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Winter Relative Abundance and Habitat Associations of Swamp Rabbits in Eastern Arkansas
Allison Fowler and Robert E. Kissell, Jr.

Southeastern Naturalist, Volume 6, Number 2 (2007): 247–258

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2007 SOUTHEASTERN NATURALIST 6(2):247–258 Winter Relative Abundance and Habitat Associations of Swamp Rabbits in Eastern Arkansas Allison Fowler1,2 and Robert E. Kissell, Jr.1,3,* Abstract - Habitat loss coupled with decline in harvest has raised concern for Sylvilagus aquaticus (swamp rabbit) in Arkansas. We assessed relative abundance and habitat associations of swamp rabbits in eastern Arkansas using presence of latrine sites. We searched for fecal pellets at randomly chosen sites during winters 2002–03 and 2003–04. Swamp rabbits were detected at 85% and 76% of sites searched during years 1 and 2, respectively. Bignonia capreolata (crossvine) stem density was significantly greater at sites inhabited by swamp rabbits (mean = 625, SE = 141) than at uninhabited sites (mean = 0.00, SE = 0.00) in year 1, but no differences were found in year 2. We found no relationship between woodland tract size and pellet-group density. Logistic regression predicted presence of swamp rabbits 88.3% of the time based on crossvine density and percent ground cover of grasses. To understand the potential for conserving swamp rabbit populations in Arkansas, it will be necessary to assess remaining bottomland hardwood stands in relation to the quality of swamp rabbit habitat. Introduction Sylvilagus aquaticus Bachman (swamp rabbit) is primarily found in bottomland hardwood forests along rivers, streams, and swamps (Allen 1985, Chapman et al. 1982, Kjolhaug and Woolf 1988). Forest tract size is positively related to presence of swamp rabbits (Scheibe and Henson 2003). The forested wetlands of the Mississippi Alluvial Valley (MAV) have been reduced from 8.5 million ha prior to European settlement to < 2.0 million ha (Creasman et al. 1992). Wetlands and bottomland hardwood forests have decreased steadily since the 1960s, with more than 2.5 million ha being lost from Arkansas and Louisiana during that time (Harris 1984). The swamp rabbit is the least studied member of its genus despite its popularity as game (Allen 1985, Zollner et al. 2000). Rabbit harvest data from 1970 through 1990 from southern and midwestern states indicates that populations of both S. floridanus Allen (cottontail) and swamp rabbits declined during the last 30 years in all states except Georgia; the harvest of rabbits in Arkansas has followed the same trend (Dickson 2001). Swamp rabbits have been monitored periodically throughout the species’ range, but few surveys have been conducted in the lower MAV during the last 20 years (Dailey et al. 1993, Woolf 1998). Most studies have been conducted on the periphery of the range, including Indiana 1University of Arkansas-Monticello, Monticello, AR 71656. 2Current address - The Nature Conservancy Arkansas Field Office, 601 North University Avenue, Little Rock, AR 72205. 3Current address - School of Forest Resources, University of Arkansas at Monticello, Monticello, AR 71656. *Corresponding author - 248 Southeastern Naturalist Vol. 6, No. 2 (Terrel 1972, Whitaker and Abrell 1986), Missouri (Dailey et al. 1993, Korte and Fredrickson 1977, Scheibe and Henson 2003), Kentucky (Sole 1994), South Carolina (Platt and Bunch 2000), and Illinois (Barbour et al. 2001, Kjolhaug et al. 1987, Porath 1992, Woolf 1998). Presence of a species at the periphery of its range is expected to vary due to ecological tolerances (Odum 1971). Furthermore, population densities should have increased variability under negative anthropogenic influences in the core of the species range. Most swamp rabbit research has been an assessment of distribution (Dailey et al. 1993, Kjolhaug et al. 1987, Platt and Bunch 2000), and few studies have been conducted in the core of the range (McCollum and Holler 1994, Zollner 1993). Several habitat factors, such as canopy closure, herbaceous cover, and shrub cover, have been suggested as ecologically important to the species (Zollner et al. 2000). However, no studies have addressed the habitat characteristics that predict presence-absence across the landscape. Given the lack of information on swamp rabbit distribution and habitat associations in the core of its range, the objectives of this study were to: (1) determine the relative abundance of swamp rabbits at selected sites in eastern Arkansas, (2) examine the relationship between relative abundance of swamp rabbits and tract size, and (3) model the probability of swamp rabbit presence based on selected habitat variables. Study Area This study was conducted in bottomland hardwood forests in the MAV of eastern Arkansas. Land use was predominately agricultural, and major crops included rice, soybeans, wheat, and cotton. The majority of remaining forests in eastern Arkansas are bottomland hardwood forests, with the exception of those found at higher elevations on Crowley’s Ridge, which are characterized by upland species including Pinus echinata Miller (shortleaf pine) and Liriodendron tulipifera Linnaeus (tulip poplar). Several public land properties are managed primarily for waterfowl and are inundated during winter. In southeastern Arkansas, land was primarily private industrial forests dominated by P. taeda Linnaeus (loblolly pine), with some forests consisting of mixed pines and hardwoods. Study site selection We randomly chose 30 study sites, each at least 30 ha in size, from a list of public and private lands within the MAV of Arkansas. We used a minimum distance between sites of 20 km to assure independence between sites each year. The sample included 25 public and 5 privately owned sites (Fig. 1). Public sites included 16 wildlife management areas (WMA), 3 national wildlife refuges (NWR), 4 natural areas (NA), 1 national forest (NF), and 1 state park (SP). Private sites were owned by Plum Creek Timber Company, Inc., Anderson-Tully, and Potlatch Corporation timber companies. 2007 A. Fowler and R.E. Kissell, Jr. 249 Methods We conducted pellet surveys during winter (November–April) 2002– 2003 (year 1) and 2003–2004 (year 2). Sites were visited once per year and were treated as replicates. We searched sites on foot using 15 parallel-strip transects at each site. Strip transects were 100 m in length, 5 m wide, and 50 m apart. Following procedures by McCollum and Holler (1994), we placed strip transects perpendicular to the main flow of a local watercourse to maximize the chance of detecting swamp rabbits. Transects were placed at least 25 m from the water’s edge. We searched logs, stumps, and the ground for swamp rabbit latrines. Swamp rabbit fecal pellets were distinguished from cottontail pellets based on their significantly larger size (Whitaker and Abrell 1986). For each latrine detected, we recorded the type of latrine (log, stump, ground) and the number of pellets in each group. When pellets were found on a log or stump, the diameter of the log or stump was estimated ocularly to the nearest centimeter. When pellets were found on the ground directly adjacent to a log with pellets, it was assumed the pellets had fallen from the log, and all were recorded as one pellet group. The presence or absence of moss on a latrine was also recorded. We collected habitat data along transects the same day pellet surveys were conducted using nested, circular plots (0.10 ha, 0.01 ha, and 0.001 ha) (Bowman et al. 1998). One plot was located on each transect. Locations of plots (0, 50, or 100 m) alternated among transects so that the plot center was at 0 m on the first transect, 50 m on the second transect, and at 100 m on the third transect, and so on. Grasses and forbs were sampled within a 0.001-ha circular plot. They were identified to species, and the number of individuals of each species was Figure 1. Locations of study sites randomly selected for swamp rabbit fecal-pellet searches during winters 2002–2003 and 2003–2004 in eastern Arkansas. 