title: Modeling biotic and abiotic influences on population size in small mammals.
abstract: Most previous work in population ecology has modeled density-dependent effects in isolation. In this paper, we concurrently modeled the effect of density-dependent and density-independent factors on the rate of population change (Rt) in Peromyscus leucopus (white-footed mouse), using a Ricker equation expanded to include weather and seasonality. From 1973 to 1996, we live-trapped P. leucopus monthly in a 2-ha Ohio woodlot. Population peaks (July to August) varied from 27 to 181 individuals, while troughs (December to March) varied from 4 to 46 individuals. We used time-delayed densities to act as surrogates for unobserved density-dependent factors, and principal components to represent 12 highly collinear weather variables. We identified time-delayed correlations by season between Rtand the independent variables (i.e., previous densities and weather principal components) using transfer function analysis. In summer, when P. leucopus densities were above the seasonal mean for the month, Rtwas lower in the following 2 months; however, in winter, Rtwas greater in the first but lower in the second succeeding month. Rtalso correlated positively in autumn with contemporaneous precipitation, and was negatively correlated with `extreme' weather in summer with 2- and 3-month lags and in winter with a 3-month lag. We hypothesize that precipitation affected juveniles by influencing food resources and that `extreme' weather affected breeding. Our model explained 65% of the variability in Rt, and density-dependent and density-independent factors explained an equal percentage of that variability. This model created good forecasts of population density up to 12 months in the future.
authors.affiliation: Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403, USA Fax: (419) 372-2024; e-mail: svessey@bgnet.bgsu.edu, , , , , , US.
authors.forename: Ruth H
authors.initials: RH
authors.lastname: Lewellen
authors.affiliation: Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403, USA Fax: (419) 372-2024; e-mail: svessey@bgnet.bgsu.edu, , , , , , US.
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Vessey
keywords: Key words Transfer function analysis; Multivariate time series analysis; Peromyscus leucopus; Ricker equation; Weather