Introduction. Animal consumers have important roles in ecosystems (Chew 1974, 1976), determining plant species composition and structure (Harper 1969, Pacala and Crawley 1992, Crawley 1983, 1989), regulating rates of plant production and nutrient cycling (Naiman 1988, McNaughton et al. 1989, Holland et al. 1992), and altering soil structure and chemistry (Milchunas et al. 1993, Huntly 1991). Desertification of semi-arid grasslands in the Southwest United States by domestic livestock provides an important example of herbivore regulation of ecosystem structure and function (Schlesinger et al. 1990). The species composition and physical structure of these desert grassland ecosystems were significantly altered by alien herbivores about 100 years ago (Bahre 1991, York and Dick-Peddie 1968, Gardner 1951, Hastings and Turner 1980, Buffington and Herbel 1965, Dick-Peddie 1993). To what extent the spatial patterns of semi-arid shrubland and grassland plant production and soil characteristics are currently controlled by plant resource use, abiotic factors, or consumers is not known. Desertification is an ecosystem-level phenomenon occurring on a global scale with great relevance to human welfare (Nelson 1988). In order to understand the processes that contribute to desertification, we must fully understand interactions among the components of arid-land ecosystems. Schlesinger et al. (1990) suggest that in the absence of continued livestock perturbations, plant resource use and abiotic factors appear to be the principal factors accounting for the persistence of desert shrublands in desertified semi-arid grasslands. However, Brown and Heske (1990a) provide evidence that indigenous small mammal consumers may also have a major role in determining vegetation structure in those desert ecosystems. Brown and Heske (1990a, Heske et al. 1993) found that the exclusion of rodents from Chihuahuan Desert creosotebush shrubland areas resulted in a significant increase in grass cover over a 12 year period. Brown and Heske (1990a) concluded that rodents were keystone species in those desert shrub communities, greatly influencing vegetation structure. Rodents are also known to have significant influences on plant species composition and diversity in desert communities (Inouye et al. 1980, Heske et al. 1993, Brown et al. 1986). Several species of granivorous rodents (Family: Heteromidae, genera: Dipodomys, Perognathus, Chaetodipus) appear to have the greatest influence on vegetation herbivory. Soil disturbance through the digging activities of rodents can have profound local effects on plant species composition and vegetation structure in the Chihuahuan Desert (Moroka et al. 1982). Digging activities of desert rodents intermix surface soils with subsurface soils (Abaturov 1972), and increase rainfall infiltration (Soholt 1975). Reported measures of the percentage of desert soil surface areas disturbed by rodent digging activities in desert enviroments range from 10% (Abaturov 1972) to 4.5% (Soholt 1975). Burrowing activities increase local soil nutrient and water status, creating favorable sites for increased plant densities, biomass production, and increased species diversity (Morehead et al. 1989, Mun and Whitford 1990). Rabbits (Lagomorpha: Black-tailed jackrabbits, Lepus californicus, and desert cottontail rabbits, Sylvilagus aduboni) are also important consumers of desert vegetation (Brown 1947, Johnson & Anderson 1984, Steinberger and Whitford 1983, Ernest 1994). Rabbits can have significant effects on plant species composition and structure resulting from selective herbivory (Gibbens et al. 1993, Clark and Wagner 1984, Norris 1950, Zeevalking and Fresco 1977). Gibbens et al. (1993) found that excluding rabbits from Chihuahuan Desert creosotebush (Larrea tridentata) communities over a period of 50 years increased the canopy cover of some grasses, and also increased canopy cover of some shrub species. Small mammal (rodent and rabbit) populations may fluctuate considerably with variation in climate and annual plant production (Brown et al. 1979, Brown & Heske 1990, Brown & Zeng 1989, Whitford 1976, Johnson & Anderson 1984). Reproduction in desert rodents is known to be induced by plant foliage production (Reichman and Van De Graff 1975, Beatley 1969). If small mammals are keystone species affecting plant species composition and structure in desert ecosystems, then the impacts of small mammals on vegetation are probably linked with variation in climate and plant production. A reciprocal plant-herbivore/granivore feedback system may result, where small mammal populations and thus impacts on vegetation, are initially determined by climate influences on plant food resource availability to the small mammals. Thus, the effects of small mammals during dry years will probably be different from the effects during wet years because of different population sizes. If this is so, one should be able to measure differential effects of small mammals on plant communities over series of wet or dry years, such as El Nino and La Nina cycles (Nicholls 1988). Such reciprocal interactions should also occur in relation to long-term (decades) climate change. The effects of any one small mammal species population