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Distribution and Habitat Use of the Big Cypress Fox Squirrel (Sciurus niger avicennia)
Danielle A. Eisenberg, Reed F. Noss, Jane M. Waterman, and Martin B. Main

Southeastern Naturalist, Volume 10, Issue 1 (2011): 75–84

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2011 SOUTHEASTERN NATURALIST 10(1):75–84 Distribution and Habitat Use of the Big Cypress Fox Squirrel (Sciurus niger avicennia) Danielle A. Eisenberg1,2, Reed F. Noss1,*, Jane M. Waterman1,3, and Martin B. Main4 Abstract - Human population growth and development reduce the area and quality of natural communities and lead to a decline of associated wildlife populations. Sciurus niger avicennia (Big Cypress Fox Squirrel), a state-listed threatened subspecies endemic to south Florida, appears sensitive to habitat fragmentation and fire regime. This research assessed the distribution and habitat use of the Big Cypress Fox Squirrel from interviews with biologists, private land owners, and golf course managers and by transect sampling in natural areas on public and private lands in southwest Florida. Our findings indicate that the distribution of fox squirrel populations is influenced by land use and understory height. Conservation of this species in natural areas will require land management practices that maintain open landscapes. Introduction Habitat loss and fragmentation are widely recognized as leading causes for declines in biodiversity (Groom and Vynne 2006, Noss et al. 2006). Florida supports many endemic taxa that evolved in relative isolation for thousands to millions of years (Adams et al. 2000, James 1961). With a growing human population of 17 million and tourist population of 40 million per year, Florida is losing native habitats due to demands for development (Kautz and Cox 2001). As a result of this rapidly changing landscape, Florida faces one of the highest risks of biodiversity loss of any US state, with hundreds of rare and imperiled native species (Adams et al. 2000, Kautz and Cox 2001, Noss and Peters 1995). Certain species can be useful indicators or “focal species” for determining the quality of ecosystem remnants and for developing management strategies to restore and conserve biodiversity (Carroll et al. 2001, Lambeck 1997). Habitat fragmentation is a principal cause of endangerment for some tree squirrels because it creates a mosaic of isolated patches and reduced habitat area (Koprowski 2005). Because patches vary in suitability for survival and reproduction of individuals, habitat quality is a key determinant of patch occupancy and the probability of local extinction. Sciurus niger avicennia L. (Big Cypress Fox Squirrel) is a state-listed threatened subspecies endemic to south Florida (Humphrey and Jodice 1992). It is found in southwestern Florida, south of the Caloosahatchee River and west of the Everglades (Ditgen et al. 2007). Jodice and Humphrey (1992) reported Big Cypress Fox 1Department of Biology, 4000 Central Florida Boulevard, University of Central Florida, Orlando, fl32816. 2Current address - Department of Science/Wellness Education, 1000 Coconut Creek Boulevard, Broward College, Coconut Creek, fl33066. 3Current address - Department of Biological Science, 186 Dysart Road, University of Manitoba, Winnipeg, MB R3T2N2. 4Southwest Florida Research and Education Center, 2685 State Road 29 North, University of Florida, Immokalee, fl34142. *Corresponding author – rnoss@ mail.ucf.edu. 76 Southeastern Naturalist Vol. 10, No. 1 Squirrel populations as rare and highly scattered in Collier, Lee, Hendry, and Monroe counties. Fox squirrels are sensitive to fragmentation, roads, fire regime, and other aspects of the structure and function of ecosystems (Koprowski 2005). The decline of the Big Cypress Fox Squirrel mirrors the decline in mature Pinus spp. (pine) and Taxodium spp. (cypress) forests as south Florida is increasingly transformed by human activities (Wooding 1997). Pinus elliottii Engelm. (Slash Pine) flatwoods declined by 79% from 1936 to 1987 (Wooding 1997), and remaining habitat has been degraded due to fire exclusion and suppression (Doren et al. 1993). Although Big Cypress Fox Squirrels are generally found in pine flatwoods, their specific habitat