Year of Award


Document Type

Thesis - Campus Access Only

Degree Type

Master of Science (MS)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Thomas E. Martin

Commitee Members

Beth Hahn, Richard Hutto


aspen, nest predation, bird diversity, habitat selection


University of Montana


Aspen forests hold a high diversity of organisms, but these forests are declining in western North America. A major correlate of aspen decline is conifer encroachment, where conifer trees gradually replace aspen trees through competition for space and light. Conifer encroachment on an aspen stand may increase bird diversity by increasing vegetation complexity. The presence of conifers may also affect the reproductive success of birds through changes in vegetation density and structural complexity around nest sites or the addition of conifer-dependent nest predators. We examined the effects of conifer encroachment on the diversity and reproductive success of birds in fourteen aspen stands that differed in the extent of conifer encroachment in Montana. Bird species richness increased with aspen stand size, but was not correlated with conifer or aspen tree density within a stand. Nest predator richness and abundance also increased with aspen stand size, and also increased with conifer tree density. Despite the increase in nest predator abundance and richness with conifer density, nest predation rates were not related to conifer density. Increased vegetation complexity apparently offset increased predator abundance with increased conifer density. Nest predation also generally was lower at nest sites with greater density of aspen trees around a nest, but varied among species. Based on my results, aspen stand size and aspen tree density within a stand affected bird diversity and reproductive success, which emphasizes the value of increasing aspen habitat quality and stand size in the landscape.

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© Copyright 2010 Amy B. Johnson