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Habitat Use by a Dense Population of Southern Fox Squirrels
James C. Lee, David A. Osborn, and Karl V. Miller

Southeastern Naturalist, Volume 8, Number 1 (2009): 157–166

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2009 SOUTHEASTERN NATURALIST 8(1):157–166 Habitat Use by a Dense Population of Southern Fox Squirrels James C. Lee1, David A. Osborn1,*, and Karl V. Miller1 Abstract - We determined seasonal macro- and microhabitat preferences of radiocollared Sciurus niger niger (Southern Fox Squirrel) in a high-density population. Fox squirrels preferred hardwood, live oak, and mixed hardwood-pine macrohabitats to pine, early successional, and turf macrohabitats. During winter, early summer, and late summer, they preferred mixed hardwood-pine, live oak, and hardwood microhabitats to pine microhabitats. Fox squirrels preferred an open or moderate crown spacing to a dense crown spacing during all seasons, and during the summer, they preferred microhabitats with at least 1 cone-bearing pine tree. Preferred microhabitats had a short-open or leaf-litter understory structure. Although the hardwood and hardwood-pine habitats on our barrier island study site differed from typical pinedominated southeastern fox squirrel habitat, our results demonstrating preference for these types indicated that they are capable of supporting an abundant fox squirrel population when managed by mowing, burning, and light timber harvesting. Introduction Sciurus niger niger L.(Southern Fox Squirrel) is 1 of 4 southeastern fox squirrel subspecies that have declined in abundance and distribution during recent years, primarily because of habitat modifications (Ha 1983, Loeb and Lennartz 1989, Loeb and Moncrief 1993, Taylor 1973, Weigl et al. 1989). In the southeastern US, fox squirrels prefer mature Pinus palustris Mill. (Longleaf Pine), mixed Pinus spp.- Quercus spp. (pine-oak), bottomland hardwood, and Carolina bay (small pine-hardwood wetlands) forests with relatively open or herbaceous understories. However, these studies focused on low-density (<38 fox squirrels/km2) populations (Edwards et al. 1989, Ha 1983, Hilliard 1979, Kantola and Humphrey 1990, Perkins and Conner 2004, Perkins et al. 2008, Weigl et al. 1989). On Spring Island, SC, a dense population (>75/km2) of Southern Fox Squirrels has increased in size during the past 10 years concurrently with habitat modifications associated with residential development (Lee et al. 2008). Although short-term increases in squirrel numbers are common after years of food abundance, Southern Fox Squirrel populations rarely are sustained at the densities observed on Spring Island (Weigl et al. 1989). Because squirrel abundance is usually regarded as an indicator of habitat quality, characterization of the preferred habitats on Spring Island may alter wildlife management practices throughout the Southeast. Subspecies of southeastern fox squirrels are ecologically similar (Weigl et al. 1989), so these recommendations probably apply to other southeastern fox squirrel subspecies. Our objective was to examine seasonal macro- and microhabitat use by this unusually dense Southern Fox Squirrel population and to characterize preferred habitats. 1Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602. *Corresponding author - Osborn@warnell.uga.edu. 158 Southeastern Naturalist Vol. 8, No. 1 Study Area Spring Island is a 1200-ha sea island located between Beaufort, SC and Savannah, GA. About 1000 ha of the island are forested, and the remainder is fallow fields, wildlife plantings, and a golf course. During the study, there were about 100 residential, administrative, or recreational buildings on the island. The topography of Spring Island is mostly level, and its soils are characterized as fine sand or fine sandy loam (USDA 1980). Spring Island, which formed as a dune ridge during the late Pleistocene Epoch, is separated from the mainland and other sea islands by 200–500 m of salt marsh and tidal rivers. The climate is subtropical, with mild winters and hot, humid summers. Yearly rainfall averages about 125 cm, with most precipitation falling between April and October (Kovacik and Winberry 1987). Forests on Spring Island are a heterogeneous mixture of mixed hardwoods and pines with numerous, small (<3 ha) stands of pine, Quercus virginiana Mill. (Live Oak), and mixed hardwoods. Most overstory trees are >40 years old. Pinus taeda L. (Loblolly Pine), Carya spp. (hickories), Sabal palmetto Walter (Cabbage Palm), Q. haemispherica Michx. (Laurel Oak), Live Oak, Liquidambar styracifl ua L. (Sweetgum), Q. falcata Michx. (Southern Red Oak), Q. nigra L. (Water Oak), and Nyssa sylvatica Marsh. (Blackgum) are common trees. Most stands lack mid-stories, but have brushy understories with intermittent openings where they are burned, mowed, or