250 Southeastern Naturalist Vol. 6, No. 2 recorded. Percent ground cover of grasses and forbs was estimated ocularly within the 0.001-ha plot. Shrubs (< 2 m in height and < 7.5 cm dbh) and vines were sampled within a 0.01-ha circular plot. Shrubs and vines were identified to species, and the number of individuals of each species was recorded. Percent ground cover of shrubs and vines was estimated ocularly. We used the point-quarter method to estimate tree density and composition (Brower and Zar 1984, Zollner et al. 2000). Using this method, we measured distance from the plot center to the nearest tree in each quadrant (NE, SE, NW, SW) to the nearest 0.1 m. We identified trees to species and measured dbh (cm). Within a 0.1-ha circular plot, we recorded the number of downed logs and stumps (􀂕 7.5 cm in diameter). We also recorded presence, depth, and percent cover of water. We estimated angular canopy cover (%) at the center of each plot using a concave densiometer. Tract size (ha) was estimated for each study site. In ArcView® GIS 3.2 (ESRI, Redlands, CA), differentially corrected GPS locations of searched transects were overlaid on color infrared digital orthophotographic quadrangles (UTM Zone 15N NAD83) for each study site. We digitized a new polygon theme around the perimeter of the woodland tract containing transects. We then calculated the area (ha) of each digitized tract. Statistical analyses The area of a transect was calculated in hectares. Pellet-group densities (number of pellet groups/ha) were then determined for each transect at each site. We compared mean pellet-group densities using 95% confidence intervals between years for each site surveyed (Payton et al. 2003, Schenker and Gentleman 2001). We compared mean numbers of pellets per group across latrine types, habitat variables between years and between inhabited sites and uninhabited sites, and differences in pellet-group densities between public and private sites using Mann-Whitney U tests. Because habitat variables were not independent, a Bonferroni adjustment was used (Rice 1989). We used linear regression to assess the relationship between pellet-group density and woodland tract size. We used stepwise logistic regression to estimate the probability of the presence of swamp rabbit latrines using combined habitat data from years 1 and 2. Results During year one, 20 sites were searched; flooding precluded the remaining 7 sites from being searched. Swamp rabbit latrines were observed at 17 of 20 sites (85%). During year two, 25 sites were searched. Swamp rabbit latrines were observed at 19 of these sites (76%). In year 1, logs, stumps, and the ground were used as latrines 53.5%, 13.0%, and 33.5% of the time, respectively. Mean diameter of logs used as latrines was 24.3 cm (n = 51, SE = 1.34) and mean diameter of stumps used as latrines was 34.5 cm (n = 14, SE = 4.76). The majority of latrines were covered with moss (57.3%). The number of pellets found per group ranged from 1–63 (n = 230, mean = 15.0, 2007 A. Fowler and R.E. Kissell, Jr. 251 SE = 0.96). Mean number of pellets found per group on the ground, on logs, and on stumps was 16.1 (n = 78, SE = 0.19), 13.6 (n = 121, SE = 1.29), and 18.8 (n = 29, SE = 2.99), respectively. In year 2, logs were observed as latrines most often (79.9%). Stumps and the ground were used as latrines 10.5% and 9.6% the time, respectively. Mean diameter of logs and stumps used as latrines was 21.9 cm (n = 57, SE = 1.21) and 23.1 cm (n = 20, SE = 1.57), respectively. The majority of latrines were covered with moss (62.3%). The number of pellets found per group ranged from 1–135 (n = 239, mean = 15.9, SE = 4.79). Mean number of pellets found per group on the ground, on logs, and on stumps was 10.8 (n = 23, SE = 1.69), 14.7 (n = 191, SE = 1.37), and 29.7 (n = 25, SE = 4.28), respectively. Mean number of pellets found on stumps was significantly higher than logs for year 2 (p = 0.002). During year 1, mean pellet-group densities ranged from 4.0 pellet groups/ha to 89.3 pellet groups/ha (n = 17, mean = 18.0, SE = 4.12) (Fig. 2). Pellet groups contained a mean of 15 pellets among sites (n = 229, SE = 0.97). During year 2, pellet-group densities ranged from 1.3 pellet-groups/ha to 49.3 pellet-groups/ha (n = 19, mean = 16.8, SE = 3.00) (Fig. 3). Pelletgroup densities among sites were not significantly different. Pellet groups contained an average of 16 pellets (n = 