The interaction of habitat fragmentation, plant, and small mammal succession in an old field: Patterns and mechanisms

The fragmentation of natural habitats is a problem of global dimensions. Spatial heterogeneity (such as that created by habitat fragmentation) has many effects on ecological processes--yet the relative impact of habitat structure at local and landscape scales is poorly understood, especially on long-term phenomena like secondary succession. Moreover, there is relatively little experimental work on the effects of fragmentation on ecological processes and patterns. I present data from a long-term experimental study of fragmentation and succession in eastern Kansas that highlights the role of spatial heterogeneity in successional dynamics.; I begin by contrasting small mammal and vegetation data from 1984-1986 and 1994-1996 using general linear models and spatially explicit tests of small mammal capture probabilities. I show that in fragmented systems, patterns of change in plant and small mammal communities vary with patch size. Therefore, some of the complexity observed in successional systems may result from the effects of the landscape mosaic. I then use live-trapping data, plant census data, and several landscape measures to examine the relative impacts of heterogeneity in within patch and landscape scale habitat characteristics on the spatial distributions of four small mammal species (Microtus ochrogaster, Peromyscus maniculatus, Sigmodon hispidus, and P. leucopus). Landscape structure accounted for most of the variation in these species distributions, which seem to reflect heterogeneity in habitat structure at spatial scales larger than a home range or patch. Finally, I use introductions of tree seedlings and analyses of patterns in woody stem and prairie vole distributions to test the hypothesis that observed patch-specific differences in successional dynamics are indirect effects of fragmentation, acting through a "prairie vole filter" of seedling herbivory. Herbivory levels are influenced by patch size; however, the effects on successional dynamics may be obscured or even reversed by other factors that also vary with patch size. Fragmentation may be an "interaction modifier", directly affecting individual vole behavior and modifying the strength of the interaction between small mammals and woody plants. My results suggest that without experimental, multiscale approaches, many effects of patch size on resource-consumer dynamics might be difficult to discern.