Showing papers by "Thomas J. Valone published in 1995"

Effects of competition, colonization, and extinction on rodent species diversity

Abstract: Analyses of long-term experimental data from the Chihuahuan desert revealed that species diversity of other rodents was higher on plots from which kangaroo rats (Dipodomys spp.) had been removed. The difference was due to consistently higher colonization and lower extinction probabilities of small granivorous rodents in the absence of competitively dominant kangaroo rats. The results of this ecosystem experiment demonstrate the importance of both competitive exclusion and metapopulation dynamics for biological diversity in a natural community.

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

Abstract: Guo, Q., Thompson, D.B., Valone, T.J. and Brown, J.H. 1995. The effects of vertebrate granivores and folivores on plant community structure in the Chihuahuan Desert. - Oikos 73: 251-259. 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.

Catastrophic decline of a desert rodent, dipodomys spectabilis: insights from a long-term study

Abstract: We describe the catastrophic decline of a species of keystone rodent at a Chihuahuan Desert site. The population density of Dipodomys spectabilis averaged 7.6 individuals/ha from 1977 to 1983, but declined to near extinction over the winter of 1983–1984. We propose hypotheses to explain this decline: 1) disease; 2) damage to seed stores by heavy precipitation; 3) contamination of seed stores by mycotoxins. Available data cannot conclusively distinguish these possibilities, but they implicate tropical storm Octave and favor hypotheses 2 and 3. The decline of D. spectabilis was followed by increased abundance of three species of a rodents and a sharp decline in another, Perognathus flavus . Long-term studies can provide unique insights into the nature, magnitude, and consequences of infrequent catastrophic disturbances.

Effects of grazing on the abundance and diversity of annual plants in Chihuahuan desert scrub habitat.

Abstract: We assess the impact of release from cattle grazing on the abundance and diversity of both winter and summer annual plant communities at an upper Chihuahuan Desert scrub site in south-eastern Arizona. In contrast to previous studies, we found that removal of herbivores (cattle) had little impact on ephemeral plant assemblages at our site. The total number of summer annual individuals per quadrat did not differ significantly, but there were significantly more winter annual plants on ungrazed quadrats. The number of species per quadrat, however, did not differ significantly between sites exposed to, or protected from, grazing in either season. Of 79 annual species recorded (34 in winter, 45 in summer), only 2 species, 1 in each season, responded significantly to the removal of cattle: Stephanomeria exigua and Polygala tweedyi were more abundant on ungrazed plots. Three additional species, Eriastrum diffusum and Cryptantha micrantha in winter, and Mollugo cerviana (summer), approached statistical significance. Differences in the effect of cattle grazing on annual plants between our results and those at other sites in the arid southwest most likely reflect differences in the speed of response by annuals in different areas. Comparisons of this with other studies underscores recent calls for studies at broader spatial and greater temporal scales.