The New England cottontail (NEC) Sylvilagus transitionalis is strongly associated with shrubland and early successional habitat and is the only cottontail native to the U.S. Northeast. The distribution and abundance of young forest habitat and NEC populations have declined. The eastern cottontail (EC) Sylvilagus floridanus was introduced into the U.S. Northeast in the early 1900s and uses similar habitat as NEC, but is expanding in distribution and abundance. Little information exists on spatial use, survival, and competition in sympatric populations of NEC and EC. Understanding differences in population demographics may identify important factors or relationships influencing population trends and aid in developing effective management strategies. Our objectives were to quantify home range and core area sizes, annual survival rates, minimum population densities, and range overlap for sympatric populations of NEC and EC at four sites in Connecticut. We monitored spatial use and survival rates of 107 radio-collared rabbits over a 10-y period. Mean annual home ranges and core areas were 10.9 and 2.5 ha for NEC and 5.6 and 1.6 ha for EC. Overlap in home range and core areas was greater within species than between species (NEC-EC). For both species and sex, home range size expanded from winter to breeding seasons. Survival rates were greater for NEC than for EC at all four sites, with predation as the major cause of mortality for both species. Space-use patterns suggest that the potential for EC to interfere with NEC reproduction is limited and avoidance or resource partitioning between species in the same patch may be occurring.
The New England cottontail (NEC) Sylvilagus transitionalis is native to the U.S. Northeast and is dependent on shrubland and young regenerating forest habitat. The U.S. Fish and Wildlife Service determined that listing the NEC as threatened or endangered pursuant to the U.S. Endangered Species Act (ESA 1973, as amended) was not warranted (50 CFR Part 17, Sept 15 2015), but conservation concerns still existed. Land use and land management have limited the distribution and abundance of young forest habitat in the Northeast (Barbour and Litvaitis 1993). Available data on NEC population abundance and distribution suggest a decline across its range (Chapman and Stauffer 1981; Litvaitis et al. 2006), primarily due to habitat loss and habitat fragmentation (Litvaitis and Villafuerte 1996). Recent genetic research suggests that landscape fragmentation has isolated NEC into five disjunct populations, which has increased the risk of extinction (Fenderson et. al. 2011).
Declines in NEC abundance have also been attributed to increased competition (Chapman and Morgan 1973) from the more adaptable eastern cottontail (EC) Sylvilagus floridanus. The EC is not a habitat specialist like the NEC and are able to utilize a wider range of habitat types (Smith and Litvaitis 1999) The EC, which was introduced into the U.S. Northeast in the early 1900s, uses similar habitat as NEC, but is expanding in distribution and abundance. However, Probert and Litvaitis (1996) suggest that EC do not outcompete NEC for resources when sympatric populations exist, and it is more likely that EC are better suited for dispersal into new habitats with limited understory cover.
Examining the population demographics of sympatric populations may provide insight into potential interspecific competition between species. The EC might have a competitive advantage over NEC but the exact cause is unclear (Fuller and Tur 2012). However, Smith and Litvaitis (1999) found that EC have an ability to see and react to predators at greater distances than NEC, allowing better predator aversion in open areas. Both species of cottontails occupy the same patches of habitat. As EC continue to expand in distribution, NEC continue to become more limited in distribution. Understanding differences in population demographics, survival, and movements may identify important factors or relationships influencing population trends and aid in developing effective management strategies that will benefit NEC specifically. We are unaware of any published reports on home range sizes or annual survival rates for NEC. Studying both species at the same time and location, using the same data collection methods, provides a rare opportunity to identify differences in movements or survival that may affect population demographics. Our objectives were to quantify home range and core area sizes, annual survival rates, minimum population densities, and range overlap for sympatric populations of NEC and EC.
