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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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