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 environments 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 heterogeneous 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.
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?
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.
This study is now being initiated at the Jornada and Sevilleta Long-Term Ecological Research (LTER) sites as a cross- site research project that is integral to the objectives of both programs. We plan to establish an additional site in the center of the Chihuahuan Desert at the Mapimi Biosphere Reserve (United Nations Man and Biosphere Program) research site in Mexico.
A regionalized research approach encompassing the Sevilleta, the Jornada, and Mapimi will provide a comparison of sites from a cooler, moister, predominately grama grassland region in the northern Chihuahuan Desert/Great Plains grassland transition in central New Mexico (i.e., the Sevilleta) to a hot, arid, predominately creosotebush desert shrubland region at the center of the Chihuahuan Desert in Mexico (i.e., Mapimi)(Figure 1). Establishing this project across such a climate gradient will provide a powerful test of small mammal/climate interaction effects on Chihuahuan Desert plant communities. Many of the same typical Chihuahuan Desert plant and small mammal species are present at all three research sites (Montana 1988, Gernot & Serrano 1981, Anderson 1972, Findley et al. 1975, MacMahon & Wagner 1986).
Research Sites. The Sevilleta LTER site is located at the US Fish and Wildlife Service, Sevilleta National Wildlife Refuge in central New Mexico (Figure 1). The Sevilleta is located at 34 north latitude, and the grasslands and shrublands are at about 1,550 meters in elevation. The grassland and shrubland areas have long- term mean annual precipitation of about 280 mm and mean annual temperature of about 13 C. The Sevilleta is predominantly black grama (Bouteloua eriopoda) and blue grama (Bouteloua gracilis) grassland, with less extensive creosotebush shrub communities.
The Jornada LTER site is located at the US Department of Agriculture, Agricultural Research Service, Jornada Experimental Range in southern New Mexico (Figure 1). The Jornada is located at 32 north latitude, and the grasslands and shrublands are at about 1,300 meters elevation. The long-term mean annual precipitation is 230 mm, and the mean annual temperature is 15 C. The Jornada is primarily creosotebush and mesquite (Prosopis glandulosa) shrubland with less extensive black grama grass communities.
The Mapimi Biosphere Reserve is located in the south-central Chihuahuan Desert in the northeastern portion of the state of Durango, Mexico (Figure 1). The Mapimi site is located at 27 north latitude, and the creosotebush shrublands are at 1,100 meters elevation. Mapimi has a long-term mean annual precipitation of 271 mm, and the average annual temperature is 21C. The Mapimi research site is primarily creosotebush shrubland with mesquite. Tabosa (Hilaria mutica) grasslands occur in the bottoms of basins.
A creosotebush shrub study site and a black grama grassland study site have been established at both the Jornada and Sevilleta, for a total of four study sites. The study sites were subjectively chosen to represent typical creosotebush shrub and black grama grassland communities on lower bajada or peidmont slopes at both the Jornada and Sevilleta. The study sites are located in areas that are not grazed by domestic livestock (except the Jornada grassland site; see experimental design below), to eliminate the confounding effects of livestock on vegetation and soil.
Each of the four study sites are approximately 1 km by 0.5 km in area. Three rodent trapping webs and four replicate experimental blocks of plots were randomly located at each of the four study sites to measure vegetation responses to the exclusion of small mammals (Figure 2). Treatments within each block include one unfenced control plot, one fenced plot to exclude rodents and rabbits, and one fenced plot to exclude rabbits only. The three treatments were randomly assigned to each of the four possible plots in each block independently. The Jornada grassland site is grazed by cattle, so an additional treatment plot of cattle fencing was randomly assigned to one plot in each of the four blocks. Each of the three or four plots in a replicate block are separated by 20 meters. Each block of plots is situated near a rodent trapping web. Distances between the four replicate blocks of plots at each study site varies among sites from 30 meters to 800 meters, depending upon the random coordinates.
Each block of experimental measurement plots consists of one unfenced control plot, and two (or three if cattle are present) fenced animal exclosure plots (Figure 2). Each experimental measurement plot measures 36 meters by 36 meters. A grid of 36 sampling points are positioned at 5.8-meter intervals on a systematically located 6 by 6 point grid within each plot. A 3- meter wide buffer area is situated between the grid of 36 points and the perimeter of each plot. A permanent one-meter by one- meter vegetation measurement quadrat is located at each of the 36 points (Figure 3). The control plots are not fenced.
One year of pre-treatment or pre-fencing measurement data will be collected in 1995 from all of the plots. Analysis of the pre-treatment data will reveal any differences between plots that are independent of the treatments. Fences will be constructed in the winter of 1995, and the long-term small mammal exclosure experiment will commence in spring of 1996.
