6. STEWARDSHIP AND MANAGEMENT STATUS OF PREDICTED SPECIES DISTRIBUTIONS

Species Distributions and Management Status
Limitations and Discussion

Species Distributions and Management Status

For this gap analysis, we limited the predicted distributions of terrestrial vertebrates to habitats within each species’ range that were rated as 4 or 5, i.e., most of the land-cover/habitat polygon was classified as moderate to high suitability. The full table of the proportions of each species’ distribution by management status level is contained in Appendix 6-1. The table also lists the total predicted distribution of each species and the percent of the total land area of the state occupied by this suitable habitat. For instance, 30.3% of the state (123,727 km²) was predicted to have suitable habitat for the Black-headed grosbeak (Pheucticus melanocephalus, WHR code B475—see Figure 3-1). Over 9% of this habitat is currently in status 1 or 2 managed areas, 31% is in status 3, and almost 60% is in status 4, which is primarily private land.

Only three species have no protection whatsoever. These unrepresented species are the Inca dove, Bronzed cowbird, and Woodhouse’s toad, all of which prefer urban or agricultural habitats near the Colorado River and thus do not require permanently protected reserves for their persistence.

Several species have very low levels of representation (<5 %) in managed areas. These species fall into four general groupings: 1) adapted to human-dominated habitats (similar to the unrepresented species mentioned in the previous paragraph), 2) very limited ranges, 3) marginal to the state, and 4) diminishing or degraded habitats.

Species adapted to human-dominated habitats are generally not vulnerable to extinction. Most of the poorly-represented birds are in the first category (Black phoebe, Western kingbird, American crow, Red-winged blackbird, Hooded oriole, and American goldfinch). Black- and Yellow-billed magpies commonly feed in agricultural habitats but also depend on riparian or oak woodland habitat for breeding and cover.

The very narrowly endemic species are best evaluated in a "fine-filter" approach to complement the "coarse-filter" gap analysis. The subspecies of Townsend’s pocket gopher found in California is restricted to the Honey Lake area in Lassen County (Zeiner et al. 1990). The White-footed vole is restricted to a small section of the north coast of the state. The state- and federally-listed Fresno (or San Joaquin) kangaroo rat is similarly restricted to remnant patches of alkali desert scrub in the San Joaquin Valley. Among the amphibians, these least well-represented taxa tend (Tiger salamander, Redbelly newt, Siskiyou Mountains salamander, Shasta salamander, and Spotted frog) to be those that have very small ranges that happen to be outside of existing managed areas. The Spotted frog may, in fact, never have occurred in California but may be misidentified specimens. The Grasshopper sparrow has a wider range than these other species, but its abundance is erratic from year-to-year.

Species that are widespread outside California should be re-assessed in ecoregional gap analyses or over their entire range. Marginal breeding species in California include the Gila woodpecker, Cordilleran flycatcher, and Cedar waxwing. Northern pocket gopher and pronghorn are restricted in California to the Great Basin but are widespread outside the state. The only poorly-represented reptile is the Short-horned lizard. This species is the most widely distributed lizard in North America but in California only occurs in the Modoc Plateau region at the margin of its range (Zeiner et al. 1990).

Those that occur in habitats that are diminishing or are degraded from neighboring land uses, such as on the south coast or the Great Central Valley, are generally the most vulnerable group of species when they have low levels of representation. Swainson’s hawk and Black-chinned hummingbird were predicted in the Central Valley and other degraded habitats. The California kangaroo rat is the most widespread of the mammals in this group, but its range is limited to grassland in northern California. The range of the Western spadefoot is relatively large but its grassland habitat has been severely modified.

Table 6-1 summarizes the status information from Appendix 6-1 to show the number of species in each taxonomic group with <10, 10-20, 20-50, and >50 % of the predicted suitable habitat in status 1 and 2 managed areas. Of all native vertebrates, the largest group has 10-20 % in managed areas, with about equal numbers with <10 and 20-50 %. Very few species have > 50 % protection and these tend to be species with very small ranges, such as the Alpine chipmuck (Tamias amoenus), which was predicted over only 444 km², or <0.1% of California’s land area, in the rocky habitats above 2800 m in the High Sierra.

Table 6-1. Summary of species at different levels of biodiversity management.

