Small mammals are among the most abundant vertebrate inhabitants of the Holt Research Forest and have wide-ranging effects on forest ecology. For example, several species are important predators of tree seeds and gypsy moths. Small mammal populations are monitored by live-trapping twice per year: in April to observe a low point in their annual fluxes, and August to observe a high point. Captures and recaptures of over 15,000 small mammals during 20 years have revealed some interesting patterns.
The connection between mice and mast is undisputed based on numerous studies. For example at the Holt Forest, based on 10 years of data, there were significant positive correlations between fall acorn crop production and the number and weights of white-footed mice (Peromyscus leucopus) the following spring. Their propensity for storing acorns during acorn-rich falls is probably responsible for this pattern.
With 20 years of data, it was found that summer populations of white-footed mice went through 4 abundance cycles with a cycle period of 4.0 years. Abundance always increased after large acorn crops and a low phase in the abundance cycle. However, in in 1992, 1996, and 1998 (abundance decreased following high abundance phases despite large acorn crops (see bold arrows in figure above). The index of fluctuation indicated a stronger fluctuation in Holt Forest P. leucopus than in populations in Indiana, Virginia, Ohio, and Pennsylvania. This periodicity may be mediated through as-yet unidentified factors such as predation.
Red-backed voles (Clethrionomys gapperi) demonstrated a 3.6-year cycle. Abundance increased following three of four large white pine seed crops, leaving some doubt as to whether the white pine seed-vole connection was spurious or biologically meaningful. Regardless, cycling and density-dependence in Holt voles is consistent that found in vole populations in Scandinavia and elsewhere.
Habitat vs. Harvest
White-footed mouse cycles on the harvest and control sides of the forest corresponded closely across 18 years (1983-2000), but voles presented a different story. Harvest-side voles increased in the first year following harvest, then declined in the 2nd and 3rd years following harvest compared to control side voles. After five years, the harvest-side vole cycle again corresponded well with the control-side cycle. If cover is at issue, it is possible that voles found cover in first-year but not 2nd- and 3rd-year slash piles, then in subsequent years found cover in regeneration. We hope to test this hypothesis in the near future. The 1987-88 harvest coincided with a shift from predominantly northern flying squirrels on the study area to the southern flying squirrel. However, that this shift probably occurred because of regional population changes rather than changes to the forest structure.