• The Yellow-bellied sapsucker, as currently classified, does not occur in California. Populations formerly assigned to Sphyrapicus varius are now called S. nuchalis, the Red-Naped Sapsucker.
• The Western flycatcher (Empidonax difficilis) has been split into two distinct species, the Pacific-slope flycatcher (which retains the original scientific name) and the Cordilleran flycatcher (E. occidentalis). Because the two species occupy distinct geographic ranges in California, we were able to associate the portions of the overall range with the appropriate species and predict their habitats in this gap analysis. E. occidentalis was assigned a new WHR code for this project (B990).
• A similar situation occurred with the Black-tailed gnatcatcher. Polioptila melanura was split into a desert-dwelling Black-tailed species which retained the scientific name, and the California gnatcatcher (P. californica) in coastal sage habitats. The latter was assigned a WHR code of B991, and the two species were modeled independently in this analysis.
• Shrews along the west coast have been revised. The Pacific shrew (Sorex pacificus) no longer occurs in California. The populations formerly called S. pacificus are now assigned to S. sonomae, the Fog Shrew.
• The Pacific giant salamander (Dicamptodon ensatus) was recently split into three species, two of which occur in California (D. ensatus and D. tenebrosus). We were unable to distinguish the individual ranges of these two species and chose to leave these two species as one for this analysis. Conclusions about the management status of either species may not be reflected accurately in the results for the combination of the two.
• The Olympic salamander was split into two species. Currently, Rhyacotrition olympicus does not occur in California, but the range is occupied by R. variegatus, the Southern Torrent Salamander.
• The slender salamanders are undergoing major revisions in taxonomy. Because of the volatility of the nomenclature and uncertainties about the ranges of new species, we kept the California slender salamander (Batrachoseps attenuatus) and the Pacific slender salamander (B. pacificus) as portrayed in the California Wildlife Habitat Relationships System.
• The Spotted frog (Rana pretiosa) may not have occurred in California as specimens may have been mis-identified. We have retained this species in the predictive modeling, but readers are warned that this may be incorrect.
• The Black-collared lizard (Crotaphytus insularis) has been split into C. insularis and C. bicinctores, the Mojave black-collared lizard. We were unable to distinguish their individual ranges from the CWHR range maps and have modeled them as a single species.
• Subspecies of the Western aquatic garter snake (Thamnophis couchii) were recently elevated to full species status. Because range data is not yet provided in CWHR for the Santa Cruz garter snake (T. atratus) or the Giant garter snake (T. gigas), our modeling and analysis are for T. couchii as originally classified.

Results

Predicted Species Distributions

In this section, we present a sample of representative maps of predicted species distributions showing the relative suitability rank of each habitat polygon. (The full set of distributions can be viewed in the CD-ROM and GIS database products.) The complete table of total mapped distribution (suitability ranks 4 and 5 only) and percent area of the entire state of California, is included in the gap analysis table in Appendix 6-1.

Land birds in California tend to have relatively wide distributions. The ext ent of high and moderate suitability habitat predicted for most species was between 10-50%. Very few species had more than 50%, e.g., turkey vulture. The Black-headed grosbeak (Figure 3-1) had high and moderate suitability habitat predicted for 31% of California's land area. The highest quality habitats are found in the foothills surrounding the Great Central Valley, portions of the north coast and urban areas such as Sacramento and Los Angeles. (See the Limitations and Discussion section at the end of this chapter about the suitability of urban lands as species habitat).

Figure 3-1. Predicted distribution and suitability categories of the Black-headed grosbeak (Pheucticus melanocephalus).

Several of the mammals were predicted to have high and moderate quality habitat over much of the land area of California. These species include the Black-tailed jack rabbit, Botta's pocket gopher, Western harvest mouse, deer mouse, coyote, american badger, and bobcat, all with distributions greater than 70% of the state. The Western gray squirrel is widespread (26%) through the montane areas of the state (Figure 3-2). Its highest suitability habitat, however, occurs in the area surrounding the Great Central Valley. Habitat suitability tends to be low on the central and southern coastal regions.

