|
COPYRIGHT 2005 Wilson Ornithological Society
Grassland birds are among the most imperiled wildlife in North America (Peterjohn and Sauer 1999, Sauer et al. 2004), and, within this group, owls are considered species of conservation concern in most North American regions (Wellicome and Haug 1995, Herkert et al. 1996, Sheffield 1997, U.S. Fish and Wildlife Service 2002). Biologists visiting and working on San Clemente Island (SCI), California, have observed Barn (Tyro alba), Burrowing (Athene cunicularia), and Short-eared (Asio flammeus) owls at various times of the year and have documented breeding by Barn Owls (BLS and ELK unpubl, data). However, little else is known about the owl populations on SCI.
Grasslands compose 30% (~4,300 ha) of SCI's vegetation community; thus, there is ample habitat for grassland owls. The presence of large, open grasslands on SCI has resulted from the island's history of ranching and the introduction of feral grazers and exotic grasses in the mid-1800s (Andrew 1998). These introductions dramatically altered the landscape by changing the shrub component of the native coastal chaparral habitats to open grasslands (Coblentz 1980; BLS and ELK unpubl. data). In 1993, however, feral grazers were removed from SCI; as a result, successional change has been allowed to take place and the grasslands are reverting to more natural, shrubby communities.
Due to successional change, the conservation status of the island's owls, and our lack of knowledge about grassland owls on SCI, we examined the presence/absence, relative abundance, and distribution of grassland owls on SCI. We hope to provide a better understanding of how grassland owls use SCI and determine how the successional transition of grassland habitats may effect these owl populations in the future.
METHODS
Study area.--SCI (32[degrees] 50' N, 118[degrees] 30' W) is located approximately 109 km northwest of San Diego, California, and is the southernmost California Channel Island. The 14,603-ha island is 34 km long and 2.4-6.4 km wide. A relatively level, open plateau runs the length of the island, with elevations ranging from sea level to 599 m. Deep canyons of varying lengths incise the plateau from the east and west sides. Temperatures range from 6 to 37[degrees] C and mean annual precipitation is 17.8 cm (California State Northridge, Department of Geography unpubl, data, 1998-2002). Fog is common, especially in the summer. Prevailing winds are from the west, and windy days are frequent throughout the year (typically Beaufort scores of 2-3). SCI is administered by the U.S. Navy and is used for active military training as part of the Southern California Offshore Range; however, the U.S. Navy has an environmental program to protect natural and cultural resources (U.S. Department of the Navy 2001).
Suitable owl habitat (i.e., grasslands and maritime desert scrub) is found predominantly on, and surrounding, the island's large central plateau. Grasslands comprise native and nonnative species (Arena spp., Bromus spp., Nassella pulchra) and scattered shrubs such as coyote brush (Baccharis pilularis); however, after the removal of feral grazers in 1993, shrub cover has increased (J. Dunn pets. comm.). Maritime desert scrub is dominated by boxthorn (Lycium californicum), snake cactus (Bergerocactus emoryi), cholla (Opuntia prolifera), prickly pear cactus (Opuntia littoralis), California sagebrush (Artemisia californica), and morning glory (Calystegia macrostegia). See Raven (1963) and Kellogg and Kellogg (1994) for a more detailed description of SCI's vegetation.
Survey technique.--Because of the inaccessible nature (i.e., steep, rocky canyons) of potential nesting habitat for some owl species and the limited availability of personnel to search the vast grassland expanses, we surveyed for owls along established island roads. We established eight 10-km transects (Fig. 1). We selected transect starting points randomly while ensuring that no two transects overlapped. Although SCI roads were not established randomly and roadside surveys are associated with certain biases (Bart et al. 1995,...
Read the full article for free courtesy of your local library.
|
Bookmark this article
Print this article
Link to this article
Email this article
Digg It!
Add to del.icio.us
RSS