239, SE = 1.24). Pellet-group densities were statistically different between years for 5 of 19 sites searched during both winters (Table 1). Pellet-group densities did not differ significantly (p > 0.05) between public and private sites for either year or for both years combined. There were no differences (p > 0.05) in measured habitat variables across sites between years. For year 1, Bignonia capriolata Linnaeus Table 1. Swamp rabbit pellet-group densities for sites searched in eastern Arkansas during winters 2002–03 and 2003–04. “*” indicates difference based on 95% confidence interval. Site Winter 2002–2003 mean (SE) Winter 2003–2004 mean (SE) Bayou Meto WMA 12.00 (6.41) 34.67 (6.89)* Bayou de View WMA 5.33 (3.07) 0.00 (0.00) Casey Jones 3 WMA 4.00 (2.90) 8.00 (3.27) Casey Jones 6 WMA 10.67 (5.47) 14.67 (6.01) Cutoff Creek WMA 8.57 (5.95) 13.33 (5.75) Dagmar WMA 22.67 (5.81) 21.33 (4.56) Felsenthal NWR 5.33 (3.07) 0.00 (0.00) Hurricane Creek WMA 0.00 (0.00) 0.00 (0.00) Pine City WMA 6.67 (3.74) 5.33 (2.36) Potlatch a 8.00 (2.62) 18.67 (5.33) Potlatch b 28.00 (6.70) 13.33 (4.22) Scatter Creek WMA 0.00 (0.00) 0.00 (0.00) Seven Devils WMA 24.00 (8.55) 1.33 (1.33)* Smokehole NA 21.33 (4.13) 49.33 (10.49)* St. Francis WMA 21.33 (7.16) 10.67 (3.30) Stateline Sandponds NA 17.33 (5.47) 28.00 (5.09) Trusten Holder WMA 4.00 (2.90) 0.00 (0.00) Wattensaw WMA 89.33 (14.72) 9.33 (4.31)* White River NWR 18.18 (5.69) 4.00 (2.14)* 252 Southeastern Naturalist Vol. 6, No. 2 Figure 2. Swamp rabbit pelletgroup densities in eastern Arkansas during winter 2002–2003. Diamonds represent mean number of pellet groups/ha. Vertical lines represent 95% confidence intervals. Sites are listed from north (left) to south (right). (crossvine) stem density was significantly greater at sites where swamp rabbits were detected, but no differences between inhabited and uninhabited sites were detected in year 2 (Table 2). Tract sizes ranged from 158 ha to 27,882 ha across both winters. Mean tract size for year 1 was 6622 ha (n = 20, SE = 1612), and mean tract size was 5616 ha (n = 25, SE = 270) for 2007 A. Fowler and R.E. Kissell, Jr. 253 Figure 3. Swamp rabbit pellet-group densities in eastern Arkansas during winter 2003– 2004. Diamonds represent mean number of pellet groups/ha. Vertical lines represent 95% confidence intervals. Sites are listed from north (left) to south (right). 254 Southeastern Naturalist Vol. 6, No. 2 year 2. Tract size was a poor predictor of pellet-group density for year 1 (R2 = 0.0001) and year 2 (R2 = 0.0048). A logistic regression model selected the habitat variables crossvine density and percent grass cover to predict the occurrence of swamp rabbits (Table 3). The model predicted presence correctly 88.3% of the time and incorrectly 11.4% of the time. The model predicted presence/absence equally the remaining 0.03% of the time. Discussion Swamp rabbits occurred widely across eastern Arkansas, although during year 1, Hurricane Lake WMA and Black River WMA were flooded and Table 3. Logistic regression model parameters predicting the presence of swamp rabbits across eastern Arkansas in winter, 2002–2003 and 2003–2004. Parameter DF Estimate Standard error Wald chi-square P Intercept 1 -0.52460 0.65090 0.6496 0.4203 Percent grass cover 1 0.35530 0.24950 2.0274 0.1545 Crossvine density 1 0.00479 0.00216 4.8920 0.0270 Table 2. Results of Mann-Whitney U tests on selected habitat variables compared between sites inhabited with swamp rabbits and sites uninhabited during winter 2002–2003 in eastern Arkansas. Inhabited sites Uninhabited sites Habitat variable mean (SE) n mean (SE) n p-value 2002–2003 Basal area (m2/ha) 58.00 (8.01) 17 66.60 (5.63) 3 0.179 DBH (cm) 25.20 (1.15) 17 25.00 (1.57) 3 0.842 Canopy cover (%) 47.40 (3.64) 17 37.80 (4.08) 3 0.216 Shrub cover (%) 0.75 (0.43) 17 0.13 (0.13) 3 0.479 Vine cover (%) 2.23 (0.74) 17 1.58 (0.57) 3 0.921 Forb cover (%) 0.20 (0.05) 17 0.03 (0.00) 3 0.216 Grass cover (%) 2.77 (1.21) 17 0.01 (0.11) 3 0.012 Log density (logs/ha) 149.00 (34.7) 17 77.90 (16.6) 3 0.358 Stump density (stumps/ha) 17.60 (7.28) 17 3.02 (2.82) 3 0.146 Water cover (%) 10.50 (3.42) 17 14.90 (11.7) 3 0.842 Crossvine