WSRC-MS-99-00310

L. D. Wike and F. D. Martin
Westinghouse Savannah River Company
Aiken, SC 29802

H. G. Hanlin and L. S. Paddock
University of South Carolina
Aiken, South Carolina 29801

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An opportunity to study the response of a small mammal community to restoration of a riparian wetland was provided by the Pen Branch project at the Savannah River Site (SRS). Live trapping of small mammals was conducted on six transects at Pen Branch in 1996 and 1998 and at three transects at Meyer's Branch, an unimpacted stream at SRS, in 1997 and 1998. Distributions of rates of capture of the four most common species were both spatially and temporally uneven. Kruskal-Wallis one-way analysis of variance found no significant differences in the relationship of capture rates between species and between treatment and both the within-stream control and Meyers Branch. Habitat use and movement within stream corridors appears to be dependent primarily on species, with age and sex perhaps contributing to preference and distance moved. The lack of differences in capture rates related to transect or treatment may be due to the close proximity of sample transects relative to the movement potential of the species sampled.

Key Words: small mammals, wetland restoration, riparian habitat

In the southeastern United States small mammal populations associated with riparian wetlands have not been widely studied (Thurmond and Miller, 1994; Mitchell, et al., 1995; Mitchell, et al., 1993). Information on the response of these populations to restoration of impacted wetland areas has not been widely published. Small mammal communities in the Southeast are somewhat depauperate in number of species (Kolka, et al., 1998 and Table 1), in the region of this study, for example, there are only 10 commonly occurring species of murid rodents.

The opportunity to study the response of a small mammal community to restoration of a riparian wetland was provided by the Pen Branch project at the Savannah River Site (SRS) (Figure 1). As a receiving stream for reactor effluent, Pen Branch underwent dramatic changes between 1954 and 1988. Greatly increased flows and high temperatures severely altered all aspects of the stream corridor. When reactor operation ceased in 1988, natural processes began to influence the area and establish communities within the corridor. Remediation efforts began in 1992 and many programs were initiated to evaluate response of the various biotic communities.

As an important part of the food chain, small mammals serve as the main link between primary production and higher level consumers in the local ecosystem. In a riparian system those higher level consumers are represented by a wide variety of predators including larger snakes, various carnivores, and owls and hawks. Rapid turnover in small mammal populations influences the transfer of nutrients and energy from localized vegetative production into more mobile organisms that range widely through out the regional landscape.

The restoration activities at Pen Branch provided an opportunity to compare the response of small mammal populations between manipulated experimental areas, control areas allowed to follow the course of localized succession, and an undisturbed portion of Meyers Branch as a baseline. Because our trapping efforts caught primarily murid rodents, our analyses are focused there.

Small mammal trapping was conducted at Pen Branch in 1996 and 1998. Trapping at Meyers Branch took place during 1997 and 1998. Resources were not available to trap both areas in 1996 and 1997. Trapping was done for 18 consecutive days and began June 11th in 1996, July 22nd in 1997, and June 16th in 1998. At Pen Branch six transects were established corresponding to reptile and amphibian sample lines (Kolka, et al., 1997) at locations indicated on Figure 2. Lines were numbered from one through six and lines one, three, and six were in control areas while lines 2, 4, and 5 were in treatment zones (Figure 2). Eight sample points, labeled A through H were established on each transect. In 1996 each of the sample points roughly coincided with coverboard arrays established by the reptile and amphibian program. In 1998 the coverboard arrays no longer existed, but sample points were placed in the same general locations. During both years, wooden live trap "rabbit boxes" were placed at the ends and center of each transect to determine the presence or absence of mammals like rabbits, raccoons, and opossums. At Meyers Branch three trap lines were established with 6 sample points on each line. At Pen Branch, each transect began and ended in the adjacent riparian forest, the lines at Meyers Branch began in the adjacent woodland but did not cross the entire flood plain. In all years and at both streams four Sherman® live traps were placed at each sample point. Traps were baited with a mixture of birdseed, sunflower seed, oats, and peanut butter. Traps were checked each day for 18 consecutive days. Captured animals were identified, weighed on appropriate capacity spring scales, sexed, and marked before release. In 1996 and 1997 animals were fitted with numbered ear tags for individual identification. In 1998 animals were toe-clipped for capture location but not individual identification. During the study, 2592 trap nights were accumulated in Meyers Branch and 6840 in Pen Branch. Statistical analysis of the data was accomplished using Systat 7.0.

Table 2 contains a list of the small mammal species that we encountered during this study. The only species which were common enough to do any kind of analysis on were the four murid rodents, the eastern woodrat, the marsh rice rat, the cotton mouse and the hispid cotton rat. All other species were either represented by fewer than ten individuals or were only observed and not actually captured.

Distributions of rates of capture of animals were both spatially and temporally uneven. For the four most common species within Meyers Branch there were much lower capture rates in 1997 than in 1998 while in Pen Branch it depends on which species as to whether 1996 had higher capture rates than 1998 (Table 3). For woodrats it appears that capture rates were much higher for Meyers Branch than for Pen Branch while for cotton rats the opposite seems to be the case. There is no apparent pattern for rice rats and cotton mice. When data were analyzed using the Kruskal-Wallis one-way analysis of variance, no significance was found for capture rates of any species either between creeks or among transects within a creek.