We conducted our study at four sites within Connecticut (Figure 1). The Groton site (Bluff Point Coastal Reserve) was a 7-ha patch within a 326-ha state-owned peninsula that consisted of mature upland hardwood forest dominating the central and northern portions of the reserve with coastal shrub and tidal wetland vegetation in low-lying areas. The southern third of the reserve consisted of a diverse mix of overgrown fields, abandoned apple orchards, and second-growth hardwoods with an understory of common greenbriar Smilax rotundifolia and multiflora rose Rosa multiflora. This site was closed to hunting but open to the public. The North Stonington site (Pachaug State Forest) was a 61-ha shelterwood (conducted in 1988) cut located within mature hardwood forest. Adjacent to this cut was a 49-ha shelterwood cut that occurred in 2000. Both shelterwood cuts were dominated by sapling-stage forest with thick patches of greenbrier. This site was open to hunting between 15 October and 29 February with year-round public access. The Scotland site (private property) was a 45-ha farm that consisted of old pasture dominated by multiflora rose, autumn olive Elaeagnus umbellata, corn fields, and hayfields. This site was closed to hunting and public access. The Kent site was a 16-ha private farm dominated by fields and hedgerows with thick patches of multiflora rose along with some hedgerow habitat bordering small hayfields. This site was closed to hunting and public access. Previous sampling efforts confirmed the presence of NEC and EC at all sites (Walter et al. 2001; Kilpatrick et al. 2013).
We captured rabbits with single-door wire traps (81 × 25 × 30 cm) baited with apple slices. We placed traps along transects throughout the patch in suitable habitat (mean of two traps/ha). We checked traps daily and closed traps on weekends or for expectant inclement weather (i.e., freezing rain, heavy rain, or heavy snow). We fitted each captured rabbit with a radio collar (model M1555, Advanced Telemetry Systems, Inc., Isanti, MN) equipped with an 8-h mortality sensor. We recorded data, collected tissue samples, and released captured rabbits on site. We followed guidelines of the American Society of Mammologists for the use of wild mammals in research (Sikes et al. 2011).
The University of New Hampshire Genetics Lab conducted genetic analysis following previously published protocols to confirm species identification. From tissue samples we extracted deoxyribonucleic acid (DNA) using the DNeasy blood and tissue kit (Qiagen, Valencia, CA). The lab performed species identification using a combination of two diagnostic mitochondrial DNA restriction fragment length polymorphism assays following the methods of Litvaitis and Litvaitis (1996) and Kovach et al. (2003), as described in Kilpatrick et al. (2013). We assigned gender to each rabbit on the basis of an examination of the genitals. We submitted DNA for a subsample of captured rabbits to confirm gender assignment. The lab performed molecular sexing by two independent polymerase chain reaction (PCR) assays using previously published protocols. For the first method, the lab followed Shaw et al. (2003) for an assay based on size variation between the zinc-finger x and zinc-finger y introns, resulting in two fragments for males and one for females (Shaw et al. 2003). The second method coamplifies a microsatellite on the Y chromosome (INRACCDDV0326; Chantry-Darmon et al. 2005) with a second autosomal microsatellite as a positive control (INRACCDDV016; Chantry-Darmon et al. 2005), again resulting in two fragments for males (both microsatellites amplify) and one for females (only the autosomal microsatellite amplifies). For the latter method, the lab followed PCR protocols used previously for NEC in Fenderson et al. (2011, 2014). For both assays, PCR products were resolved in 3% agarose gels. Results were consistent between the two methods.
Spatial use and survival
We initiated monitoring for each rabbit the day after collaring. We located rabbits via triangulation from fixed locations. We calculated time to independence between consecutive fixes using Schoener's index in program RANGES V (Kenward and Hodder 1996). We used this calculation to develop a sampling protocol that ensures that location data were independent. We estimated six fixes per week (three at night and three during day) using a handheld three-element Yagi antenna and portable model TR-2 receiver (Telonics, Inc., Mesa, AZ). When a collar emitted a mortality signal, we located the collar and collected evidence to assess cause of mortality. To minimize telemetry error, we obtained telemetry readings in proximity (< 50 m) to collared rabbits.
Annually, we calculated minimum rabbit densities during the winter by dividing the total number of unique individual rabbits captured by the size of the habitat patch. We estimated 95% home ranges and 50% core areas using the adaptive kernel method with least-squares cross-validation smoothing parameter (Seaman and Powell 1996) in Ranges V (Kenward and Hodder 1996). We used the default cell size (4.1 m2). The adaptive kernel method does not assume that fixes are normally distributed (Jennrich and Turner 1969), and is less sensitive to grid size than the harmonic means method (Dixon and Chapman 1980), but may overestimate home ranges compared with the fixed-kernel method (Kernohan et al. 2001). Goodie et al. (2004) used both the adaptive and fixed-kernel method to estimate home range size of rabbits in Connecticut and found no difference in home range size between methods. To minimize the effects of telemetry error on home range estimates, we removed fixes with relatively large error polygons (> 10% of mean home range size; White and Garrott 1990). Seaman et al. (1999) recommend a sample size of at least 30 when calculating home ranges using the kernel method with least-squares cross-validation. We tested home ranges to confirm the asymptote. We calculated the asymptotes for each rabbit using 100 bootstrap replicates per rabbit, with random sampling and a tolerance of 0.5.