shaded by the canopy. Myrica cerifera L. (Wax Myrtle), oaks, Sweetgum, Sassafras albidum Nutt. (Sassafras), Ilex vomitoria Aiton (Yaupon), Serenoa repens Bartram (Saw Palmetto), and Smilax spp. (greenbriars) are common understory plants. Methods We trapped fox squirrels in wooden Mosby-style box traps (Day et al. 1980) baited with corn, pecans, or a combination of both during August 1998–October 1998, December 1998, February 1999–March 1999, and May 1999–June 1999. Trapping effort was evenly distributed throughout the island. Trapped squirrels were placed into a nylon mesh bag, where they were aged and sexed (Dimmick and Pelton 1994), and weighed to the nearest 10.0 g with a 2.0-kg spring scale (Douglas Homs Corp., Belmont, CA). Forty-seven squirrels, each weighing >900 g were immobilized with 3–5 ml of methoxyfl urane (administered by inhalation) or with 20–30 mg ketamine hydrochloride (administered by intramuscular injection) and fitted with a radio-collar (Advanced Telemetry Systems, Isanti, MN or Telemetry Solutions, Walnut Creek, CA). Collars weighed 23–27 g, had an expected battery life of 2 years, an effective range of 0.5–2.0 km, and a mortality switch that activated when the collar remained motionless for 8.0 hours. Number 1 or 3 monel fingerling tags (National Band and Tag Company, Newport, KY) were attached to both ears of each radio-collared squirrel. After recovering from the immobilant (1 hour for methoxyfl urane, 3 hours for ketamine hydrochloride), the squirrels were released at their capture sites. 2009 J.C. Lee, D.A. Osborn, and K.V. Miller 159 We monitored the activities of 17–31 radio-collared squirrels during each season. Each squirrel was located at least 30 times per season at random times between 0.5 hours after sunrise and 0.5 hours before sunset. Seasons were based on plant phenology and were defined as: fall = 1 October 1998 to 15 January 1999, winter = 16 January 1999 to 15 March 1999, spring = 16 March 1999 to 1 June 1999, early summer = June 1999 and July 1999, and late summer = August 1999 and September 1999 (Weigl et al. 1989). Locations of radio-collared squirrels were determined by using the homing method (Mech 1983). A squirrel’s location was recorded with a global positioning system (GPS) receiver (Geoexplorer II, Trimble, Sacramento, CA). Later, the positions (≥25 per squirrel location) were differentially corrected and averaged using the computer program PATHFINDER (Trimble, Sacramento, CA). Because species composition of a forest stand is not the only determinant of fox squirrel habitat suitability, we examined additional aspects of habitat use. We considered forest stands (a contiguous group of trees sufficiently uniform in species composition, arrangement of age classes, and condition to be a distinguishable unit; Smith 1962), fields, and the golf course to be macrohabitats, whereas unique habitat features within 30 m (0.3 ha) of a squirrel’s location were microhabitats. When a squirrel was located, macrohabitats classified as pine (>80% of overstory species composition was pine), hardwood (>80% was hardwoods), live oak (>80% was live oak), mixed (<80% pine and <80% hardwood), turf (golf course and shortgrass fields), and early successional (fallow fields and wildlife plantings) were recorded. To compare the use of macrohabitats relative to their availability on Spring Island, we used a geographical information system (GIS) macrohabitat coverage based on aerial photographs that were taken in 1994 (BDA Consulting, Orlando, FL). The coverage was ground-truthed and updated in 1999. We imported the locations of radio-collared squirrels into ARCVIEW (Environmental Systems Research Institute, Redlands, CA) and overlaid them onto this coverage. Seasonal home ranges (95% kernel; Worton 1989) were generated for each squirrel with the Animal Movement extension to Arcview (Hooge and Eichenlaub 1997). We intersected squirrel locations and home ranges to determine proportional use of macrohabitats by fox squirrels during each season. Microhabitat variables (Table 1) associated with squirrel locations also were recorded. To compare the use of microhabitats relative to their availability on Spring Island, we measured these variables within 100 randomly chosen 0.3-ha plots. The plots were randomly generated with the Animal Movement extension to Arcview and we subsequently navigated to each plot using a GPS receiver. Compositional analysis (Aebischer et al. 1993) was used for all habitat analyses. Multivariate analysis of variance with Wilk’s Lambda (multivariate F approximation), constructed using randomization (Edgington 1980), was used to detect deviations (P ≤ 0.05) from random habitat use. Macroand microhabitat variables were ranked according to their use, and paired t-tests were used to detect differences between habitat variables (MacComp 160 Southeastern