SMES Study: Rabbit Survey Data

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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.

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Field data forms

Night road surveys are conducted four times each year during a full moon in winter (January/February), spring (April/May), summer (July/August), and autumn (October/November). The counts can be conducted during the full moon of either of the two months for a season, when ever the weather is best. The portion of road surveyed through the creostoebush environment (Route 1) runs 6.2 miles (9.98 kilometers) across the east bajada of the Dona Ana Mountains (dominated with creosotebush with scattered mesquite), from the USDA, JER south entrance on Jornada Road at the rock pillars, to the power-line road, past the C-GRAV and C-SAND sites, to the Weather Trailer gate. The grassland portion of the road (Route 3) runs 6.3 miles (10.14 kilometers) from South Well to USDA Pasture 13, along the west boundary of Pasture 13, then along the north boundary of Pasture 13 to Jornada Road at the "wind tunnel" site. One open bed pickup truck is driven approximately 12 miles per hour along each road route. The driver (data recorder) of the truck sets the trip odometer to zero at the start of each of the road segments (creosotebush and grassland). Two people (observers) in the back of the truck scan each side of the road for rabbits, each using a 100,000 candle-power spotlight. The observers shine the light beam in a 90 degree arc from the road ahead to the area perpendicular to the truck. The observers should watch for rabbits, and the reddish reflection of rabbits eyes in the light. Each time a rabbit is observed, the truck stops, and the observer guides the truck to a position perpendicular to the rabbit. The observer then calls out information to the driver who records the following information in the data book. (1) Date (2) Road route number (1 = creosotebush; 2 = grassland) (3) Rabbit species (LECA for black-tailed jackrabbit) or SYAU for desert cottontail, or UNKN for unknown if the observer cannot identify the rabbit to species), (4) Odometer mileage (when perpendicular on the road to the rabbit) (5) Perpendicular distance from the center of the road (truck) to the rabbit measured to the nearest meter by use of a metric rangefinder. (6) Direction of the rabbit (L=left of road center; R=right of road center; C=exact center of road) (7) Time (24 hour or military time). (8) Comments In addition, the data recorder must record the road mileage from the odometer for the START and END points as well as the mileage and time for the GATE that is passed through along each route. The terms START, END, and GATE are entered into the species field on the data sheet. At the start of each route, Comments must be made by the data recorder on the Moon Phase (full moon) at the beginning of the first route. Weather conditions (for example, scattered high clouds, mostly clear, no wind) should be commented on at the beginning and end of each route since wind and cloud cover conditions may change. NOTE: Rabbit surveys are conducted only when the moon is full, the skies are mostly clear, and winds are light (< 10 mph). NOTE: Bushnell Yardage Pro Compact 800 rangefinder (laser rangefinder) used beginning January 2002. See ASCII file, Bushnell800_rangefinder.inf for specs and other information related to the unit.

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There are two roads that the survey is conducted on, one in a creosotebush habitat and the other in a grassland habitat. For the creosotebush route (Road 1), drive to the USDA-ARS Jornada Experimental Range (JER) south entrance on Jornada Road and turn left (west) at Stone Pillars. Begin the survey here. This road continues to the south boundary gate of NMSU Chihuahuan Desert Rangeland Research Center (CDRRC) located at the powerline crossing. Turn right here (north) and go through the gate. Continue north on this powerline road and veer right when the road forks. Continue past LTER sites C-GRAV, Upper Trailer, C-SAND, and LTER Weather Station. The survey is complete for the creosote habitat when you reach the gate immediately north of the Weather Station. Route is 6.2 miles long. The grassland route (Road 3), drive to USDA-ARS JER South Well on Jornada Road. The route begins 1/4 mile from the South Well cattleguard on the road going northwest just after crossing the cattleguard on the east side of South Well. There is a yucca on the right side of the road about 4 feet high that serves as the starting point. Follow this road far side of Pasture 13 (boundary fence with CDRRC is next to road on your left while crossing Pasture 13). Turn right (northeast) just before cattle guard (don't cross into Pasture 8C), and proceed along fenceline. You will need to pass through a large green gate and continue the survey on the same road on into Pasture 11A. Dale Gillette's Scrape Site will be off on your right within a quarter mile after the green gate. The survey ends at the fork in the road encountered at about mile 6.3 shortly before it joins Jornada Road.


Quarterly [winter (January/February), spring (April/May), summer (July/August), and autumn (October/November)

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