We calculated home range and core area for the annual (≥ 365 consecutive days of survival), breeding (April–September), and winter (December–March) periods. Because of small sample size, we used Mann–Whitney U test to test for significant differences (P < 0.05) in home range and core area size between species. We calculated percent home range overlap among rabbits using program RANGES V. We calculated the percentage of home range overlap between all rabbits alive at the same time at each study site.
We estimated survival rates for cottontails at each study site with the Kaplan–Meier product limit estimator, which is a nonparametric statistic that allows for staggered entry, right-censored data, and stratification (Pollock et al. 1989). We used the Tarone–Ware nonparametric log ranks to compare survival curves because it has power across a wide range of survival functions (Tarone and Ware 1977). We initiated survival analysis 14 d after we captured and equipped rabbits with radio collars to exclude any capture-related mortality. We used the last known date of monitoring to calculate time criteria in days (365 for individuals monitored 1 y) for right-censored individuals. We used stratification to evaluate survival rates between species for each study site. We performed all analyses using SYSTAT 12.0 (Systat Software Inc., San Jose, CA).
We monitored 107 radio-collared rabbits (50 NEC, 57 EC) over a 10-y period (Data S1, Supplemental Material). Collectively, mean annual home range and core areas were 10.9 ha and 2.5 ha for NEC and 5.6 and 1.6 ha for EC. Sizes of annual home range ranged from 5.5 to 19.5 ha for NEC and from 2.6 to 8.3 ha for EC. Only the Groton study site had more than two rabbits of both species survive ≥ 1 y; therefore annual home range was only calculated for this site. Annual home ranges were larger for NEC compared with EC but the differences were not statistically significant (U = 3, P = 0.18; Table 1).
Collectively, mean breeding period home range and core area were 9.7 and 2.5 ha for NEC and 13.0 and 4.4 ha for EC. At three of four study sites, more than 2 rabbits of both species survived the breeding period. We found no statistically significant differences in home range size between NEC and EC during the breeding period among three sites (23 < U < 37, 0.144 < P < 0.923; Table 1).
Collectively, mean winter home range and core area were 6.4 and 1.6 ha for NEC and 5.6 and 1.5 ha for EC (Table 1). At two of four study sites, more than two rabbits of both species survived the winter period. We found no statistically significant difference in home range sizes during the winter period (3< U <16, 0.302 < P < 0.439). Home range size of male and female NEC increased by 94% and 83%, respectively, from the winter to the breeding period. Average home range size of EC increased by 342% for males and 138% for females from the winter to the breeding period.
Overlap in home ranges and core areas between pairs of NEC and between pairs of EC was greater than between mixed pairs of NEC and EC (Table 2; Data S2, Supplemental Material). Home range overlap among all pairs of rabbits increased during the breeding period. Core-area overlap between female and male NEC increased from the winter (3.1%, V = 57) to breeding period (11.9%, V = 398). No overlap in core area existed between female NEC and male EC during both the winter and breeding periods.
Survival and population density
We calculated annual survival for 87 rabbits at our four sites (Data S3, Supplemental Material). Mean days survived was greater for NEC (183, V = 15,449) than for EC (99, V = 7,767) at all sites (Table 3). Mortalities at the North Stonington site, the only site open to hunting, were attributed to predation (n = 33), hunting (n = 3), being run over on a road (n = 1), and unknown causes (n = 4). The greatest mortality rates occurred in October, December, February, and June (Figure 2). Capture efforts (trap nights/ha) were intense and similar among all sites (214, 180, 200, 175). Minimum rabbit density across the four sites occupied by both NEC and EC ranged from 0.5 to 4.4 rabbits/ha (Table 4).
