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The effects of woody patches of habitat on grassland small mammals.

Habitat modifications (i.e., habitat loss, habitat fragmentation, and species invasion) can affect biological communities. To understand better the influence of habitat modifications on grassland ecosystems, I examined the effects o f small introduced woody patches on small mammals inhabiting a grassland. Specifically, I examined: the effects o f woody patches on the species richness, species diversity, and total density o f grassland small mammals; the role o f local (e.g., average grass height) and regional (e.g., distance to mixed-grass prairie) environmental variables in determining abundance of grassland small mammals associated with woody patches; and the effects o f edge on small mammals associated with a grassland-woody patch interface. To evaluate the effects o f woody patches, I live-trapped a series o f sites grouped by treatment (control sites, grassland only; experimental sites, woody patches surrounded by grassland) on Fort Sill Military Reservation in southwestern Oklahoma, primarily during summer months from 1994 to 1997. Total density o f small mammals was higher at sites with woody patches than at grassland sites without woody patches, in part because Peromvscus leucopus benefitted the most from woody patches, while numbers o f Microtus ochrogaster were lower. Species richness and species diversity did not differ. More animals (all species combined) inhabited sites surrounded by relatively uniform landscapes, which also were farthest from bottomland forest and tallgrass prairie. Numbers and change in abundance o f Siemodon hispidus. the most frequently encountered mammal, did not follow the same pattern as that for all species combined; higher numbers and greater change through time typically occurred at sites with taller grasses located within landscapes having cover types with more complex edges. For P. leucopus. numbers were higher during two trapping sessions at sites surrounded by relatively uniform landscapes and more distant from bottomland forest and tallgrass prairie; increases over time in its numbers were greater at these sites. Overall, regional environmental variables accounted for more variation in abundance and changes in abundance in small mammals associated with woody patches than did local factors. Edge associated with woody patches varied in its effect on grassland small mammals. Captures of all species combined were more frequent (i.e., a positive effect) within and closer to woody patches than at control sites, in part because S. hispidus. P. leucopus. and Neotoma floridana benefitted from woody patches and/or edge. Numbers o f M. ochrogaster were lower (i.e., a negative effect), likely due to interactions with S. hispidus. All species combined, S. hispidus. and M. ochrogaster exhibited ecotonal effects (i.e., unique responses due to presence of edge habitat) that extended at least 10 m but not 25 m into the surrounding grassland, while N. floridana and P. leucopus showed matrix effects (i.e., attribute changes at edge due to different habitats) that ended near the patch edge. Although edge often is discernable to an investigator through structural characteristics o f vegetation, my findings suggest that for small mammals it may be more useful to employ a functional description based on attributes o f those species, such as abundance. Overall, the addition o f woody patches o f habitat to grasslands had a significant influence on the small-mammal community. In particular woody-habitat species (e.g., P. leucopus) and habitat generalists (e.g., S. hispidus! benefitted markedly, while grassland species (e.g., M. ochrogaster) were negatively affected.