on the biotic community will be complicated by competitive interactions with other mammal species (Munger & Brown 1981, Brown & Zeng 1989, Brown & Heske 1990), however overall impacts on vegetation and soils by the combined effects of all small mammal species may be closely linked with variation in precipitation and plant production. Depending upon the persistence of plant food resources such as foliage or seeds, lag times in consumer impacts may be expected following periods of precipitation and plant production. In desert ecosystems, widely scattered shrubs produce a patch pattern of fertile islands with high plant biomass production and soil nutrients, surrounded by relatively unproductive barren soil (West and Klemmedson 1978, Crawford and Gosz 1982). Researchers at the Jornada Long-Term Ecological Research site in New Mexico have proposed a desertification model suggesting that perturbations caused by domestic livestock grazing and climate change initiated processes transforming grasslands with relatively homogeneous resource distributions to shrubland environments with relatively heterogenous resource distributions (Schlesinger et al. 1990). This patchy vegetation/resource distribution pattern is stable under present climate regimes, and appears to be maintained by plant resource use and abiotic soil processes (Schlesinger et al. 1990). However, Wagner (1976, page 195) suggested that small mammals were probably maintaining shrubland dominated ecosystems at the Jornada by suppressing grasses through selective herbivory. Research Hypotheses. The purpose of this study is to determine whether or not the activities of small mammals regulate plant community structure, plant species diversity, and spatial vegetation patterns in Chihuahuan Desert shrublands and grasslands. What role if any do indigenous small mammal consumers have in maintaining desertified landscapes in the Chihuahuan Desert? Additionally, how do the effects of small mammals interact with changing climate to affect vegetation patterns over time? This study will provide long-term experimental tests of the roles of consumers on ecosystem pattern and process across a latitudinal climate gradient. The following questions or hypotheses will be addressed. 1) Do small mammals influence patterns of plant species composition and diversity, vegetation structure, and spatial patterns of vegetation canopy cover and biomass in Chihuahuan Desert shrublands and grasslands? Are small mammals keystone species that determine plant species composition and physiognomy of Chihuahuan Desert communities as Brown and Heske (1990a) and Gibbens et al. (1993) suggest? Do small mammals have a significant role in maintaining the existence of shrub islands and spatial heterogeneity of creosotebush shrub communities? 2) Do small mammals affect the taxonomic composition and spatial pattern of vegetation similarly or differently in grassland communities as compared to shrub communities? How do patterns compare between grassland and shrubland sites, and how do these relatively small scale patterns relate to overall landscape vegetation patterns? 3) Do small mammals interact with short-term (annual) and long-term (decades) climate change to affect temporal changes in vegetation spatial patterns and species composition? Other Consumers. Ants are important consumers in Chihuahuan Desert ecosystems (MacKay 1991), and granivorous ants are known to have competitive interactions with rodents (Brown & Davidson 1977, Brown et al. 1979) for plant seed resources. Termites are important detritivores in Chihuahuan Desert ecosystems (MacKay 1991) and appear to have key roles in plant litter decomposition and nutrient cycling (Whitford et al. 1982, Schaefer & Whitford 1981), and in altering soil structure and hydrologic processes (Elkins et al. 1986). Grasshoppers are important herbivores in Chihuahuan Desert ecosystems (Rivera 1986, Wisdom 1991, Richman et al. 1993), with various species specializing on most of the different plant species present in any location (Otte 1976, Joern 1979). Since manipulations of small mammals will probably affect these arthropod consumers, we will monitor these other consumers on the measurement plots to document any changes. Documentation of changes or lack of changes in ant, termite, and grasshopper consumer groups will be needed to interpret the results of small mammal manipulations on vegetation and soils. For example, if removal of rodents results in an increase of seed-harvesting ants, changes or lack of changes in vegetation and soils may be attributed to compensatory granivory from the increase in ants. Small mammals are the consumer group that appears to have the greatest influence on Chihuahuan Desert communities (see literature citations above). Given the known ecological importance of small mammals and the complexity and difficulties that would be associated with manipulating small mammals and arthropods, we have chosen to start with experiments on small mammals first. If these other consumer groups appear to have important interactions with small mammals, we will pursue additional experiments in the future to focus on those interactions, and to elucidate the ecological roles of these arthropod consumers.