density (stems/ha) 625.00 (141) 17 0.00 (0.00) 3 0.004* 2003–2004 Basal area (m2/ha) 72.2 (6.03) 19 72.60 (14.9) 6 0.877 DBH (cm) 26.3 (0.92) 19 24.70 (1.88) 6 0.400 Canopy cover (%) 38.9 (4.73) 19 44.00 (10.5) 6 0.733 Shrub cover (%) 1.85 (0.45) 19 1.58 (0.93) 6 0.642 Vine cover (%) 9.74 (1.65) 19 5.78 (2.78) 6 0.106 Forb cover (%) 4.02 (2.48) 19 4.35 (2.80) 6 0.877 Grass cover (%) 4.96 (1.26) 19 1.57 (0.75) 6 0.176 Log density (logs/ha) 147 (8.44) 19 127.00 (26.7) 6 0.642 Stump density (stumps/ha) 17.1 (3.10) 19 14.60 (4.16) 6 0.877 Water cover (%) 2.16 (0.83) 19 5.75 (5.55) 6 0.333 Crossvine density (stems/ha) 769.00 (163) 19 1001.00 (934) 6 0.106 *Indicates a significant difference at 􀁟 = 0.05, adjusted for multiple pair-wise comparisons using a Bonferroni adjustment (􀁟 = 0.005). 2007 A. Fowler and R.E. Kissell, Jr. 255 drained prior to the searches, and pellets were not found. Rabbits may have been undetected at those two sites because they moved to higher ground and pellets were washed away by flood water. Rabbits were undetected both years at Hurricane Lake WMA, and year 2 at Bayou de View WMA and Trusten Holder WMA. The absence of rabbits at these sites is puzzling given that the habitat composition of these sites was comparable to other sites. It is possible that the specific sites searched within these large areas were not used by swamp rabbits, but rabbits may occur elsewhere in the areas. Logs were used most often as latrine sites during both years of the study. In south-central Arkansas, swamp rabbits used logs as latrines 91% of the time (Zollner et al. 1996). Heuer and Perry (1976) found pellet groups on the ground only rarely in Louisiana, but in Alabama, 60% of pellet groups were located on the ground (McCollum and Holler 1994). Zollner et al. (1996) found that increasing decay, percent cover of moss, and height of a log were correlated with use as latrine sites. Ground latrines were found most often during the spring and summer months, indicating that seasonality also has an effect on latrine choice. As in other studies, the majority of latrine logs we found were covered with moss. Zollner et al. (1996) suggested that rotten and mossy logs provide an absorptive surface on which olfactory depositions can persist longer. If latrines facilitate olfactory communication, individuals may be exchanging information about sex, breeding status, and physical condition. Zollner et al. (1996) detected latrines throughout the year, implying that females, subordinate males, and juveniles in addition to adult males may deposit feces on logs. This observation supports the idea that latrines facilitate olfactory communication. The behavior of swamp rabbits at latrines should be investigated further to ascertain the importance of latrine use. Mean number of pellets found on stumps was significantly greater than the mean number found on logs for year 2. Whitaker and Abrell (1986) also reported a greater mean number of pellets on stumps compared to logs. The surface area of stumps is higher, and the surface is often flatter. This would make pellets rolling off of stumps less likely than pellets falling off of logs, resulting in a longer persistence of pellets. Pellet-group densities were similar for all sites searched during year 1, with the exception of Wattensaw WMA. Mean pellet-group density was significantly higher at Wattensaw WMA than any other site. Wattensaw WMA had one of the highest percentages of grass and sedge ground cover. Also, the area surveyed was a Colinus virginianus Linnaeus (quail) restoration area, where hunting of quail and rabbits was prohibited. During year 2, all sites had similar pellet-group densities. A different portion of Wattensaw WMA was searched during year 2, yielding a lower pelletgroup density. This site was also closed to hunting; however, there was less ground vegetation in the form of sedges and grasses. Even though there was no rabbit hunting on the area, the higher amount of grasses and sedges 256 Southeastern Naturalist Vol. 6, No. 2 for forage was likely the reason for higher pellet counts, as hunting has not previously been shown to negatively impact rabbit