Because the four species we examined have quite different habitat requirements, we examined the distribution of captures relative to distance from the streams (Table 4). Woodrats have no easily discernable pattern. Rice rats are captured most frequently one or two stations away from the stream channel while cotton rats are captured most frequently two stations away from the stream channel. Cotton mice, on the other hand, are captured most frequently at the stations farthest from the stream channel at Pen Branch, indicating a preference for the woods as opposed to the bottomlands. The pattern for cotton mice at Meyers Branch is less clear because, except for the station nearest the access road, the rest of each transect is on the wide, flat floodplain with little contour variation.

Because of the high percentage of recaptures (Table 5) we feel that we were sampling a large proportion of the populations in these areas that are susceptible to these trapping techniques. For this reason we believe that we should be able to assess whether there are real differences in these populations between treated and untreated areas. We used the Kruskal-Wallis one-way analysis of variance to assess relationship of capture rates between species and between treatment and both the within-stream control and Meyers Branch and found no significant differences.

Vagility of the species involved may explain the lack of differences. As one measure of vagility we examined the capture locations of marked animals that were caught two or more times. Table 6 summarizes this data. By this measure cotton rats are the least vagile with almost 75 percent of all recaptures being at the same trap station where the immediately previous capture had occurred. Rice rats, on the other hand, appear to wander more so that only about half of recaptures were at the same trap station as the immediately previous capture.

Another possible measure of vagility is the percent of individuals that are recaptured at trapping stations on a different transect. By this measure cotton rats appear to be marginally more vagile (Table 7) with about 8 percent of individuals trapped more than once being captured on a transect other than the one where first capture occurred.

The four species considered here vary greatly in their habitat requirements. The rice rat is primarily a marsh or swamp species that occasionally goes into nearby grasslands to feed, the cotton rat is a grassland or oldfield species and the woodrat and cotton mouse are primarily woodland species which readily utilize bottomlands and swamps (Webster, et al., 1985).

Suitable habitat for cotton rats was quite fragmented in the Pen Branch corridor. This may have had an effect on the frequency of movement and distance moved. Diffendorfer, et al. (1995) reported that when patches of suitable habitat for cotton rats are large, movement distances are moderate. In that study smaller patches resulted in larger movement distances, but very small patches (32 m² patch size) resulted in almost no resident cotton rats to capture.

Subadult male cotton rats appear more likely to use wetlands than adults of either sex or subadult females (Lidicker, et al., 1992). For our data and for those individuals with gender identified, the sex ratios for cotton rats were 2.8 male :1 female in 1996 and 1.8 male : 1 female in 1998. This may be indicative of the higher probability of subadult males to occur in wetlands, but because of the large number of animals for which gender is unknown there can be no firm conclusion.

That no significant differences were found in capture rates of any of these species related to either trapping transect or treatment type is not remarkable. The control areas for Pen Branch are only 100-150 m wide and the entire Pen Branch study area is approximately 2.5 km in length. The degree of movement among transects together with the variances in trapping rates is certainly high enough to erase any predicted differences. This is consistent with the literature. Pournelle (1950 in Wolfe and Linzey, 1977) reported movements by individual cotton mice of up to 853 m with an average movement by males of 145 m and by females of 115 m. Cotton rats in other studies showed much less movement. Cameron, et al. (1979 in Cameron and Spencer, 1981) reported average daily movements of only 13 m. Debusk and Kennerly (1975 in Cameron and Spencer, 1981) reported that cotton rats displaced up to 300 m were still on familiar territory and thus still able to easily home. While most of the cotton rats in our data set moved little if at all, certain individuals moved large distances with one being captured at both transects 1 and 2.

Habitat type has little effect on movement distances for cotton rats. Slade and Swihart (1983) report that comparisons between pasture and old field habitat showed significant effects on movements in adult males and juvenile females but not for adult females, juvenile males or subadults of either sex. However, they did report that a dirt road running between the two habitat types was a very effective barrier to movements. The differences in habitat within the Pen Branch corridor were probably smaller than between pastures and old fields.

Stafford and Stout (1983) showed that dispersal in cotton rats, as opposed to ordinary movement, was not influenced by gender or by size class. We cannot say for sure whether our larger movements were ordinary movements or dispersal, but no sex or size dominates.

Another factor that might have changed the species makeup of our samples is delayed response to traps by particular species. Our traps only took a few short-tail shrews and golden mice. Both of these species have been shown to be trap neophobic (Smith, et al., 1980) and, had we extended the sampling period a few more days each year, would have probably been better represented in our samples.

Small mammal movement capacities have proved to be too large for our analysis to show differences among the treatments analyzed here. Further studies at larger scales, possibly augmented by radiotelemetric methods might prove valuable in ascertaining the effects of different treatments upon the dynamics of small mammal populations in restored wetlands.

This research was funded by DOE contract number DE-AC09-96SR18500 W. M. Fulmer and R. J. Roseberry provided exceptional assistance with fieldwork and took responsibility for its safe conduct. Numerous students and educators assisted with the field work; thanks to A. Corely, M. Fleirmans, J. Gass, C. Hesse, T. Parker, E. Stieve, N. Ta, L. Virgo, and H. M. Westbury. Special thanks to B. Bryant and M. Nix of WSRC Education Programs for helping to make this possible.

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