Naturalist Vol. 8, No. 1 0.90; J. Carroll, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, pers. comm.). Macrohabitat analyses included 2 spatial scales: habitat proportions within the home range versus habitat proportions within the study area, and habitat proportions of individual radio-locations versus habitat proportions within the home range. These scales are equivalent to second-order and third-order selection processes, respectively (Johnson 1980). This experimental design provides for the recognition of habitat types that are important, but not often used. Microhabitat variables available on the study area were compared to individual squirrels’ radio-locations. We used sub-compositional analysis (N. Aebischer, The Game Conservancy, Fordingbridge, Hampshire, UK, pers. comm.) to determine crown-spacing class preferences within overstory composition classes. For example, crown-spacing classes within the pine overstory composition class were analyzed separately from crown-spacing classes within the live oak overstory composition class because average tree crown diameter varied greatly, especially between live oaks and pines. Results We captured 154 fox squirrels during August 1998–June 1999. Trapping success ranged from 1 capture/160 trap days during August 1998–October 1998 to 1 capture/4 trap-days during February 1999–March 1999. Thirtynine fox squirrels were located frequently enough (≥30 times) during at least one season to include in the analyses, and 17–29 radio-collared squirrels were used for analyses during each season. We recorded 3049 radiolocations between October 1998 and September 1999. Table 1. Variables used to characterize microhabitats (0.3-ha plots) on Spring Island, SC, 1998–1999. Variable Description Overstory composition Overstory species composition classified as hardwood (>80% hardwood tree species), pine (>80% pine tree species), mixed (<80% pine and <80% hardwood tree species), and live oak (>80% live oaks). Crown spacing Overstory crown spacing classified as open (tree crowns >3 m apart), moderate (tree crowns <3 m apart but not overlapping) and dense (tree crowns overlapping). Crown spacing classes correlated to our perception of how easily a squirrel could travel through the canopy (e.g., open was difficult or impossible to travel through, moderate offered some difficulty but still allowed travel, and dense offered unrestricted travel through the canopy). Understory structure Understory structure classified as short-open (ground cover <1 m tall and <50% dense, grass, or bare ground), tall-open (ground cover >1 m tall and <50% dense), short-dense (ground cover <1 m tall >50% dense), tall-dense (ground cover >1 m tall and >50% dense), and leaf litter. Cone-bearing pine Presence or absence of a cone-bearing pine. 2009 J.C. Lee, D.A. Osborn, and K.V. Miller 161 Macrohabitat use was non-random during all seasons (Table 2). Mixed, hardwood, and live oak macrohabitats were generally preferred over turf, early successional, and pine. During early and late summer, turf ranked higher than if it was selected randomly. Pine macrohabitats generally were avoided, although loblolly pines were easily accessible to squirrels as components of mixed stands or scattered through predominately hardwood stands. We did not notice any strong seasonal differences in macrohabitat selection. Microhabitat use, as measured by the overstory composition variable was non-random during winter, early summer, and late summer (Table 3). Hardwood and mixed microhabitats were preferred during winter and mixed and live oak microhabitats were preferred during early and late summer. The pine microhabitat was not preferred during any season. For hardwood, live oak, and mixed microhabitats during all seasons, and pine microhabitats during spring, open and moderate crown-spacing classes were preferred (P < 0.05) to the dense crown-spacing class, but differences between open and moderate crown-spacing classes were rarely significant (P > 0.05). Short-open and leaf-litter understories were generally preferred to tall-open, tall-dense, and short-dense understories, but they were rarely significantly different from each other (Table 3). Microhabitats containing a cone-bearing pine were either not preferred or avoided during fall, winter and spring. However, they were preferred during early and late summer (P = 0.001). The mean number of tree species in 100 random 0.3-ha sample plots was 4.34. Thirty-five percent of the sample plots had leaf-litter or short-open understory classes, whereas the remaining plots’ understory structure classes were tall-open, short-dense, or tall-dense. At least one cone-bearing pine was present on 82% of the sample plots. Discussion The typically sparse populations of southeastern fox squirrels have been associated with large, continuous, low-diversity forests of fire-maintained Table 2. Seasonal preference rankings of macrohabitats used by fox squirrels on Spring Island, SC, October 1998–September 1999. Comparison Ranking1,2,3 P-value Study area vs. home range Fall HA > M > LOBC > T > ESC > PAB 0.05 Winter MA > HE > LOABCD > TC > ESD > PBE <0.01 Spring M > H > LOAB > T > ES > PAB <0.01 Early summer MA > H > TA > ESD > LOABCD > PB <0.01 Late summer MAE > LOABCD > TC > HE > PB > ESD <0.01 Home range vs. radio-locations4 Fall LOAB > HCD > PD > T > MAC > ESB 0.03 Winter LOA > M > HB > ESA > T > PB <0.01 Spring M > LOA > T > H > P > ESA <0.01 1> denotes preference. 