populations (Connelly et al. 2005). It should be noted that there was no measure of harvest by site available for analyses. Therefore, the significance of hunting versus habitat quality is unknown. We found crossvine, an important winter food item, at all but two sites where swamp rabbits occurred during year 1. Crossvine was not found where swamp rabbits were not detected. Crossvine stem density was significantly higher at sites with swamp rabbits present during year 1. Smith (1982) found crossvine to be the most important food item for rabbits during winter in Mississippi. Crossvine was also identified by Toll et al. (1960) as an important winter food item because it retains its green leaves throughout the winter. Crude protein of crossvine averaged 13.5% in February and May in Mississippi (Smith 1982). When grasses and crossvine are the sole forage for swamp rabbits in winter, they provide 11–12% crude protein. Smith (1982) suggested this was the minimum amount needed for normal growth and development. The logistic regression analysis indicated a significant relationship between swamp rabbit presence and crossvine density and grass cover. As crossvine density and percent cover of grasses increased, so did the probability of detecting swamp rabbits. During winter in Arkansas, an adequate amount of grasses and crossvine is critical for swamp rabbit persistence. Thus, areas that remain inundated for extended periods during winter, or have closed canopies, resulting in a reduced amount of ground vegetation, may provide poor quality winter habitat for the species. Woodland tract size was not related to pellet density. This may indicate swamp rabbit populations were not denser on large tracts. The minimum tract size of 100 ha suggested by Korte (1975) does not appear to hold true in fragmented areas (Dailey et al. 1993, McCollum and Holler 1994, Sole 1994). Dailey et al. (1993) found tract size of private land was not significantly related to swamp rabbit presence. Habitat quality of woodland tracts is probably more important than size. For instance, large areas that are inundated for the duration of the winter may not be as suitable as smaller areas that remain relatively dry. Large, mature, uncut tracts may not be used by swamp rabbits because there may be fewer canopy openings, resulting in less herbaceous ground vegetation (Terrel 1972). Also, corridors and short distances to adjacent stands may make smaller tracts suitable for some habitat use. The use of small tracts is indicative of the habitat plasticity of swamp rabbits. Long-term conservation of swamp rabbit populations will require increased monitoring, continued management of remaining habitat, and creation of additional habitat (reforestation). Collecting harvest data and/or surveying rabbit hunters is one option for monitoring population trends. The use of pellet counts to assess swamp rabbit abundance will require validation through careful analysis of population density and pellet abundance. This important 2007 A. Fowler and R.E. Kissell, Jr. 257 step would provide a more reliable and inexpensive way to estimate rabbit populations. In light of the anthropogenic pressures on swamp rabbits, it is critical to assess the habitat quality of remaining bottomland hardwood stands, and determine their ability to sustain swamp rabbit populations. Acknowledgments We thank S. Fowler, E. McCammon, K. Bogart, and C. Guffey for assistance in the field. E. Sundell and A. Grell assisted in plant identification. A special thanks to M. Staten of Anderson-Tully Company, G. Thornton of Potlatch Corporation, and R. Stich of Plum Creek for allowing us access to private lands. We also wish to thank T. Gabor and J. Scheibe for comments that improved the paper. Support for this project was provided by the Arkansas Game and Fish Commission and the School of Forest Resources, University of Arkansas-Monticello. Literature Cited Allen, A.W. 1985. Habitat suitability index models: Swamp rabbit. US Fish and Wildlife Service, National Wetlands Research Center, Lafayette, LA. Biological Report 82 (10.107). 20 pp. Barbour, M.S., A. 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