THE EFFECTS OF VERTEBRATE GRANIVORES AND FOLIVORES ON PLANT COMMUNITYSTRUCTURE IN THE CHIHUAHUAN DESERT

We examined the effects of vertebrate granivores and folivores on wint er and summer plant communities in the Chihuahuan Desert by selectivel y excluding different vertebrate combinations of birds, rodents, and l arge folivorous mammals from small experimental plots continuously sin ce 1982. Few differences were observed across treatments during the fi rst two yr of the experiment. The influence of vertebrates on winter a nd summer plant communities became apparent after three yr, and change s continued to occur after 11 yr of exclusion. Multivariate repeated-m easures analyses indicated that removal of birds and both birds and ro dents caused significant increases in the total density of winter annu als. The winter annual plant density was highest on bird removal plots and this increase was significant by 1985. Canonical Discriminant Ana lyses 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. Th e exclusion of birds had the greatest effect on composition of summer plant communities followed in importance by the exclusion of rodents a nd mammalian folivores. Most of the plant species that responded signi ficantly to the treatments were large-seeded winter annuals. Our resul ts both complement earlier experimental work documenting the impacts o f the granivorous rodents and folivorous mammals on desert plant commu nities and now demonstrate the important influence of avian granivores .