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Dissertation/Thesis Abstract

The influence of an ecosystem engineer on nutrient subsidies and fish invasions in southwestern Montana
by McCaffery, Magnus Leask Mann, Ph.D., University of Montana, 2009, 107; 3389408
Abstract (Summary)

Streams and associated riparian zones are among the world’s most highly valued yet threatened ecosystems. Interest in using the ecosystem engineering behavior of beaver to meet watershed conservation goals is becoming more pervasive, owing to substantive work documenting the physical effects of beaver impoundments on freshwater ecosystems. However, it is unclear how beaver modify ecological processes linking abiotic factors to changes in the surrounding biotic community. I investigated how beaver impoundments influence local food web processes, as well as impact native fish species threatened by nonnative invasions.

I showed that beaver impoundments enhanced aquatic resource availability to terrestrial consumers. Beaver impounded watersheds had increased densities of emerging aquatic macroinvertebrates and higher levels of aquatic carbon in terrestrial consumer tissues, resulting in higher terrestrial consumer abundances.

Beaver impoundments also had measurable effects on invasion dynamics between nonnative brook charr and native cutthroat trout populations. Brook charr are native to the eastern U.S., and are a key factor in native cutthroat declines in western watersheds. Streams with beaver had potential negative impacts for cutthroat, with higher brook charr densities, and increased spatial overlap between these species. In contrast, young-of-the-year cutthroat in invaded streams maintained high growth rates with beaver present, but showed growth reductions without beaver. Thus beaver conveyed both negative and positive impacts to cutthroat trout.

At the population level, I found that cutthroat in the non-beaver invaded watershed exhibited low survival rates, negative population growth, and a short median time to extinction. With beaver present in invaded streams, cutthroat exhibited 40% higher survival rates relative to the non-beaver control. This led to cutthroat population growth rates 5–20% higher than in non-beaver streams, with longer median times to extinction. Therefore, beaver impoundments had positive implications for cutthroat persistence in brook charr invaded streams.

My research links the habitat altering effects of beaver to changing ecological processes that influence community and population structure of other elements of the system, with implications for persistence of native species. Understanding the ecosystem effects of a highly interactive species like beaver is crucial to predicting repercussions of using beaver in a restoration context.

Indexing (document details)
Advisor: Eby, Lisa
Commitee: Lowe, Winsor, Mills, Scott, Stanford, Jack, Woods, Scott
School: University of Montana
Department: Fish & Wildlife Biology
School Location: United States -- Montana
Source: DAI-B 71/01, Dissertation Abstracts International
Subjects: Wildlife Conservation
Keywords: Beaver, Brook trout, Cutthroat trout, Fish invasions, Nutrient subsidies, Stable isotopes
Publication Number: 3389408
ISBN: 978-1-109-55971-2
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