An evaluation of the proposed international biological program census method for estimating small mammal populations

Abstract

Polish workers have proposed a method of estimating small rodent populations based on removal trapping. Using a linear regression method, they obtained reliable population estimates with five days of prebaiting followed by five days of removal trapping. The aim of our study was to compare the results obtained, using the same method at the Atomic Energy Commission's Savannah River Plant in the Southeastern United States, with the results of the Polish investigators. Our study area consisted of 5.8 ha over which a grid of 256 trap stations was arranged with two-snap traps at each station. Since by the fifth day of trapping, the number of daily captures had not declined, the trapping period was extended to cover 27 days. An increase in daily captures was observed between the 13th and 15th day in three of the four most common species. These fluctuations in capture rates were believed to be due to behavioral changes in the population. The results from this study indicated that at least nine days of trapping were necessary to obtain reliable estimates for the population as a whole. Even so, reliable estimates could not be calculated for two of the four most common species after 27 days of removal trapping.

Determining density for small mammal populations using a grid and assessment lines

“…Large variations in P c for given species over the first few nights means that no population estimate can be obtained (e.g. Gentry et al, 1968). Hayne (1949) did not give r values in his original paper and our recalculations of his data indicate a nonsignificant fit of his data to his model for the example.…”

“…6 & 9). The Polish recommendation of five days of removal can result in an entirely erroneous picture of the mammal community (Gentry, et al 1968;Faust et al 1971;Kaufman, et al 1971). Another approach might be to develop techniques to measure the magnitude of mortality.…”

“…A 5.06 ha grid, consisting of 256 stations (16X16) spaced 15 m apart was laid out in a lowland mesic-hardwood forest (see Gentry et al, 1968 for description of habitat). One Museum Special and one Victor mouse snap-trap were placed at each station.…”

“…First, the probability of capture is not constant for all animals (e. g., T a n t o n, 1965& 1969Smith, 1968) or at different times during the sampling interval (Calhoun, 1963;Gentry et al, 1968Gentry et al, , 1971 a, b, & c; Kaufman et al, 1971). Secondly, the population at risk of capture does not stay constant but increases through the sampling interval due to immigration.…”

The use of stratified traps in estimating density: Peromyscus and Blarina

“…The development of a standard way to determine the density of small mammals (Grodziriski, Pucek & Ryszkowski, 1966) has been hampered by the fact that, during the first few days of removal trapping, some species of small mammals may be taken at very low rates relative to their actual abundance (Gentry, Golley& Smith, 1968. Calhoun (1959Calhoun ( , 1964 suggested that such lags in catch were due to community organization -the presence of a psychologically dominant species seems to inhibit the movements of some subordinate species, in effect preventing them from encountering traps until inhibitions are relased by the removal of the more dominant species.…”