SMES rabbit survey data

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 and 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 and Heske 1990, Brown and Zeng
1989, Whitford 1976, Johnson and 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 and Brown 1981,
Brown and Zeng 1989, Brown and 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 and 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 and 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.