2Macrohabitats with the same superscripted letter are significantly different (P < 0.05). 3H = hardwood, M = mixed, LO = live oak, T = turf, ES = early successional, P = pine. 4 Seasons without differences in preference (P > 0.05) are excluded from this table. 162 Southeastern Naturalist Vol. 8, No. 1 Longleaf Pine with a scattered hardwood component and a uniformly open or herbaceous understory (Bangs 1899, Moore 1957, Perkins and Conner 2004, Weigl et al. 1989). Because large-scale conversion of this forest type presumably has led to the decline of fox squirrel distribution and abundance (Weigl et al. 1989), many researchers have suggested that large tracts of fire-maintained Longleaf Pine with a uniformly open understory is the optimal habitat for fox squirrels. However, fox squirrels on Spring Island, and in other areas of the Southeast (Edwards et al. 1989, Hilliard 1979, Kantola and Humphrey 1990), appear to prefer areas with a higher diversity of tree species than primitive Longleaf Pine forests contained. Furthermore, on Spring Island, the forest understory is not uniformly open. The structure of habitat on Spring Island is similar to that reported for populations of S. n. rufiventer Geoffrey St. Hilaire (Midwestern Fox Squirrel) of similar densities in the northern and midwestern United States (Allen 1942, Baumgartner 1943, Nixon and Hanson 1987). The high-density population (75/km2) and smaller than normal home ranges (mean annual areas of 3.4 ha for females and 9.6 ha for males) of fox squirrels on Spring Island refl ect high habitat quality (Dasmann 1964, Lee et al. 2008). Habitat characteristics on Spring Island are partially the result of a history of mowing and burning. These activities maintained an open understory to facilitate the management and hunting of Colinus virginianus L. (Northern Bobwhite). Between the 1960s and 1990, annual burning was conducted over most of the island. Fires caused many of the hardwood trees to develop butt scars and then, cavities (Carey 1983), which may be important as fox squirrel denning and litter-rearing sites (Weigl et al. 1989). Prescribed fires caused some tree mortality that thinned the overstory and may have contributed to the lack of a midstory over much of the island. Since 1990, less burning was conducted, but mowing increased, which resulted in a mosaic of understory structure classes. The fox squirrel population on Spring Table 3. Seasonal preference rankings for overstory species composition (OSC) and understory structure (US) microhabitat variables by fox squirrels on Spring Island, SC, October 1998–September 1999. Microhabitat variable Season1 Ranking2,3,4 P-value Overstory composition Winter HA > MB > P > LOAB 0.05 Early summer MBC > LOACD > HAB > PD <0.01 Late summer MB > LOABC > HAB > PC <0.01 Understory structure Fall SOAB > LL > SD > TDA >TOB <0.01 Winter LLABC > SODEF > TOCF > TDBE > SDAD <0.01 Spring LLABC > SODEF > SDAD > TOCF > TDBE <0.01 Early summer LLAB > SOCD > SDA > TOBD > TDC <0.01 Late summer SOE > LLAB > TOCDF > SDACD > TDBEF <0.01 1Seasons without differences in preference (P > 0.05) are excluded from this table. 2OSCs or USs with the same superscripted letter are significantly different (P < 0.05). 3H = hardwood, M = mixed, P = pine, LO = live oak, SO = short-open, LL = leaf litter, SD = short-dense, TD = tall-dense, TO = tall-open. 4> denotes preference. 2009 J.C. Lee, D.A. Osborn, and K.V. Miller 163 Island has certainly not declined, and may have increased substantially since then (Lee et al. 2008). Therefore, maintaining about 35% of the understory in leaf-litter or short-open understory structure classes seems to be at least partially responsible for the abundance of fox squirrels. Ecotone areas are important to fox squirrels (Edwards et al. 1989, Kantola and Humphrey 1990, Weigl et al. 1989). Because of the high degree of interspersion on Spring Island, ecotones are nearly always ≤200 m apart. Furthermore, macrohabitats tend to blend over large areas so that ecotones are indistinct. The result of this interspersion is an environment in which fox squirrels have an abundance of resources nearby during all seasons. Certain habitat features such as wildlife plantings, cone-bearing pines, and the golf course