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Document Type

Open Access

Degree Program

Wildlife & Fisheries Conservation

Degree Type

Master of Science (M.S.)

Year Degree Awarded

2003

Month Degree Awarded

May

Abstract

Investigations in various fields of research require reliable population density estimates of small mammals. Such estimates allow the direct comparison of independent experimental results and statistical synthesis via meta-analyses, thereby broadening our ecological knowledge. The reliability of traditional density estimation techniques is uncertain, because procedures by which the effective area of open, unbound populations may be determined have not been satisfactorily developed. To circumvent this problem, Anderson and others (1983) proposed a distance-sampling method (the "trapping web") that provides a direct estimate of population density without requiring an area estimate; yet, its use has been infrequent.

A literature review suggests that non-use or unsuccessful use of the trapping web is due to the large effot required (big grids, many traps) and/or low individual captures; as such, further work is needed to establish how small a trapping web will perform well. Nonetheless, other more fundamental work seems more important initially. Using data representing a wide range of murid population densities from seven forested sites in central Massachusetts, 1996-98, I assessed the movement of individual white-footed mice (Peromyscus leucopus) captured on trapping webs to determine the effect of the methods inherent trap/bait gradient. I also evaluated variation in capture probability (heterogeneity) for these mice and sourthern red-backed voles (Clethrionomys gapperi) to elucidate this behavior's effect on trapping web estimates. Finally, I compared trapping-web estimates to density estimates derived from mark-recapture grids for the same populations to assess method performance.

Mice were apparently displaced toward trapping web centers; such movement likely exacerbates edge-effect, limits the duration of trial periods, complicates use of web recapture data, and potentially biases density estimates. Heterogeneity had no effect on estimates for either murid, making the post-stratification of data unnecessary. Comparison of density estimates suggests systematic differences in method performance relative to population levels; CAPTURE estimates appeared positively-biased at low- to mid-level densities, whereas DISTANCE estimates appeared positively-biased at the highest population densities observed. Although not without problems, the trapping web method, with its amplitude of trap distances, may prove useful in determining optimal trap spacing; thus providing a long-sought, empirically-defined trap-spacing index specific to small mammal species and habitats.

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