Figure 3-2. Predicted distribution and suitability categories of the Western gray squirrel (Sciurus griseus).

Figure 3-3. Predicted distribution and suitability categories of Western spadefoot (Scaphiopus hammondii).

Many of the amphibians have very narrow ranges in California, generally less than 10%. The widest predicted occurrence for any amphibian was for the Pacific chorus frog (Pseudaris regilla, WHR code # A039) with highly suitable habitat in nearly half the land area of the state. Several species were predicted to occur in no quadrangles with a suitability rank of at least 4. These species all had very small ranges, such as the Black toad (Bufo exsul) which only occurs in a single wetland in Deep Springs Valley in Inyo County. When the only suitable habitat for the species was wetland or riparian and this habitat was below the mapping resolution of CA-GAP, our method predicted a suitability rank of 1, which was not included in calculating species richness or in the gap analysis of predicted species distributions. The predicted distribution of the Western spadefoot is shown in Figure 3-3 which includes all suitability ranks. Although its range extends across the Great Central Valley and the central and southern coastal regions of the state, its most suitable habitat is relatively limited (roughly 11% of the state) to a narrow band around the valley.

Reptiles tend to have wider distributions than the amphibians. Many species' distributions were predicted to exceed 20% of the state. With the exception of the gopher snake and the common kingsnake, however, no reptiles had highly suitable habitat over more the 50% of the state. The predicted distribution shown in Figure 3-4 is for the California mountain kingsnake. Although it is relatively widespread, there is virtually no high quality habitat (rank = 5) for this species.

Species Richness Patterns

Richness was determined by tallying all vertebrates with at least one polygon with a suitability rank of 4 or 5 in a quadrangle. Thus the entire quadrangle should not be assumed to be high suitability, only that some Medium or High quality is present. Similarly, richness is based on predicted habitat suitability, not on known occurrences of species. So richness itself is only an estimate of the actual number of species present. Because richness patterns are seldom the same among taxonomic groups (Prendergast et al. 1993), we computed it separately for the four orders of terrestrial vertebrates (Figures 3-5 through 3-8).

The patterns of species richness at the scale of 7.5 minute quadrangles are quite distinct among the four taxonomic groups. Land birds (summer ranges of breeding birds only) are most diverse in the lower elevations of the Sierra Nevada and Cascade ranges and in the mountains of southern California. The maximum number of species in a quadrangle is 126, or 61% of all species of land birds. Mammals have a similar pattern except that the richest quadrangles tend to be at higher elevations than birds. They also tend to have greater turnover in composition than birds, with a maximum of only 49% (66 species) of all mammals in the richest quadrangle. Amphibians are most diverse in the cooler, wetter parts of California, primarily on the northern and central Coast Ranges and the central Sierran foothills. Of the 45 native amphibians in the state, as many as 15 (33%) are predicted with highly suitable habitat in the richest sites , while many quadrangles in the deserts contain none. Reptiles, in contrast, thrive in the hot, arid deserts of southeastern California. Because of their concentration in just a portion of the state, the richest quadrangle for reptiles contains 73% (52 species) of all reptiles. Clearly, it would be impractical to base a conservation strategy solely on protecting the richest sites for one of the se groups, since it would seldom be a site that was rich for other taxa. If all terrestrial vertebrates are combined, the number of birds dominates the totals that the pattern is essentially the same as seen in Figure 3-5. As a result, the richest sites for all vertebrates, in the northeastern part of the state would capture few amphibians and reptiles.

Figure 3-6. Species richness of mammals by 7.5 minute quadrangle. (Only species with at least category 4 or 5 suitability are counted).

Figure 3-7. Species richness of amphibians by 7.5 minute quadrangle. (Only species with at least category 4 or 5 suitability are counted).

Figure 3-8. Species richness of reptiles by 7.5 minute quadrangle. (Only species with at least category 4 or 5 suitability are counted).

