Dispersal Dynamics of White-footed Mice in a Patchy Landscape
Richard S. Ostfeld
Institute of Ecosystem StudiesPI: R. Ostfeld; coPIs: F. Keesing (Siena College); and C. Canham (IES)
In this project, which is beginning in winter of 1999, we will link two key ecological processes that until now have been treated as conceptually distinct phenomena: dispersal dynamics of mobile animals in a patchy landscape, and the effects of seed consumers on tree regeneration in forested landscapes. Our primary goals are twofold: (1) to determine whether populations of white-footed mice (Peromyscus leucopus) demonstrate source-sink dynamics in the patchy, postagricultural landscapes typical of the northeastern U.S.; and (2) to reveal the consequences of dispersal patterns by mice (whether source-sink or an alternative model) for the survival of tree seeds and ultimately for forest regeneration. This will allow us to link dispersal dynamics of animal populations with community dynamics of plants within a patchy landscape. Our new research builds on the following observations, generated from recently completed research at IES.
1. White-footed mice are the predominant seed consumers in forests and oldfields of the eastern U.S. Intensity of seed predation by mice is correlated with mouse density such that nearly 100% of experimental seeds are consumed when mouse density approaches multiannual peaks.
2. Density of mice within oak-dominated forest patches is determined by mast (acorn) production, reaching a peak the summer following a heavy mast year. Nonoak patches, such as maple-dominated forests and oldfields, show more regular production of foods for mice. Determinants of the patterns of population dynamics by mice, and of forces regulating their density, in these patches are unknown.
3. Mouse populations in the three dominant patch types in natural landscapes of this region (oak forests, maple forests, and oldfields) appear to be linked via dispersal. Two lines of evidence suggest that population declines following peak density in oak forests may result largely from emigration. First, annual peaks in mouse population density within oldfields lag 1-3 months behind peaks in adjacent oak forest. Second, studies of the abundance and infection rates of Ixodes ticks, which are specialists on mice, suggest that they are dispersed via mouse hosts from oak forest to maple forest patches and oldfields during years of high mouse density. If mouse populations in nonoak patches are maintained by dispersers from oak patches, then dispersal dynamics by mice would be described accurately by a source-sink model, with oak patches as source and nonoak patches as sink habitats.
4. Rodents in simulated sink populations (i.e. those composed largely of immigrants after removal of residents) show fundamentally different foraging behavior than do those in source populations (i.e. those composed largely of residents), probably due to transience and the lack of established home ranges. Our recent studies show that foraging by meadow voles and white-footed mice in "immigrant" populations results in lower per-rodent consumption rates of both tree seedlings and moth pupae, respectively, compared to foraging by "resident" populations. Whether "immigrant" populations of mice also relax their predation rates on seeds, resulting in an increase in tree regeneration rates, remains to be determined. Therefore, oak masting may cause both a numerical and a functional response by mice that may penetrate to adjacent patch types and strongly influence the dynamics of forest regeneration.
Supported by NSF
References:
Ostfeld, R.S., F. Keesing, C.G. Jones, C.D. Canham, and G.M. Lovett. 1998. Integrative ecology and the dynamics of species in oak forests. Integrative Biology 1:178-186.
Van Buskirk, J. and R.S. Ostfeld. 1998. Habitat heterogeneity, dispersal, and local risk of exposure to Lyme disease. Ecological Applications 8:365-378.