longterm

Long-term dataset

Dataset: 

Study number: 

11

Data set ID: 

210011002

Abstract: 

Standing biomass is sampled three times a year: in winter (February - March), before shrubs begin spring growth; in spring (May), when shrubs and spring annuals have reached peak biomass; in fall (late summer; October), when summer annuals have reached peak biomass but before killing frosts. At each sample date, each site is visited (order of sampling may vary, according to phenological stage of sites) and the dimensions of each plant on each quadrat are measured and recorded. Recorded for each observation are: date, zone, plot, quadrat #, species (4 letter acronym), observation # (one for each measurement of that species in that quadrat), cover (percentage of quadrat covered by canopy of that individual or species), height (vertical extent of that individual or species), count (if multiple individuals with the same dimensions are present), and phenological stage (Flowering/fruiting or Vegetative). Attention: These data are not appropriate for estimates of percentage cover. NPP-associated percent cover measurements were developed for and are used solely to provide the best estimate of biomass production. Becuase the methodology results in measurements of overlapping subcanopy systems and canopies of adjacent individuals, NPP percent cover measurements are not an appropriate measure of actual aerial plant cover. Doing so will result in inflated numbers for the "actual" vegetative cover.

Data sources: 

JornadaStudy_011_npp_quad_measurement_data

LTER Core Area(s): 

Keywords: 

Dataset: 

Study number: 

11

Data set ID: 

210011001

Abstract: 

These data sets contain calculated aboveground biomass values, by species, for each quadrat in each site for a given season. They are constructed (as outlined below) from the field data which are measurements of the physical dimensions (horizontal cover, vertical height) of plants or plant parts in the quadrats.

Objective is to monitor patterns (both temporal and spatial) of aboveground biomass across a range of ecosystem types; to allow the estimation of net primary production and its variability in those ecosystems; and to provide a quantitative description of plant community structure over time in those ecosystems.

Please refer to these publications to evaluate the appropriateness of these data for your intended use prior to contacting Debra Peters, Responsible Investigator, with a data request.

    Attention: These data are not appropriate for estimates of percentage 
    cover. NPP-associated percent cover measurements were developed for 
    and are used solely to provide the best estimate of biomass production. 
    Becuase the methodology results in measurements of overlapping subcanopy 
    systems and canopies of adjacent individuals, NPP percent cover 
    measurements are not an appropriate measure of actual aerial plant cover. 
    Doing so will result in inflated numbers for the "actual" vegetative cover. 
 
    Attention: Data through 2003 was replaced online per below on 9/22/2011. 
    Analyses and results for ANPP differ from previous uses of the data from 
    1989-1998 (Huenneke et al., 2002) in three ways: 
      (1) Yucca elata was removed prior to analysis because its growth form 
          results in large errors in biomass estimates from year-to-year, 
      (2) regressions between biomass and plant volume used an intercept equal 
          to 0 to be consistent with a recent study in a similar system 
          (Muldavin et al., 2008), and 
      (3) reference harvests obtained in extreme years resulted in adjusted 
          regression coefficents through time that reflect year-to-year variation 
          in ANPP. These changes result in ANPP values that are smaller compared 
          (Peters et al. submitted) with previous studies (Huenneke et al., 2002).

LTER Core Area(s): 

Dataset: 

Study number: 

86

Data set ID: 

210086003

Abstract: 

A creosotebush shrub study site and a black grama grassland study site have been established at each of the Sevilleta, Jornada and Mapimi research locations. Each study site is 1 km by 0.5 km in area. Three replicate experimental blocks of plots are randomly located at each study site to measure vegetation responses to the exclusion of small mammals, including rodents and lagomorphs. This dataset is for the Jornada Experimental Range, which also contains a cattle exclosure.  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 permanent one-meter by one-meter vegetation measurement quadrat is located at each of the 36 points. The percent of a quad covered in cryptograms was estimated by determining the percent of each 10 cm square within a quad containing cryptogams (See methods for a detailed explanation). Cryptogams include lichens, algae, and moss. This study is complete.

Data sources: 

data_JornadaStudy_086_smes_cryptogam_crust

LTER Core Area(s): 

Dataset: 

Study number: 

13

Data set ID: 

210013002

Abstract: 

        NOTE: 1. This dataset should only be used as a reference from this 
                         point forward (12/14/2071).
                    2. This dataset has been replaced by the NPP Soil Volumetric 
                        Water Content dataset 
                        (https://jornada.nmsu.edu/content/soil-volumetric-water-content-15-npp-sites-jornada-basin-lter-1989).
                        which contains the recalculated VWC values in addition to the 
                        adjusted raw count data from the hydroprobe.

