The effects of vertebrate granivores and folivores on plant community structure in the Chihuahuan Desert

We examined the effects of vertebrate granivores and folivores on winter and summer plant communities in the Chihuahuan Desert by selectively excluding different vertebrate combinations of birds, rodents, and large folivorous mammals from small experimental plots continuously since 1982. Few differences were observed across treatments during the first two yr of the experiment. The influence of vertebrates on winter and summer plant communities became apparent after three yr, and changes continued to occur after 11 yr of exclusion. Multivariate repeated-measures analyses indicated that removal of birds and both birds and rodents caused significant increases in the total density of winter annuals. The winter annual plant density was highest on bird removal plots and this increase was significant by 1985. Canonical Discriminant Analyses revealed that the exclusion of birds and both birds and rodents had the strongest influence on species composition of both winter and summer plant communities, although the winter plant community was more sensitive to vertebrate exclusion than the summer plant community. The exclusion of birds had the greatest effect on composition of summer plant communities followed in importance by the exclusion of rodents and mammalian folivores. Most of the plant species that responded significantly to the treatments were large-seeded winter annuals. Our results both complement earlier experimental work documenting the impacts of the granivorous rodents and folivorous mammals on desert plant communities and now demonstrate the important influence of avian granivores.