We examined spatial use and survival of sympatric populations of NEC and EC to assess differences that may explain why EC populations appear to be increasing while NEC populations appear to be decreasing in distribution. We found that seasonal and annual home ranges were similar between species and home range size varied greatly among individual rabbits, likely due to patchiness of habitats. Overlap in home ranges and core areas between species was limited, inferring spatial segregation between species within patches and limited opportunities for EC to interfere with NEC reproduction. Survival rate was greater for NEC; predation was the most significant factor affecting survival, and impacts of hunting were minimal. When establishing new NEC populations, removing EC from potential release sites and reintroducing NEC after June may improve success of establishing new populations.
We found that annual and seasonal home ranges were similar for sympatric populations of NEC and EC. NEC tended to have larger home ranges than EC during the annual and winter periods; however, these differences were not statistically significant and likely were due to wide variations in the size of individual rabbit ranges at the same site. Using mark and recapture data, Dalke (1942) showed similar variability with annual home ranges of 0.2 to 8 ha in Connecticut. However, it is unclear if Dalke's data were based on recaptures of NEC or EC, or a combination of both. Radiotelemetry studies using diurnal fixes estimated home range size from 0.1 to 7.2 ha for EC (Trent and Rongstad 1974; Briggs et al. 1983; Althoff and Storm 1989). Only Bond et al. (2001a) used both day and night telemetry fixes to estimate home ranges of EC (0.3-6.9 ha). Great variability in annual home range sizes for cottontail rabbits in our study and other published studies could reflect uneven patchiness of early-succession/thicket-type habitats.
We observed differences in home range size by season and sex, but not between species. For both species and sex, home range sizes during the breeding period were more than double the home range sizes during the winter period. Previous studies consistently documented increased home range size for male cottontails and similar or increased home range size for female cottontails during the breeding period (Haugen 1942; Trent and Rongstad 1974; Althoff and Storm 1989; and Bond et. al. 2001b). Bond et al. (2001b) also suggested that increased home range size for females could be a result of energetic demands caused by lactation. We suspect that movements were restricted during winter when cover from aerial predators was more limited, and expanded during the breeding season because of improved habitat quality and breeding-related activities. Expanding movements during the breeding season would be beneficial for species that occupy patchy habitat and have relatively small home ranges to increase encounter rates with potential mates.
Spatial use of patches by sympatric populations of NEC and EC suggest that some mechanism is causing these species to segregate within patches. We observed that home range overlaps between mixed pairs of EC and NEC were much less than overlap between pairs of EC and between pairs of NEC, especially during the breeding season. We observed a similar pattern with core-area overlap for both species, suggesting some type of avoidance or resource partitioning between species in the same patch. However, Eabry (1968), using capture data, concluded that overlaps between EC and NEC were considerable, although no statistical test was provided to support this conclusion. We found overlap between species to be < 15% for home ranges and < 2% for core areas. Spatial segregation may limit the potential for EC to interfere with NEC reproduction within the same patch of habitat. Spatial segregation may also limit success of establishing new populations of NEC if the patch is already occupied by EC. If establishing new populations of NEC, removal of resident EC may improve success.
Many factors can influence rabbit survival. In our study, survival was greater for NEC than EC and predation was the primary cause of mortality. In Massachusetts, Boland and Litvaitis (2008) found that survival rates for EC over an 8.5-mo period were lower on hunted sites (0.05–0.33) than on nonhunted sites (0.19–0.40). In New Hampshire, along the north edge of the NEC range, winter survival rates over a 70-d period were lower on small patches (0.29–0.37; patches ≤ 2.5 ha) than on large patches (0.6–0.7; patches ≥ 5 ha; Barbour and Litvaitis 1993; Brown and Litvaitis 1995; Villafuerte et al. 1997). Several factors (length of monitoring period, access for small game hunting) appear to have some effect on survival rates of cottontails; however, patch size seems to have the greatest effect on survival. At our study sites, with sympatric populations of cottontails on large patches (> 5 ha) in the core of the NEC's range, survival was greater for NEC than for EC at all four sites. Differences were not statistically significant at two sites. NEC's proclivity to cover and aversion to open areas (Fay and Chandler 1955; Barbour and Litvaitis 1993; Probert and Litvaitis 1996) may aid in predator evasion, resulting in greater survival compared with EC. The major cause of mortality in our study was predation. Confirmed predation included bobcat Lynx rufus, coyote Canis latrans, and avian predators. Of known mortalities at the site open to small-game hunting, most (87%) were killed by predators and less than 10% were killed by hunters. In Massachusetts, Boland and Litvaitis (2008) also found predation to be the leading cause of mortality (> 70%), with hunters harvesting 10% of their monitored rabbits. Predation, the most significant source of mortality for cottontails, may be reduced by enhancing or expanding existing habitat used by NEC. We believe that, on the basis of the small contribution of overall cottontail mortality due to hunting, manipulating or restricting small-game hunting likely would have little impact on cottontail populations.