Once a month soil water content measurements are made at 10 depths (where possible) at each of 10 access tubes at each of the 15 LTER-II sites using a neutron probe (Campbell Model 503DR Hydroprobe). Measurements are taken at 30cm, 60cm, 90cm, 120cm, 150cm, 180cm, 210cm, 240cm, 270cm, and 300cm when possible or to the greatest depth it was possible to install the access tubing before hitting impenetrable caliche. If fewer depths were measured, the missing depths have a zero in the raw data set of count values. These are changed to "." in converted water content data set. Calculated water content values equal to less than zero are changed to zero in converted water content data set. Converted water content values are a volume/volume relationship and represent cm3 water/cm3 soil. Data from neutron probe data logger is dumped to disk. Raw count data is then converted to water content using a Fortran program called WC2.FOR (water content for LTER-II). Water content data is then sorted by i.d. number using a LOTUS 123 macro found in WC2_SORT.WQ1 which creates final version of water content data set for the month in a separate file. Two files per month are saved: raw count data (mmddyy-2.RAW; ex. 121189-2.RAW where -2 indicates LTER-II NPP site neutron probe readings) and calculated/ sorted soil water content data appended to data file containing that year's data (NPPSWCyy.DAT; example NPPSWC89.DAT contains NPP soil water content data for 1989). Regression equation was derived by Mahlia Nash, and after datalogger upgrade was added to hydroprobe, by David Hudson, both working for Dr. Peter Wierenga (as of 1994 at university at Tucson (Hudson's phone # is 602-621-3236). See PROBE.HIS file for probe history and regressions used for different data periods when different probes were used. The hydroprobe used whenever possible is Hydroprobe Model CPN503DR (Campbell Pacific Nuclear, Pacheco, CA) with data logger. This probe has a 50mCi241 Am-Be source and a hydrogen detector. Neutrons encountering hydrogen become thermalized. The detector totals returning thermalized neutrons over a 16 second sample time which is the raw count value displayed. The raw count value is then substituted into the proper regression equation for cm3 of water per cm3 of soil.

Data sources: 

data_JornadaStudy_013_npp_soil_water_content

LTER Core Area(s): 

Dataset: 

Study number: 

13

Data set ID: 

210013001

Abstract: 

        NOTE: 1. This dataset should only be used as a reference from this 
                         point forward (12/14/2071).
                    2. This dataset has been replaced by the NPP Soil Volumetric 
                        Water Content dataset 
                        (https://jornada.nmsu.edu/content/soil-volumetric-water-content-15-npp-sites-jornada-basin-lter-1989).
                        which contains the recalculated VWC values in addition to the 
                        adjusted raw count data from the hydroprobe.

Once a month soil water content measurements are made at 10 depths (where possible) at each of 10 access tubes at each of the 15 LTER-II sites using a neutron probe (Campbell Model 503DR Hydroprobe). Measurements are taken at 30cm, 60cm, 90cm, 120cm, 150cm, 180cm, 210cm, 240cm, 270cm, and 300cm when possible or to the greatest depth it was possible to install the access tubing before hitting impenetrable caliche. If fewer depths were measured, the missing depths have a zero in the raw data set of count values. These are changed to \\".\\" in converted water content data set. Calculated water content values equal to less than zero are changed to zero in converted water content data set. Converted water content values are a volume/volume relationship and represent cm3 water/cm3 soil. The hydroprobe currently used is Hydroprobe Model CPN503DR (Campbell Pacific Nuclear, Pacheco, CA) with data logger. This probe has a 50mCi241 Am-Be source and a 3He detector. Neutrons encountering hydrogen become thermalized. The detector totals the returning thermalized neutrons over a 16 second sample time which is the raw count value displayed. The raw count value is then substituted into the proper regression equation for cm3 of water per cm3 of soil. Data from neutron probe data logger is dumped to disk. Raw count data is then converted to water content. Two files per month are saved: raw count data (mmddyy-2.RAW; ex. 121189-2.RAW where -2 indicates LTER-II NPP site neutron probe readings) and calculated/sorted soil water content data appended to data file containing that year's data (NPPSWCyy.DAT; example NPPSWC89.DAT contains NPP soil water content data for 1989). See PROBE.HIS file for probe history and regressions used for different data periods when different probes were used.

Data sources: 

data_JornadaStudy_013_npp_soil_water_content_raw

LTER Core Area(s): 

Dataset: 

Study number: 

86

Data set ID: 

210086002

Abstract: 

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?

Data sources: 

data_JornadaStudy_086002_smes_quad_plant_cover

LTER Core Area(s): 

Dataset: 

Study number: 

86

Data set ID: 

210086001

Abstract: 

This is data for perennial plant vegetation canopy cover measured from all SMES study plots, fall 1995. The purpose of this data is to provide ground-truth data for comparison with low-level aerial photographs of each study plot. Three, 29 meter lines were measured along three of six rows of the permanent vegetation measurement quadrats. Each line was measured at 10cm resolution for intercepts of perennial plant live canopy cover, and for bare ground. 10cm resolution is comparable to the resolution of the aerial photos. All plants were identified to the species level. These line-intercept measurements are taken once every ten years, at the same time that low-level aerial photographs are taken. These data will be compared to both decadal air photos, and annual measures of vegetation from one-meter2 quadrats on each plot to provide information on vegetation change over time relative to the various animal exclosure treatments.

Data sources: 

data_JornadaStudy_086_smes_plant_cover_line

LTER Core Area(s): 

Dataset: 

Study number: 

86

Data set ID: 

210086009

Abstract: 

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? Data collected for each captured rodent: habitat, trap night, trap web, recapture, species, sex, age, weight, reproductive status, reproductive condition.

Data sources: 

data_JornadaStudy_086_smes_rodent_trapping

LTER Core Area(s): 

Dataset: 

Study number: 

86

Data set ID: 

210086004

Abstract: 

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.

Data sources: 

data_JornadaStudy_086004_smes_quad_leaf_litter

LTER Core Area(s): 

Keywords: 

Dataset: 

Study number: 

86

Data set ID: 

210086006

Abstract: 

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.

Data sources: 

data_Jornada_086006_smes_rabbit_survey

LTER Core Area(s): 

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