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Since the 1964 earthquake and geologic uplift, beavers (Castor canadensis) have expanded their historic range from the inland (outwash plain) Copper River Delta, Alaska to the once tidally influenced areas of the outer area (uplifted marsh). The uplift presents a unique opportunity to observe beaver expansion, habitat selection and influence on hydrologic structure of a coastal wetland ecosystem after a major disturbance.
First, I examined the relationship between the sites of active beaver lodges and associated water and vegetative features (Chapter 2). I hypothesized that active lodge sites would be associated with distinct water features and woody vegetation types in each landscape. I used aerial photographic and ground survey methods to compare sightability and to determine habitat associations. I developed an a priori set of models to determine active beaver lodge occurrence using logistic regression with presence and absence as the response variables and water features and vegetation cover as the dependent variables. My results were similar between survey methods but differed between the two major landscape types. In the uplifted marsh, odds of an active lodge occurrance increased with the presence of former tidal sloughs and an increase in mixed shrub coverage. The odds of an active lodge occurrence increased with the presence of streams and decreased with increasing closed cottonwood-alder-willow coverage in the outwash plain. Post earthquake uplift, beaver now exploit a unique topographic setting through the use and modification of former tidal sloughs and a transitioning mixed shrub vegetation community.
Second, I quantified the influence of beaver on the surface water of the West Copper River Delta using remote sensing (Chapter 3). Water features were documented from digitized aerial photographs from 1959, 1974, 1986, 1992/1993, and 1996. My results showed that surface water area in beaver influenced water impoundments increased from 0 to 20% from 1959 to 1996, concurrent with beaver colonization of the uplifted marsh. These results suggest that beavers may have altered the trajectory of wetland change on the West Copper River Delta by raising the water table within open water basins and flooding those areas that would have otherwise drained. Beaver may be used an effective management tool in wetland restoration where key habitat features are available. |
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