Overall rabbit densities for sympatric populations of NEC and EC were highly variable by patch but were similar to other published studies of allopatric NEC populations. Our trapping data indicate that minimum rabbit densities for sympatric populations of EC and NEC at our four study sites ranged from 0.5 to 4.4 rabbits/ha. Our densities were similar to those reported by Barbour and Litvaitis (1993) of 0.3 to 7 rabbits/ha for small and large patches combined for allopatric NEC. Barbour and Litvaitis (1993) extrapolated rabbit densities on large patches (> 5 ha) by stratifying the trapping effort on the basis of rabbit activity and comparing the results with the amount of similar habitat in the patch. We used minimum captures over a 40-d period throughout the entire patch to estimate rabbit densities. We suspect that differences in habitat quality between patches and winter conditions influence winter rabbit densities. Brown and Litvaitis (1995) found that the percentage of rabbits killed by predators during the winter increased as the number of days with snow cover increased. At our study sites, NEC comprised 40 to 90% of the sympatric population of cottontails.
The Conservation Strategy for NEC (Fuller and Tur 2012) was developed to restore NEC populations to healthy levels throughout their range by 2030. Habitat and population goals have been established in the Conservation Strategy for NEC to create or maintain 17,175 ha of shrub/young forest habitat with the expectation of it supporting about 1.25 NEC/ha of habitat (Fuller and Tur 2012). In regions where both species of rabbit are present, we found that the density of rabbits (2.5 rabbits/ha) and ratio of NEC in the population (approximately 1:1) are in agreement with the population goals of the Conservation Strategy. When reintroducing or establishing new populations of NEC, success may be improved by trapping and removing any resident EC that may be occupying the release site. Removing EC will increase the amount of space available for NEC to occupy and become established. Relocating or releasing captive-bred NEC into new patches of habitat after June is recommended to avoid periods of high predation. We recommend follow-up studies to identify possible differences in microhabat use of patches co-occupied by NEC and EC that may explain the apparent segregation within patches.
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Data S1. Annual and seasonal home range and core area data for eastern cottontail Sylvilagus floridanus and New England cottontail Sylvilagus transitionalis in Connecticut (2001–2010).
Found at DOI: https://doi.org/10.3996/082016-JFWM-062.S1 (22 KB XLSX).
Data S2. Overlap data for home range (95%) and core area (50%) use between species and gender for eastern cottontail Sylvilagus floridanus and New England cottontail Sylvilagus transitionalis in Connecticut (2001–2010).
Found at DOI: https://doi.org/10.3996/082016-JFWM-062.S2 (33 KB XLSX).
Data S3. Survival data and summary table for eastern cottontail Sylvilagus floridanus and New England cottontail Sylvilagus transitionalis at four sites in Connecticut (2001–2010).
Found at DOI: https://doi.org/10.3996/082016-JFWM-062.S3 (22 KB XLSX).
We thank J. Litvaitis and T. Rittenhouse for comments on earlier drafts of this manuscript and W. Embacher, J. Brooks, P. Lewis, and R. Wilbur for assistance in data collection. We thank Andrea Petrullo for mapping assistance. We thank A. Kovach and L. Federson from the University of New Hampshire for genetic analysis. We appreciate the valuable comments from three anonymous reviewers and the Associate Editor that improved this manuscript. This project was funded by State Wildlife Grant and Federal Aid in Wildlife Restoration.
Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Citation: Kilpatrick HJ, Goodie TJ. 2020. Spatial use and survival of sympatric populations of New England and eastern cottontails in Connecticut. Journal of Fish and Wildlife Management 11(1):3–10; e1944-687X. https://doi.org/10.3996/082016-JFWM-062
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.