Range-wide abundance and fluctuating asymmetry patterns of sagebrush-obligate passerine birds Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xd07gx206

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  • North American sagebrush (Artemisia spp.) ecosystems are suffering from reductions in habitat extent and quality. Only about 50% of sagebrush remains from pre-settlement conditions, and much of the remaining habitat is fragmented or degraded by invasive species, fire suppression and overgrazing. Sagebrush-obligate species are experiencing population declines as a result of these ecosystem changes. To effectively conserve this ecosystem, it is essential to understand patterns of abundance and stress of the inhabitants at the landscape-level. Abundance of species across their geographic range is not uniform. Instead, abundance often decreases towards the periphery of the range where resources and habitat conditions become less suitable. In addition, stress in populations closer to the periphery of the range may be expressed in condition-dependent traits where suboptimal environmental conditions occur. Fluctuating asymmetry, random deviations from perfect symmetry in bilateral body parts, may reveal increased stress in these populations. I examined patterns of abundance and fluctuating asymmetry of Brewer’s Sparrow (Spizella breweri), Sage Sparrow (Amphisipiza belli), and Sage Thrasher (Oreoscoptes montanus), in the western United States, to determine how these responses vary with proximity to range periphery of sagebrush. Using negative binomial regression and Akaike’s Information Criterion, I investigated associations between species abundance from North American Breeding Bird Survey count data and several local- and landscapelevel variables derived from digital maps depicting the distribution of sagebrush throughout the United States. Abundance for these three sagebrushobligate passerine birds was greatest in mid-elevation (1,200-2,300 m) areas and increased with sagebrush cover. I found little support for abundance declining as a function of proximity to range periphery. Using mixed model regression, I assessed the relationship between fluctuating asymmetry estimates in the tarsi of juveniles and the proximity of specimens to the periphery of sagebrush distribution. I predicted higher levels of fluctuating asymmetry in individuals nearer the range periphery. However, fluctuating asymmetry decreased with proximity to the range periphery for Sage Sparrow, and, although present Sage Thrasher, fluctuating asymmetry did not differ significantly across the geographic range for this species. Fluctuating asymmetry could not be estimated with confidence for Brewer’s Sparrow. While this study revealed stress in these species, a more logistically complex study to evaluate fluctuating asymmetry patterns across the landscape is necessary for determining areas of conservation priority. Loss of areas of high percent sagebrush cover due to habitat fragmentation and degradation will result in continued declines in abundance of sagebrush-obligate passerine birds. Knowing high abundances of sagebrush-obligate passerine birds occur in locations with high sagebrush cover at mid-elevations will aid land managers and conservation biologists in designing effective conservation strategies for these species.
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