One set of exclosure plots will be fenced with three-foot high wire hardware cloth of 1/4-inch mesh size to exclude rodents, and four-foot high poultry-wire fencing of two-inch mesh size to exclude rabbits. The hardware cloth screen will be buried inches to prevent animals from burrowing underneath the fence. The four-foot high poultry-wire fence will be positioned inside of and flush with the screen fence to exclude rabbits from the plot. A six-inch wide strip of metal flashing will be placed along the top of the hardware cloth, and attached outside of the poultry-wire, to prevent rodents from climbing over the fences. Steel fence posts and reinforcing bar (rebar) will be used to support the fences.
The second set of exclosure plots will be fenced with poultry-wire to exclude rabbits, but not rodents. Two-inch diameter poultry-wire mesh will allow access for all local rodent species at each site. A four-foot high poultry-wire fence will be positioned around the perimeter of each plot to exclude rabbits. Steel fence posts and rebar will be used to support the fences.
Fencing with hardware cloth and poultry wire will impede the ground surface movement of organic litter. Rainfall runoff transports plant litter in ground-surface sheet-flow, and in small ephemeral surface drainages or rills. To alleviate the problem of fences interfering with transport, litter accumulations on the up-slope sides of fences will be manually removed and lifted over the fences once every two months. This procedure will allow for the natural movement of soil surface organic materials on to and off of the fenced plots.
Litter catchment screens made of hardware cloth and poultry wire will be placed in all small drainages crossing the up-slope perimeters of each plot, including the unfenced control plots. Litter caught on the screens will be manually lifted over the fences of treatment plots, and placed just across the perimeters of control plots. Use of catchment screens should allow normal litter transport in rills onto and off of the plots. Use of catchment screens on the control plots will account for differences in water flow that the screens might create on the fenced plots.
Cattle are present (one-month low intensity winter grazing) at the Jornada black grama grassland site. Barbed-wire strands will be added to the rodent and rabbit, and rabbit only exclosure fences to exclude cattle from those plots. Additionally, a fourth measurement plot was added to each replicate block that will be fenced with barbed-wire to exclude cattle only to measure cattle effects on vegetation and soil. The cattle exclosure treatment is not intended to be part of the overall experimental design, but rather a way a accommodate for the presence of cattle at the site. There are no ungrazed black grama grassland sites at the Jornada that are large enough in area to support the small mammal exclosure study.
The same experimental design is proposed for the Mapimi research site, except that a creosotebush study site and a tabosa grassland study site will be installed instead of creosotebush and black grama grassland sites as at Sevilleta and Jornada. Tabosa is the dominant grassland at Mapimi, and no black grama grasslands occur at Mapimi. Additionally, the Mapimi site is grazed by cattle, so a fourth cattle exclosure plot will be included in each block of plots at the creosotebush and tabosa grassland study sites.
Rodent trapping webs are being used to determine the composition of rodent species at each study site, and to estimate densities of each species over time. The use of webs and distance measures to estimate rodent densities is statistically more robust than grid plot sampling and mark-release indices (Anderson et al. 1983, Buckland et al. 1993). Each rodent trapping web consists of a series of 12 equally spaced lines radiating from a central point. Each line consists of 12 trap stations. The first trap station is located 5 meters from the center, the next three at 5 meter intervals, and the remaining 8 at ten meter intervals. Each trap line is 100 meters long, and each web is 200 meters in diameter. The above rodent trapping web design has been used for six years at the Sevilleta LTER, and has recently been adopted by the US Centers for Disease Control and Prevention, as a standard technique for monitoring rodent populations.
Sampling and Measurements.
Rodent populations will be sampled from each of the three webs at each of the study sites twice each year, in the early (April-May) and late (September-October) summer. Sherman (H. B. Sherman Traps, Inc., Tallahassee, FL) live-traps are left open for three consecutive nights, and captured animals are recorded for three consecutive mornings. Each animal caught is identified, measured, and released at the same location where it was captured. Each animal is temporarily marked with a marking pen to determine recapture status for a given three-night sampling period. No permanent marking techniques are used. Rodent trapping is conducted at all 6 webs at a given research site over the same 3 night period. Rodent trapping at the Sevilleta, Jornada, and Mapimi will be conducted at the same time of year.
Rodent traps will be placed in all of the rodent exclosure plots for two nights, once every two months to remove any animals that may gain access to the exclosure plots. Animals caught in the exclosure plots will be released outside of the plots.
Persons working with rodents in the field will follow safety guidelines developed by the US Centers for Disease Control and Prevention (Mills, et al. in press), to reduce exposure to hantavirus, plague, and other rodent-vectored diseases.
Rabbit densities will be estimated from road transect surveys (Buckland et al. 1993) near each study site at the same times of year that rodents are sampled. Rabbit fecal pellets will be counted and removed from each of the 36 vegetation quadrats on each of the measurement plots. Rabbit pellets will be collected and saved for potential diet studies. Rabbit pellet census will provide a relative measure of rabbit activity on each of the control plots. The presence or absence of pellets in the exclosure plots will provide an indication of the effectiveness of the fences as a barriers to rabbits.
