This large complex of wetlands, dune and riparian habitats extends up to five miles inland from the coast to the base of Nipomo Mesa and is bounded by Oso Flaco Lake to the north and by the Pt. Sal headlands/Casmalia Hills (within Vandenberg Air Force Base) to the south. Ecologically, the area should extend north to the bluffs north of Pismo Beach, but the habitat in the north has been tremendously degraded within the Pismo Beach State Vehicular Recreation Area, and has a totally separate management regime. This IBA may be divided into three main habitats: riparian corridors along creeks (incl. the Santa Maria River, Oso Flaco Ck.); freshwater marshes and willow thickets around dune lakes, and the coastal estuary at the Santa Maria River mouth itself. This IBA has long been a focus of conservation groups and public agencies, and now receives protection as patchwork of reserves, dominated by the 600-acre Rancho Guadalupe Dunes Preserve (Santa Barbara Co.).
In addition to the thousands of shorebirds that utilize the wetlands at the river mouth during spring and fall migration, this site protects many sensitive taxa, including large nesting colonies of Snowy Plover and Least Tern. The riparian vegetation supports good numbers of breeding riparian passerines, including Warbling Vireo, Swainson's Thrush, Yellow and Wilson's warblers, and Yellow-breasted Chat. Though physiographically separate from the river mouth system, the coastal bluffs on either side of this IBA (rocks off Shell and Pismo Beach; Pt. Sal) are regionally important for nesting seabirds, including Rhinoceros Auklet and Pigeon Guillemot (Carter et al. 1992).
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Disturbance by human visitors, including trampling of nests and nest sites and discarded trash that attracts scavenging predators remain chronic problems to nesting Snowy Plovers and Least Terns (fide W. Wehtje). Off-leash dogs were a major threat, but are less of a concern now that dogs have been totally banned from the area. Cattle grazing in unprotected riparian areas within the IBA (including within the Santa Maria River estuary) may be impacting riparian and potential freshwater marsh breeders. Exotic plants such as European Beach Grass and various species of ice plant continue to depress natural diversity. The habitat just north of this IBA receives virtually no resource management, and much of it is taken up by an OHV park. The effect of these activities so close to this IBA deserves investigation.
The Rancho Guadalupe Dunes Preserve is managed for the county by the Center for Natural Lands Management, a conservation non-profit that employs a full-time on-site manager and relies on a network of volunteers for visitor education.
This IBA may be divided into three main habitats: riparian corridors along creeks (incl. the Santa Maria River, Oso Flaco Ck.); freshwater marshes and willow thickets around dune lakes, and the coastal estuary at the Santa Maria River mouth itself.