This IBA is dominated by Temecula and Wilson creeks, which drain the north face of Palomar Mountain and the southwestern slope of the San Jacinto Mountains, and include Vail Lake. Floristically, it is one of the most biologically diverse areas of southern California, supporting both coastal and desert elements, as well as several narrow endemics (particularly around Vail Lake). Many biologists find it reminiscent of northwestern Baja California: extensive coastal sage scrub (including Jojoba), alluvial fan scrub, and mesquite bosque mix with Coast Live Oak woodland and, along Temecula Creek, mature Fremont Cottonwood-willow woodland (one of the finest stands south in southern California). Though most of the land is in private hands, it is bounded on the south by the Agua Tibia Wilderness area and the Cleveland National Forest, and on the east by several Indian Reservations.
The broad floodplain of Temecula Creek east of Vail Lake supports Least Bell's Vireo among the expected riparian breeders (though much of the best habitat remains unexplored). Fascinating desert-meets-coast bird assemblages of the scrub habitats occur along Wilson Creek and Tule Creek, where Bell's Sage Sparrow from the coast breeds side-by-side with Black-throated Sparrow from the desert, California and Black-tailed Gnatcatcher maintain adjacent territories (Weaver 1998), along with all three California orioles: Scott's, Hooded and Bullock's. Other interesting sibling species in the area include Nuttall's and Ladder-backed woodpecker and California and Gambel's quail. Localized species found here include breeding Yellow-headed Blackbirds in Lancaster Valley (RE). The canyons running south from Palomar Mountain support a wide variety of foothill and montane birds, as well as several rare non-birds, including Arroyo Southwestern Toad.
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The uniqueness of this region (at a global scale) and its unprotected status would make it a high priority for conservation whatever the threats. The Aguanga Area is one of the few areas of the state where conservation attention is urgently needed to avoid massive habitat alteration. Already, the boom in "ranchettes" has left large lots denuded of native vegetation (cleared by landowners worried about snakes and fire). Current threats include grazing along Temecula Creek that has nearly eliminated the understory. Efforts by the Western Riverside Habitat Management Plan (WRMSHCP) to protect the wide floodplain of Temecula Creek, and to retain its linkages with more extensive habitat to the north and south, should be monitored and supported. Due to the threat of the urban sprawl from Temecula (housing tracts) and the booming casino construction on Indian tribal lands to the east, a "Critical" level of threat is warranted. The land around Vail Lake is slated for massive housing developments, with similar developments planned for the Wilson Valley area. If approved, these will necessitate the widening of Hwy. 79, which cuts through the center of this IBA, which will lead inexorably to the channelization of at least portions of Temecula Creek (a pattern that has been repeated virtually throughout California).
Though most of the land is in private hands, it is bounded on the south by the Agua Tibia Wilderness area and the Cleveland National Forest, and on the east by several Indian Reservations.
Many biologists find this IBA reminiscent of northwestern Baja California: extensive coastal sage scrub (including Jojoba), alluvial fan scrub, and mesquite bosque mix with Coast Live Oak woodland and, along Temecula Creek, mature Fremont Cottonwood-willow woodland (one of the finest stands south in southern California).