response of marsh rice rat (oryzomys palustris) to ... - BioOne
76 The Southwestern Naturalist vol. 52, no. 1 FIG. 1—Population densities (estimate 6 SE) and total monthly precipitation (cm). Graph shows population densities for Oryzomys palustris (diamond) and Sigmodon hispidus (square) for comparison, and total monthly precipitation for each trap session. Low abundances and recaptures of S. hispidus precluded the use of MARK for population estimations; therefore, minimum known number alive (MKNA) was used. Standard error bars for August 2003 were smaller than other sessions due to a large number of recaptures per individual. August and December 2002 were trapping sessions during the inundation period. METHODS—Study Area—Our study was conducted from May 2002 to August 2003 at Peach Point Wildlife Management Area in southeastern Brazoria County, Texas. Peach Point Wildlife Management Area encompasses 4,174.5 ha and is bordered by the Intercoastal Waterway. The study grid was located about 2 km from this waterway and was ,10 cm above sea level, which contributed substantially to inundation of the area. Vegetation on and near the grid was dominated by coastal prairie grasses and forbs, including spikegrass (Distichlis spicata), wiregrass (Spartina patens), sea oxeye daisy (Borrichia frutescens), and saltwort (Batis maritima). Other species occurred on ,5% of the grid and included bulrush (Scirpus robustus), marsh elder (Iva frutescens), glasswort (Salicornia), and Carolina wolfberry (Lycium carolinianum). Inundation of Study Area—The average yearly precipitation of the area is 133.35 cm/yr (data from the National Oceanic and Atmospheric Administration, Freeport 2NW weather station, located ca.10 km from Peach Point Wildlife Management Area). The study area received normal levels of precipitation during our May 2002 sampling trip; however, our August and December 2002 sampling trips occurred during a 6month period of inundation (to the depth of 12 to 18 cm). Inundation was caused by heavy rainfall, including a tropical storm in September 2002 (Fig. 1). By March 2003, the area was no longer flooded and the ground was dry. Rodent Sampling—The rectangular grid was approximately 1.14 ha and comprised of 111 stations, spaced 10 m apart, with one Sherman live-trap per station. The southern border was irregularly shaped by the adjacent watercourse that fed into the Intercoastal Waterway. Due to inundation, not all trapping stations could be used during every trapping session (only 108 in December 2002). Mark-recapture sampling was conducted during 6 sessions (seasonally, ca. 3 to 4 mo apart) from May 2002 to August 2003. The population was sampled for 6 nights each trap session, except August and December 2002 (4 and 5 nights, respectively), by using a mixture of rolled oats and peanut butter as bait. All rodents captured were assigned a unique identification number by using toeclipping or passive integrated transponder (PIT) tagging. Upon capture, we recorded identification number, weight, age (adult, subadult, or juvenile; determined by examining pelage, size, and weight), sex, reproductive condition, and trap station. Blood samples also were collected as part of another study (McIntyre et al., 2005). Animals were then released immediately at their sites of capture.
March 2007 Abuzeineh et al.—Response of the marsh rice rat to inundation of habitat 77 Rafts—Due to inundation of the terrain during sampling sessions in August and December 2002, we attached all of our traps to 30.5-cm 3 15.2-cm 3 3.8-cm Styrofoam platforms or ‘‘rafts’’. Each trap was secured to the raft with wire to ensure that it did not move or fall off. To prevent it from floating away, the raft was anchored to the wire survey flag marking the station. Each raft offered a 7-cm to 8-cm platform in front of the trap door, allowing individuals to climb completely onto the raft before entering the trap. All traps in August and December 2002 were on rafts. Population Statistics—We used the Pollock and Otto (1983) population abundance estimation model (Mbh)in the software program MARK (www.warnercnr.colostate. edu/,gwhite/mark/mark.htm) to account for individual heterogeneity and trap response. Population densities were estimated by dividing the estimated population abundance by the estimated effective grid area. Effective grid area was estimated by extending the presumed grid boundaries in each direction (except into the watercourse, where there was no emergent vegetation) by a distance equal to one-half the mean maximum moved for that sampling session (using individuals with $3 captures) and then calculating the grid area based on the expanded boundaries. One-way analyses of variance (ANOVA) were used to determine whether maximum distances traveled were significantly different among the 3 sex-age groups (adult male, adult female, and subadult) during any trap session. If no significant difference was demonstrated, the same effective grid area was used for all 3 groups. Home Range Estimation—Home range areas of individuals were estimated using the 100% minimum convex polygon (MCP) method (Mohr, 1947) in the CALHOME software program (Kie et al., 1996). We included only those individuals captured 3 or more times in one sampling session. These included a few individuals captured 3 or more times at the same trap station, resulting in an estimated home range area of 0. We included these instances of 0 home range so as to not upwardly bias home range estimates. Differences among sex-age groups (adult male, adult female, and subadult) and among sampling sessions were analyzed using unbalanced 2-way ANOVA, with probabilities estimated by 2,000 permutation iterations for each analysis after the model of Anderson and ter Braak (2003). If results of 2-way ANOVA were significant, we used the Hommel-Shaffer (improved Bonferroni) adjustment (Thorpe and Holland, 2000) to determine which sex-age group, session(s), or both significantly differed from the others. Statistical analyses were done using programs written in Matlab, version 6.5 (www. biol.ttu.edu/Strauss/Matlab/matlab.htm). Population Turnover—We examined longevity and site fidelity indirectly. Sex and age of individuals encountered in more than one trap session were noted, as were any changes in their observed home-range size or location. RESULTS—Abundances and Densities—The dominant small mammal species captured on our study grid was O. palustris. Sigmodon hispidus also was captured on the grid; however, its presence was minimal and sporadic (,2% of captures). We captured 162 O. palustris over 3,717 trap nights. The one-way ANOVA of maximum distance traveled by adult males, adult females, and subadults revealed no significant differences among groups or trapping sessions. Therefore, the effective grid size was assumed to be equal among groups during each trapping session and ranged from 1.18 ha in May 2002 to 1.26 ha in December 2002 and March 2003. Estimated densities of O. palustris ranged from a low of 28.8 6 8.9/ha (mean 6 SE), which occurred during an inundated period, to a high of 49.6 6 0.6/ha during a non-inundated period (Fig. 1). Home Ranges—Overlap among home ranges occurred both within and between the sexes and ages during all of the trapping sessions. There was no significant difference in home range size among these groups (F 5 2.23, df 5 2, P 5 0.108) or interaction between group and season (F 5 1.13, df 5 10, P 5 0.270). However, there was a significant difference among trap sessions (F 5 2.93, df 5 5, P 5 0.021); the Hommel-Shaffer test revealed that home ranges during the May 2003 trap session (post-inundation) were significantly larger than those in all other months/sessions. Population Turnover—We captured 162 O. palustris during this study: 147 (90.7%) were caught in only one trap session, indicating a high turnover rate. Of 14 individuals captured in more than one session, only one adult male was captured during 3 seasons. When the movements of these 14 recaptured individuals were examined, we observed no apparent pattern with respect to direction or distance for either sex. Although not significant (likely because of low sample sizes), males were nearly 4 times as likely as females to be encountered during more than one trap session. DISCUSSION—Oryzomys palustris maintained a substantial population in a habitat that experienced long-term inundation (ground entirely submerged). Population densities did not differ significantly among sampling sessions or sex-age groups, although estimates were lower during and immediately following inundation. Maximum distance moved did not differ significantly among sessions or groups. Although home range sizes significantly differed among sessions, they did not differ among sex-age groups, and there was no significant interaction effect. An a posteriori examination of the among-session differ-