appeared seasonally important to fox squirrels (Lee et al. 2001). Many of the early successional fields on Spring Island were planted in wheat, which was available as food to fox squirrels during May and June 1999. Although fox squirrels often were seen feeding in these wildlife plantings, no radio-collared fox squirrels were observed to travel outside their normal ranges to take advantage of these wheat plantings, and the early successional macrohabitat was not preferred during any season. However, wheat may be an important food source for fox squirrels during the spring after a fall mast failure (Lee et al. 2001). Cone-bearing pines were an important microhabitat feature during early and late summer, when pine seeds were an important fox squirrel food source (Lee et al. 2001, Schultz 1997, Weigl et al. 1989). Because of the number of fox squirrel sightings, the golf course initially appeared to be important to fox squirrels. Sciurus niger avicennia Howell (Big Cypress Fox Squirrel) in Florida are more abundant near golf courses than in natural habitats (Jodice and Humphrey 1992). However, the turf macrohabitat ranked high only during early and late summer, when mushrooms may have attracted fox squirrels. It is likely that our initial perception associated with the golf course was related to the greater visibility of fox squirrels there. Fox squirrels can be observed from afar while they are foraging on or around the golf course and they probably habituate to humans (Jodice and Humphrey 1992). Loeb and Lennartz (1989) described habitats occupied by fox squirrels as forests with large pines, sparse ground cover, and an association of mature, mast-producing oaks. The oaks could occur as a midstory component or as patches within pine stands. Because of the typically large home ranges of southeastern fox squirrels (9–19 ha for females and 20–32 ha for males; Edwards 1986, Hilliard 1979, Kantola and Humphrey 1990, Weigl et al. 1989), researchers conclude that fox squirrels need large tracts of land to satisfy their habitat needs. However, because fox squirrels on Spring Island have much smaller home ranges than previously reported for southeastern fox squirrels (Lee et al. 2008), we suspect that large homerange sizes are not an inherent characteristic. Rather, they reflect a need by fox squirrels in inadequate habitats to traverse a large area to satisfy their 164 Southeastern Naturalist Vol. 8, No. 1 resource requirements. Resource needs may be met in a smaller area if a high interspersion of habitat types is present. The most important tree species are mast-producing hardwoods, especially oaks. A mixture of red oaks and white oaks, as on Spring Island, helps prevent the loss of the entire mast crop if either group of species fails to produce acorns. Because Spring Island fox squirrels were located within 30 m of a cone-bearing pine more often than expected during early and late summer, cone-bearing pines should be present at a minimum density of 1.0/0.3 ha. In areas where hardwoods are a component of pine stands, they should be dominant or co-dominant, but not part of the midstory. The canopies of the trees should not overlap, although they need not be greater than 3 m apart. A sparse understory is important, but it probably does not need to cover more than 35% of the area. In the event of a fall mast failure, spring food sources should be available (i.e., wildlife plantings, and trees such as Acer rubrum L. [Red Maple], and Morus rubra L. [Red Mulberry]. These habitat conditions have been encouraged on Spring Island through land management practices such as mowing, burning, and light timber harvesting. In areas where the conservation of Southern Fox Squirrels is a priority, quality habitat may be created or maintained through these methods. Acknowledgments The Daniel B. Warnell School of Forestry and Natural Resources, the Turner Foundation, Inc., The Spring Island Trust, and McIntire-Stennis Project Number GEO-0093-MS provided funding for this study. John P. Carroll provided assistance with statistical analyses. Helen J. H. Whiffen, Brian R. Chapman, and Robert J. Warren provided helpful comments on this manuscript. Literature Cited Aebischer, N.J., P.A. Robertson, and R.E. Kenward. 1993. Compositional analysis of habitat use from animal radio-tracking data. Ecology 74:1313–1325. Allen, D.L. 1942. Populations and habits of the fox squirrel in Allegan County, Michigan. American Midland Naturalist 27:338–379. Bangs, O. 1899. The land mammals of peninsular Florida and the coast region of Georgia. Proceedings of the Boston Society of Natural History 28:157–236. Baumgartner, L.L. 1943. Fox squirrels in Ohio. Journal of Wildlife Management 7: 193–202. Carey, A.B. 1983. Cavities in trees in hardwood forests. Pp. 167–184, In B. Melder (Ed.). 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