. The focus of landscape ecology is the spatial relationships between structural and functional elements of the land. Any type of landscape at any scale, whether natural or modified by human action, can be described as a mosaic: a background matrix and patches connected by corridors. This model provides a base for analysis and comparative evaluation, plus the potential for detecting general patterns and principles. Qualifications to the generalized elements can provide evaluations of the landscape. Change in the landscape ecological pattern of a region can cause a change in the biodiversity of the area, and planning initiatives that maintain the landscape pattern may preserve biodiversity.

Single Species Potential Habitat

The single species potential habitat models map the possible home ranges for selected vertebrates based on food and nesting requirements, and on behavioral characteristics. While single species management has been criticized by wildlife biologists and planning professionals as being too narrowly focused, there are several reasons for integrating this type of modeling into a biodiversity study.

First, several species in the study area are on the federal lists of threatened and endangered species. The California gnatcatcher, least Bell's vireo, and arroyo toad are examples. Still other species are candidates for federal listing, or are listed as California Species of Special Concern. Consideration of those species is legally mandated. Some impact assessments, mitigation, or recovery management strategies clearly need to be species-specific.

Second, one species, the California cougar, is in danger of regional extinction because development and roadways are splitting the existing population into two increasingly isolated sub-populations. Without a habitat linkage, neither of these populations will be large enough to maintain genetic viability beyond the next 100 years. There are obvious species-specific planning, design, and management dimensions to this problem.

Third, some species are particularly susceptible to changes in the environment and, as such, are good indicators of environmental change associated with development. The least Bell's vireo, for example, is very sensitive to changes in hydrology, channel morphology, and riparian vegetation. In contrast, the brown-headed cowbird populations increase with suburban development.

The habitat information presented for each wildlife species has been formatted according to Standards for the Development of Habitat Suitability Index (HSI) Models of the U.S. Fish and Wildlife Service (1981). The HSI models are an outgrowth of the Habitat Evaluation Procedures (HEP)(USFWS, 1980). HEP is a widely used methodology for evaluating the various types of impacts on wildlife habitat and wildlife species associated with changes in water and land use.

Species Richness

Biologists have long used knowledge of species' life history attributes to model animal ecology. One common method is to model habitat by linking known needs and use patterns with maps of existing vegetation, thereby identifying the spatial extent of important habitat features. This information can then be used in conservation and management (see Verner, et al., 1986). For California, a complete set of wildlife habitat relation (WHR) models has been developed that links all terrestrial vertebrates to specific habitat types (Mayer, et al., 1988). By mapping the abundance, or richness, of species associated with each habitat type as derived from these relation models, it is possible to understand better the spatial implications of biodiversity in a region. The species richness approach does not focus on any particular species. Rather, it is an indicator of the properties of the set of all species associated with a pattern of vegetation.

The study region is currently an area of high biodiversity. Figure 7 shows the terrestrial vertebrate species richness of California. Each hexagon is 640 sq. km. The species richness values were made by assigning all individual species distribution maps to the hexagonal grid and then counting the number of species. Note the area of high species richness covering Camp Pendleton and its context region.