Richness alone can be a misleading indicator of biodiversity. In Figures 3-9 through 3-11, we illustrate one of the dilemmas of relying on species richness as a conservation criterion. Each of these 3 maps depicts the similarity in species composition of mammals of all quadrangles in relation to a reference quadrangle. For this set of maps, we selected 3 reference quadrangles that each contain 52 mammals (top 5% of all quadrangles in mammal richness), Taylorsville in the northern end of the Sierra Nevada range, Cascadel Point in the low- to mid-elevations of the southern Sierra, and Lake Arrowhead in the San Bernardino Mountains in southern California. All 3 quadrangles are characterized by a mix of montane conifer and hardwood forest habitats with some shrub types. Yet they show little similarity in composition. The two Sierran sites only share 2/3 of their combined species. The Taylorsville and Lake Arrowhead quadrangles share only 1/3. In fact, it is striking how rapidly the similarity of species composition declines as one moves away from the reference quadrangle. The average similarity of all other quadrangles with these 3 ranged from only 36-42% Selecting one site to represent biodiversity, simply because the site has high species richness, would tend to miss many species. This result emphasizes the importance of the complementarity of sites, since each contains species not represented by the others.

Figure 3-9. Jaccard similarity index of mammals by 7.5 minute quadrangle in reference to the Taylorsville quadrangle in the northern Sierra Nevada region. (Only species with at least category 4 or 5 suitability are counted).

Figure 3-10. Jaccard similarity index of mammals by 7.5 minute quadrangle in reference to the Cascadel Point quadrangle in the southern Sierra Nevada region. (Only species with at least category 4 or 5 suitability are counted).

Figure 3-11. Jaccard similarity index of mammals by 7.5 minute quadrangle in reference to the Lake Arrowhead quadrangle in the Southwestern California region. (Only species with at least category 4 or 5 suitability are counted).

Accuracy Assessment

When this report was written, was written, the accuracy assessment/validation of the predicted species distribution modeling had not been completed. Two approaches are being taken concurrently: 1) comparison of predicted species lists using the CA-GAP data and methodology with lists of observed species from state parks and other manages areas (in collaboration with Dr. James Quinn at University of California, Davis), and 2) comparison of predicted species with the Breeding Bird Survey (BBS) data. Dr. Quinn's group at Davis has assembled and edited species lists from a large number of managed areas. From the comparisons with his data, we hope to be able to estimated the overall quality of the CA-GAP vertebrate data and identify which groups of species tend to be predicted with greater or lesser accuracy. Because the managed areas are various sizes, we should be able to estimate the spatial resolution at which the predictions are of satisfactory accuracy. The expectation is that the predictions will be less accurate for small sites than for large ones, where many of the small local errors are overcome as one encounters more habitat types. For instance, predicted lists for the entire state should be virtually perfect. All species known to occur would be predicted to occur somewhere in the state. But in very small managed areas of 100 ha or less, generally the habitat suitability method tends to greatly overestimate the number of species (errors of commission). At the same time, the generalization of the CA-GAP land-cover map will often miss individual patches of unique habitat and therefore fail to predict species at the site (errors of omission). The difficulty in making these comparisons is standardizing the predicted and observed lists to include the same types of species. For instance, park species lists often include every species ever seen in the managed area, not just the breeding species modeled in gap analysis.

The BBS is a long-term program of the U. S. Fish and Wildlife Service to collect data on the trends in birds across the nation. Each spring, 25 mile long sections of a sample of roads are surveyed, with stops at 0.5 mile intervals. Birds that are seen or heard are recorded, both the species and the number of individuals, in a brief stay at each stop. Comparisons of the predicted bird lists and the BBS will allow us both to test the CA-GAP wildlife predictions with actual field data over a large area as well as to evaluate the significance of management status of the transects in predicting the trends in bird abundance. If the assumptions of gap analysis are valid, we should expect to see declines in the birds that are least well-represented in managed areas.