Endangered Species Recovery Program

2. Reasons for Decline and Threats to Communities

Loss and degradation of natural communities due to agriculture, urbanization, livestock grazing, water impoundment and diversion, historical predator and pest control, and other human activities have jeopardized nearly all the unique biota of the Valley below the woodland belts, and are the major causes of endangerment of the state and federally listed species (Figure 3). The delta freshwater marshes and the vast tule marshes of the Valley are nearly gone. Of the approximately 2,110,257 hectares (5,214,539 acres) of land in the southern San Joaquin Valley region (including the Carrizo Plain Natural Area and most of the Tulare Basin below the woodland belts) studied by the California Energy Commission, only 324 hectares (800 acres) of degraded wetlands were found by 1989 (Spiegel and Anderson 1992). Over 40,468 hectares (100,000 acres) of seasonal wetlands are found farther north in the San Joaquin Basin, mostly in Fresno and Merced Counties. The grassland and vernal pool communities have been reduced mostly to narrow piedmont strands, fringing the Valley floor, and their native species have been largely displaced by exotic species of weedy annual grasses and forbs. Of the original 400,000 hectares (about 1 million acres) or more of riparian communities in the Central Valley, less than 10 percent existed in 1979, mostly located in the Sacramento Valley (Warner 1979). Water diversions, stream channelization, and clearing and cultivation of riparian communities all have played roles in loss of riparian communities. Of those remaining today, most are highly degraded in quality and support few or none of their characteristic species. Extant riparian communities in the San Joaquin Valley consist of less than 2,800 hectares (7,000 acres) of narrow, degraded stands along channelized streams. Only about 269 hectares (665 acres) of relatively mature riparian forest with a well-developed understory of herbs and shrubs are found in two parks and one prserve in the San Joaquin Valley (Williams and Kilburn 1984).

Loss and degradation of natural communities in the region due to conversion to irrigated cropland have continued at much slower rates since about 1986, but still pose new threats to many additional species (Williams and Kilburn 1992, USFWS 1994a). The greatest new threats are to the biota of grassland and vernal pool communities along the eastern and northwestern edges of the Valley, where urbanization, ranchette developments, wind energy developments, and cultivation are collectively causing destruction of natural communities at an increasing pace.

Figure 3. Map showing recent land uses in three categories.