last update July 7, 2000
The most striking pattern generated by the small mammal surveys was therelatively low diversity and density of species on the OWSVRA. Habitatssampled in this survey typically supported only 1-3 rodent species anddensities of 0.89-7.11 animals/ha. This pattern seems a result ofphysical and biotic factors operating at OWSVRArather than of impacts from off-road vehicle activity. Significantdifferences between ridingand control areas were not apparent for either small or large mammals.
It is well-known that species diversity and density of desert rodents are primarily determined by plant productivity, which, in turn, is correlated with patterns of annual precipitation. Many of the small mammal plots surveyed in this study were sparely vegetated and had low diversity plant communities (e.g. Mud Hills, Barrel Springs). Plots with the highest rodent diversities and densities (e.g. Anza Borrego, San Felipe Wash) were those with diverse and abundant vegetation. The low diversity/abundance of mammals on the Tule Wash plot was somewhat unexpected, as the habitat on that plot appeared suitable for rodents. However, this plot also supported a diverse and abundant ant community. Other studies have shown an inverse relationship between ant and rodent diversity and abundance, reflecting the fact that these taxa compete interspecifically for food (seeds).
Kutilek extensively surveyed large and small animal communities in the western portion of the OWSVRA in 1989 and the results of their survey can be compared to the present survey. In both studies, Dipodomys merriami was the most abundant rodent species, followed by D. deserti and Perognathus formosus. With the exception of D. merriami, the 1989 study found no difference in rodent abundances between sites experiencing off-road vehicle use and control sites; this pattern was observed for all species is the 1997 study.
Kutilek also reported the presence of three rodent species not captured in this survey (Peromyscus maniculatus, P. crinitus, and Neotoma fuscipes). At least two factors probably account for this difference. First, the 1989 survey utilized a larger number of sites sampled (16 vs. 8) and a more extensive trapping effort (7,200 trap-nights vs. 1,200 trap nights) than the present survey. Second. Sites in the 1989 survey were trapped during three different seasons (winter, spring and fall), while the present survey was limited to a single season (spring); two of the species captured in 1989 but not in 1997 (Peromyscus maniculatus and P. crinitus) were only captured in the fall by Kutilek.
Results of spotlight surveys for large mammals are relatively comparablebetween 1989and 1997. Both surveys found the black-tailed jackrabbit (Lepuscalifornicus) to be the most abundant species in the park, followed bythe the kit fox(Vulpes macrotis) and the coyote (Canis latrans). Also,both surveys found little difference in large mammal abundance between"riding" and control areas. The desert cottontail (Sylvilagusaudubonii) was observed in both the east and west parts of the park in1997, but was not reported in the 1989 study.
RECOMMENDATIONS
Long-term monitoring of the mammal communities on the OWSVRA is important for assessing temporal trends in species diversity and density and, in turn, for making management decisions. Temporal changes in mammal communities may be due to inter- year variation in physical and/or biotic factors, impacts of OHV activity or some combination of factors, and the monitoring of suitable control plots is very important for identifying factors impacting on mammal communities. It is recommended that monitoring of both control and experimental plots continue on at least an annual basis. Semiannual sampling (e.g. spring and fall) of plots would provide a seasonal perspective that would be desirable.
While the OWSVRA presently maintains an "open riding" policy, much of the area has not been directly exposed to OHV activity and the vast majority of the OHV activity on the area appears to be confined to a few heavily-used roads, trails and washes. It is recommended that this current pattern of usage be encouraged to the fullest extent possible, so that the creation of new roads and trails into undisturbed habitats is minimized. In addition, it would be useful for park personnel to map and monitor spatial patterns of OHV use within the OWSVRA. This could be done by surveying portions of the area in a systematic fashion at regular intervals for signs of new or expanding OHV activity (e.g. new roads and trails). Steps to restrict riding in relatively undisturbed habitats, thereby protecting plant and animal communities, could then be taken.