The effects of mammalian herbivores on successional grasslands in central Illinois

No previous study has separated the effects of several types of herbivorous mammals on mid-western grassland communities at different stages of development, in spite of a substantial literature indicating the potential of mammals to impact the composition of these plant communities. Mice and voles consume large seeds, and rabbits disperse small seeds endozoochorously. Voles, rabbits, and deer prefer certain plant species, particularly leguminous forbs, over graminoids. Deer, rabbits, and voles often damage woody plants and seedlings. I therefore expected fewer small-seeded plants in areas without access by rodents and rabbits, greater abundances of palatable plants in areas without access to each herbivore, and more woody seedlings in areas without access to each herbivore. I expected effects of granivory and endozoochory to be stronger in early succession than in mid- or late succession because plant recruitment by seeds is more important in early succession. I used exc10sures that allowed access to different combinations of animals to assess the impact of mice, voles, rabbits, and deer in old fields of 3 successional stages. In addition, I conducted feeding trials to determine palatabilities of plants and seeds to the most abundant small mammal species (Peromyscus leucopus, Microtus ochrogaster, M pennsylvanicus, and Sylvilagus floridanus) and related these data to those from the exclosure experiments. To determine the kinds and numbers of plants commonly dispersed by rabbits, I also collected and germinated plants from rabbit fecal pellets. Neither rabbit endozoochory nor rodent granivory affected plant communities in any successional stage. Rodents did not prefer larger seeds. Herbivory by rabbits and rodents influenced the plant communities more than did consumption or dispersal of seeds. Both groups of animals reduced abundance of palatable forbs and increased that of unpalatable forbs. Grasses palatable to voles increased in exclosures during mid-and late succession. Rodent exclusion also significantly increased abundance of the legume Medicago sativa, which had highly palatable shoots and seeds. Cirsium arvense, a palatable forb, increased substantially with the removal of deer and the grass Festuca arundinacea declined. Woody seedlings rarely occurred in the grasslands and none of the mammals affected their abundance