	# (%) with < 10% Status 1/2	# (%) with 10-20% Status 1/2	# (%) with 20-50% Status 1/2	# (%) with > 50% Status 1/2	Total # of species
Land Birds	72 (35%)	83 (41%)	44 (22%)	5 (2%)	204
Mammals	30 (23%)	52 (39%)	40 (30%)	11 (8%)	133
Amphibians	18 (44%)	14 (34%)	7 (17%)	2 (5%)	41
Reptiles	10 (14%)	21 (30%)	35 (49%)	5 (7%)	71
All Native Vertebrates	130 (29%)	170 (38%)	126 (28%)	23 (5%)	449

As a group, reptiles appear to be the best protected according to their management status. More than half of the 71 reptiles have > 20 % status 1 and 2 habitats. Only 10 reptiles have <10 % in managed areas. This high level of protection for this group is explained not so much because conservation action has focused on them but rather by an extensive set of parks and wilderness areas designated in the California deserts where most of the reptiles reside. Amphibians are the least well-represented group in managed areas. The largest category of amphibians has <10 % status 1 and 2. Only 9 amphibians (22 %) have >20 % protection.

Limitations and Discussion

As described in Chapter 3, CA-GAP predicted species distributions on the basis of range boundaries and the suitability of habitat types with the range. It does not guarantee that a species will occur at all locations that are modeled as suitable habitat. For some species, there are additional habitat elements that further control their distribution. Elements such as the presence of snags, proximity to surface water, or the adjacency of different critical habitat types, for example, are site-specific features that can not be detected at a regional scale with remotely sensed data. Habitat structure is also a crucial factor in determining habitat suitability for many species. Structure would characterize the height of the canopy, the density of canopy cover, and the number of layers in the canopy in multi-storied, uneven-aged habitats. Those species that are dependent on either these key habitat elements or on specific seral stages are likely to be grossly overestimated by our modeling methods. Species such as the California spotted owl and the fisher require mature stands of dense forest. This level of detail was also not compiled for CA-GAP, so suitability was based only on the type of habitat, not its structure.

This overgeneralization of the distribution of some species has two possible effects. First, the overall distribution for such a species is likely to be overpredicted. More suitable habitat will be predicted than actually occurs. This will be most glaring for species dependent on habitat stages that have been most depleted, such as late seral, old-growth forest. Second, because the predicted distributions are suspect, the quality of the gap analysis results from overlaying the distribution maps with the management status maps is also uncertain. For these species, we suggest that the modeling reported here be considered an initial screening. That is, lands predicted to be unsuitable are reasonably correct. Lands predicted as suitable have passed the first test, but more detailed habitat information needs to be incorporated in a subsequent phase of the assessment for this group of species.

By limiting the analysis to the highest suitability habitats, CA-GAP predicted that several species would have no habitats of this extent and quality. These species, which were not included in Appendix 6-1, include the Vermillion flycatcher, Red fox, Desert slender salamander, Tehachapi slender salamander, Colorado River toad, and Black toad. The Vermillion flycatcher was only bird in this situation. It only occurs in a few riparian areas such as desert oases in the Sonoran Desert region. Therefore its habitat is below the resolution of the CA-GAP database. It is considered rare and is listed as a California Species of Special Concern (Zeiner et al. 1990). The native Red fox is listed as a California threatened subspecies and is limited to the Sierra Nevada and Cascades ranges and to Siskiyou County. The CA-GAP database rated all mapped habitat polygons as low suitability. Populations of Red fox in the Great Central Valley and coastal areas are introduced (Zeiner et al. 1990). The Desert (Federal and California endangered) and Tehachapi (California threatened) slender salamander species are both restricted to a very small range. The Desert slender salamander is only found at a single location in Riverside County. While the CA-GAP database did identify several potential habitat sites for these two species, they were all rated as low suitability. Similarly, the Colorado River toad (limited to a small area on the Arizona and Mexican borders) and the Black toad (a California threatened species restricted to a single wetland in Inyo County) were predicted to have only low suitability habitat in a single polygon in the CA-GAP database. Because the habitat requirements of these are more detailed than could be mapped for gap analysis at the state-level, we recommend that their conservation status be evaluated with a "fine-filter" approach, using site-specific mapping or field inventories of their habitats and populations.

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