Small
mammals such as rodents and rabbits influence the
structure and function of grassland ecosystems as
consumers of plants and arthropods, as movers of soil and
soil nutrients, and as the primary prey of raptors,
snakes, and carnivorous mammals. Because of their
intermediate trophic position and high dispersal
abilities, small mammals may track changes in biotic and
abiotic environment that result from shifts in land-use
practices and other human activities. These populations
are variable over space and time, however, and their
responses to environmental change may not be immediately
apparent because of the behavioral flexibility and
relatively long life spans and generation times of these
animals. Long-term studies of population and community
dynamics therefore are needed to fully understand the
role of small mammals in grassland ecosystems.
One of the central missions of the Long-Term Ecological Research program is to document the spatial and temporal distributions of populations representative of trophic structure. Direct consumption of primary plant production by small mammals is a relatively small component of energy flow through the shortgrass-prairie ecosystem of the central Great Plains, but these animals may still have significant effects on other consumers, in several ways. First, many prairie rodents are omnivorous, and may consume significant numbers of arthropods, including species that may be agricultural pests. Second, rabbits and other small mammals are the most important prey of hawks, eagles, owls and coyotes that live in shortgrass prairie. Third, herbivorous and granivorous species may affect vegetation structure and diversity directly by consuming taller grasses and shrubs, or indirectly, through differential seed predation and dispersal. Finally, the movement of soil and soil nutrients by fossorial rodents may influence soil hydrology and plant diversity; the burrow systems created by these rodents are critical as refuges for other small animals in a grassland with little vegetative cover. Patterns in the distribution and abundance of small mammals, thus, simultaneously reflect and affect the stability of the shortgrass-prairie ecosystem. Understanding the roles of small mammals in this system is particularly important because of the historical and current exploitation of prairie landscapes for agricultural use and for urban and residential development.
Population Monitoring Studies
In 1994, we began field studies that would permit us to monitor changes in populations of small mammals on the Shortgrass Steppe Long-Term Ecological Research (LTER) site in north-central Colorado. Our objectives are to:
- Assess spatial and temporal patterns of abundance and species composition of small mammals among representative prairie cover types;
- Provide baseline information to aid future population and ecosystem-level studies on the site;
- Establish a long-term database that could be used in comparative studies in association with data collected at other sites, including others in the LTER network.
Currently, these efforts are focused on populations of nocturnal rodents and rabbits. In addition, we collect scats from coyotes (Canis latrans) and swift foxes (Vulpes velox) in association with our rabbit studies to track temporal changes in the relative abundance of these important predators on the study area.
LTER/CPER Carnivore Scat Count
a. Nocturnal rodents
We estimate population
density of nocturnal rodents during 
late spring (April/May) and late summer
(August/September) each year. Trapping webs (3.14 ha)
contain 124 large Sherman live traps arranged in 12 100-m
transects radiating from a central point (10 m between
stations and four traps in the web center). Traps are set
each evening for 4 consecutive nights in each session. We
record the sex, age and reproductive condition of
captured animals and mark each with a felt marker to
allow us to distinguish recaptures from new animals.
Program DISTANCE is used to calculate population
densities. Currently, we sample populations on three webs
in each of two vegetation cover types:
Grassland: Rolling plains dominated by Bouteloua gracilis and Buchloe dactyloides with prickly-pear (Opuntia polycantha) and fringed sagewort (Artemisia frigida). This is the most common cover type in shortgrass prairie.
Saltbush: A mixture of grassland and shrub vegetation with four-wing saltbush (Atriplex canescens) and several species of small shrubs (A. frigida, Gutierrezia sarothrae, Eriogonum effusum) and mid-grasses (Pascopyron smithii, Stipa comata, Sitanion hystrix). Found primarily in coarsely textured soils in swales and drainages, this cover type represents less than 20% of the vegetation on the Central Plains Experimental Range.
Long-Term Monitoring of Vegetation on Small Mammal Trapping Webs (pdf)
Sixteen
rodent species are present in northern regions of
shortgrass prairie, but to date, four species make up
more than 90% of all individuals captured (Table 1). Most
species reach highest densities in saltbush areas, and
only grasshopper mice are regularly captured in both
vegetation types. Pocket gophers (northern pocket gopher,
Thomomys talpoides; plains pocket gopher, Geomys
bursarius) and thirteen-lined ground squirrels (Spermophilus
tridecemlineatus) are common on our study area, but
are not usually captured during our nocturnal trapping
studies. Other species captured include the plains
harvest mouse (Reithrodontomys montanus), prairie
vole (Microtus ochrogaster), hispid pocket mouse (Chaetodipus
hispidus), and silky pocket mouse (Perognathus
flavus).
Long-term Population Monitoring of Nocturnal Small Mammals on Shortgrass Prairie (ARS #118) and Long-Term Population Monitoring of thirteen-lined ground squirrels (Spermophilus tridecemlineatus) (pdf).
In addition, the abundance of arthropod prey during the growing season is monitored.
b. Rabbits
Population densities of rabbits are estimated on the
study area from spotlight counts conducted during
January, April, July, and October of each year. We
spotlight rabbits on one night in each season along a
32-km route on and adjacent to the study area. The route
encompasses a range of vegetation types, including
grassland, saltbush, yucca-sandstone bluffs, and some
agricultural fields. As in the rodent studies, we
calculate densities using Program DISTANCE.
Three rabbit species are present on shortgrass prairie. Desert cottontails are widespread, occurring in both native and disturbed areas and in abandoned black-tailed prairie-dog (Cynomys ludovicianus) towns. Black-tailed jackrabbits are the most common jackrabbits and are found primarily in saltbush areas. The white-tailed jackrabbit is a grassland species and may be more abundant farther east on the Pawnee National Grasslands where saltbush is less common.
For further information please contact:
Paul Stapp
Dept. of Biological Science, California State University, Fullerton, CA
92834-6850
714-278-2949, pstapp@fullerton.edu
or
Mark D. Lindquist
SGS LTER Site, Nunn, CO, 80648
970-897-2210 mark.lindquist@colostate.edu
01/22/08