An assessment of indices of relative and absolute abundance for monitoring populations of small mammals

Abstract

Knowledge of small-mammal population densities and species richness values are crucial to wildlife conservation and many ecological investigations. The relationship between estimates of absolute abundance and relative abundances of small-mammal populations is unclear. Therefore, from 1999-2001 measures of relative and absolute abundance of small mammals were compared across a terrestrial landscape using 3 vegetation communities (forest, early-successional field, and forest-field edge) in western Tennessee. We used a catch/unit effort index derived from transect sampling to estimate relative abundance and 2 estimates of absolute abundance, M t+1 (the number of unique individuals captured) and N est (a population estimate obtained from statistical models). These 2 estimates were derived from mark-recapture sampling on trapping grids. Using both sampling procedures, we conducted trapping with Sherman live traps during autumn, winter, and spring for 2 years. We tested the prediction that the pattern of total captures of small mammals across a landscape from a catch/unit effort index reflects the same pattern as those derived from M t+1 and N est. Also, we tested the prediction that the pattern of species richness of small mammals derived from a catch/unit effort index on transects corresponds with the species richness based on M t+1 determined from grid sampling. We conducted comparisons of patterns using a Spearman rank correlation coefficient. Overall, we captured 988 small mammals, representing 9 genera and 11 species. The pattern of captures from the catch/unit effort index was proportional to those derived from M t+1 and N est. (P ≤ 0.001 in each correlation analysis; rs 17 = 0.880 and rs 17 = 0.810, respectively). The pattern of species richness derived from the catch/unit effort index was similar to those derived from the mark-recapture sampling (P ≤ 0.001; rs 17 = 0.861). Capture frequencies across vegetation communities and within seasons from both transect sampling and grid sampling did not differ for 4 of 6 seasons (χ 22 = 4.4329, P = 0.1090; χ 22 = 18.2224, P = 0.0001; χ 22 = 0.2129, P = 0.8990; χ 22 = 1.6395, P = 0.4405; χ 22 = 3.8688, P = 0.1445; χ 22 = 7.1750, P = 0.0277). Results of this study suggest that measures of relative abundance provide patterns of population trends proportional to those derived from estimates of absolute abundance.

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Wildlife Society Bulletin

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