Graduate Thesis Or Dissertation

 

Multi-scale relationships between aspen and birds in the northern Yellowstone ecosystem Public Deposited

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  • I conducted a multi-scale evaluation of aspen – bird relationships in the northern ungulate winter range of the northern Yellowstone ecosystem during June 2001-03. Questions addressed were: (1) Does bird diversity increase with conifer presence in aspen stands? (2) Given known habitat selection cues, are migrating birds passively intercepted by aspen patches oriented perpendicular to migratory direction of travel? (3) Are resident (short-distance migrant) birds passively intercepted by aspen patches? and (4) Given the dynamics of wolves, ungulates, and forage plants in the northern Yellowstone ecosystem, what is the present condition of aspen and cavity-nesting birds within Yellowstone National Park (YNP) and how will they change over the next 100+ years? Regression analyses of migratory bird diversity (species richness or Shannon-Weiner index) on measures of habitat heterogeneity (ratio of conifer to aspen canopy cover, or basal area) suggested no positive relationship with intermediate levels of conifer presence. Migratory birds were most diverse in pure aspen, and least diverse in pure conifer. I found a weak, but significant, positive relationship between most measures of habitat heterogeneity and resident bird diversity. Many residents were habitat generalists or conifer-associated species. To maximize bird diversity and aspen, managers may want to manage for pure aspen stands in a matrix that includes conifer habitat. Using model selection techniques, long-distance migratory birds did not show evidence of passive interception by aspen patches oriented against northerly or elevational direction of travel. Aspen patch area was most important for migratory birds, given the data and set of models analyzed. Resident (and short-distance migrant) birds showed a marked positive response to patch orientation relative to the elevational gradient of the northern range. Migratory birds appear not to be passively intercepted at high elevation sites such as YNP’s northern range. Short distance migrants appear to be passively intercepted. Northern range aspen stands within YNP have 10cm greater mean live and dead stem diameter, 80% more snags, and greater abundance of many cavity-nesting bird species than northern range stands outside YNP. These conditions fit a conceptual framework of interactions driven by a top-down ecosystem structure that predicts aspen and cavity-nester dynamics over several decades.
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