Animal created soil disturbance from digging activities will be measured (in square centimeters) from the soil surface of each of the 20 vegetation quadrats on each of the measurement plots. The presence or absence of soil disturbance in the exclosure plots will provide an indication as to the effectiveness of the fences as a barriers to small mammals.
Vegetation will be measured two ways on each of the measurement plots. Plant species composition, cover, and above ground foliage height will be measured from each of the 36 quadrats on each plot. The maximum height (cm) and total canopy cover (cm2) of each plant species will be measured from a one- meter by one-meter measurement frame that will be placed over each permanent vegetation measurement quadrat. The plant measurement frames are constructed of 1/2-inch diameter PVC pipe. The frames are one-meter on a each side with an internal grid of 100 ten-centimeter by ten-centimeter squares made from string. Four height-adjustable legs are attached to each corner of the frame to measure vegetation from ground level up to two meters in height.
This vegetation measurement frame design has been used successfully at the Jornada LTER site to measure plant canopy cover and heights for the past six years. Biomass estimates will be made for each plant species by use of species specific regression equations that have been developed at the Jornada LTER based on clipped foliage harvests. Vegetation measurements will be taken twice each year in the late spring (April-May) to include spring annual plants, and again in the late summer (September-October) at the end of the growing season to include summer annuals and peak perennial biomass.
Low-level aerial photographs will be taken to record total vegetation canopy cover and plant species composition throughout controlled camera positioned above each 36-meter by 36-meter study plot on a small weather balloon that will be held in position by tether lines. The weather balloon has been used successfully for low-level aerial photographs at the Sevilleta LTER site. Four separate aerial photographs will be taken of each 13 meter by 13 meter quarter of each plot to increase photographic resolution. Resolution of such photographs should be less than 1 centimeter, based on similar photographs taken at the Sevilleta LTER site. Complete plot vegetation map overlays will be prepared from the four photographs for each plot. Photographs will be ground-checked to verify plant species. Balloon photographs will be taken once each year at the end of the growing season in the autumn.
Seed harvester ants in the genus Pogonomyrmex are the principal granivorous ants at the Sevilleta, Jornada, and Mapimi research sites. Pogonomyrmex ants form large colonies and construct large soil mounds that are easy to observe. We will mark and map all Pogonomyrmex ant mounds, of each species, on each of the measurement plots, and inventory mounds once each year when the autumn vegetation measurements are made. In this way we can monitor relative abundance of granivorous ant colonies on all of the study plots.
Several species of termites occur at the Sevilleta, Jornada, and Mapimi. Gnathamitermes tubiformans, Paraneotermes simplicicornis, and Reticulitermes flavipes are the common termites of the Chihuahuan Desert, and all of these species construct earthen or mud casing from soil and saliva over dead plant material on which they are feeding above ground (Mackay & Whitford 1988). We will measure the surface area of mud casing on each of the one-meter by one-meter vegetation quadrats on each of the study plots, when vegetation is measured in the autumn. Termites in the Chihuahuan Desert are most active above ground after the summer rainy season, and accumulations of casing reach a peak in the autumn. Casing surface areas will be calculated from three-dimensional field measures. Measure of gallery casing will provide us with relative index of termite above-ground foraging activities on all of the study plots. We will not be able to identify casing to species, because we would have to destroy the casing to capture soldier and alate individuals that are necessary for species identifications.
We will visually sample grasshoppers on all study plots by walking six 30 meter by 1 meter strip transects along each of six vegetation quadrat lines on each study plot. Grasshoppers that are flushed from the transect line by the observer will be recorded by species, to provide data on species composition and abundance. Grasshopper sampling will be done twice each year when vegetation is measured. The flush-transect procedure is currently used at the Sevilleta, Jornada, and Mapimi research sites to monitor grasshopper populations.
The principal goals of data analysis will be to compare the species composition, canopy cover, and spatial distribution patterns of vegetation between animal exclosure plots and control plots in creosotebush and black grama grass communities, over time. In combination with mammal density data, and climate data, these analytic goals will provide information to address the above hypotheses.
Small mammal species densities will be estimated by the use of existing software packages developed for rodent trapping web and rabbit transect (road) data (Buckland et al. 1993). Standard parametric or nonparametric statistical procedures, including repeated measures analysis of variance and time series analysis will be used to test for differences in animal abundances and species composition between research sites and over time. Data for the arthropod consumers will also be analyzed by use of standard parametric and non-parametric procedures to compare treatments over time.
Vegetation quadrat data will be analyzed by use of standard parametric and nonparametric statistical techniques to test for differences in plant species composition, canopy cover, and biomass. Spatial patterns in vegetation canopy cover will be analyzed for each entire plot from the aerial photographs. Any number and size of systematic or random sample points may be taken from the aerial photographs for analysis. Quadrat variance techniques (Ludwig and Reynolds 1988) and geostatistical techniques (Clark 1979, Henley 1981, Legendre and Fortin 1989, Robertson 1987) will be used to document vegetation spatial and temporal patterns between treatments and sites. Computer image processing may also be used to measure vegetation canopy cover from aerial plot photographs.
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