requirements are poorly understood (Humphrey and Jodice 1992, Williams and Humphrey 1979). Approximately 384,000 ha of potential Big Cypress Fox Squirrel habitat may remain, with 223,000 ha (58%) within conservation lands (WilsonMiller, Inc. 2002). Potential fox squirrel habitat was estimated by WilsonMiller, Inc. (2002) using January 1995 USGS digital ortho quarter-quad (DOQS) imagery to map pine, Quercus spp. (oak), and cypress forests, which are consistent with potential fox squirrel habitat as described by Humphrey and Jodice (1992). Areas with less than 10% canopy cover were excluded from the potential habitat estimate. Understory characteristics that are important to fox squirrels (Ditgen et al. 2007) apparently were not factored into the WilsonMiller, Inc. (2002) analysis; moreover, this GIS-based study identified potential habitat only and did not evaluate presence/absence of squirrels. Even though a large amount of potential fox squirrel habitat may remain, squirrels avoid many areas because of dense understory growth or other factors (Wooding 1997). Big Cypress Fox Squirrel populations are also reported to occur in pasture and ranch lands that were historically pine flatwoods and upland prairie (Wooding 1997). Previous research found higher fox squirrel abundance in suburban areas than in natural habitats (Ditgen 1999, Jodice and Humphrey 1992). Golf courses and rural residential lands have little to no understory, may serve as habitat corridors, and, somewhat ironically, may provide more suitable habitat for fox squirrels than many remaining natural areas, at least in the short term while large trees remain available in these developed landscapes (Ditgen et al. 2007, Jodice and Humphrey 1992, Wooding 1997). This research evaluated the distribution and habitat use of the Big Cypress Fox Squirrel in urban greenspace, private ranch lands, and conservation lands throughout southwest Florida. Assuming fox squirrel occurrence depends on the availability of open understory, and open understory is more prevalent in intensively managed landscapes such as golf courses and grazed ranchlands, we predict (H1) that Big Cypress Fox Squirrels are present more often in intensively managed landscapes than in natural areas. If fox squirrel occurrence depends on understory characteristics, we predict (H2) that Big Cypress Fox Squirrels will be found more often in areas with lower understory growth than in areas with dense understory. Methods Study sites This study attempted to conduct a range-wide status survey of the Big Cypress Fox Squirrel; therefore, field sites were located in multiple locations in southwest 2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 77 Florida (Fig. 1). The climate of the region is humid subtropical with warm, rainy summers and cool, dry winters (Chen and Gerber 1990). Field surveys were conducted between January 2005 and October 2007 along 1-km transects to census fox squirrels and measure habitat variables. We identified transect locations in a stratified random manner from areas of potential squirrel habitat identified through a combination of geographic location, land use, and accessibility (Bourgeron et al. 2001). From a pool of 40 potential sites in natural areas, private ranch lands, and urban greenspace, we randomly selected 20 transect locations (12 in natural areas, 5 in golf courses, and 3 in private ranches). Transect orientation (N–S, E–W) was randomly assigned to each transect by the flip of a coin. Universal Transverse Mercator (UTM) coordinate locations for each transect were recorded using a Garmin 12 Global Positioning System (GPS, Garmin 12, Olathe, KS). Field surveys Big Cypress Fox Squirrel surveys were conducted along 1-km x 20-m belt transects (Avery and Burkhart 2001), resulting in a surveyed area of 0.02 km2 along each transect. Three surveys were conducted along each transect, with each survey comprising a 2-hr walk to search for fox squirrels. If no fox squirrels were observed, the site was characterized as fox squirrel “absence”. If at least one fox squirrel was observed, the site was characterized as fox squirrel “presence”. Presence/absence methods are useful when the species of interest exists in low numbers or is difficult to detect (Joseph et al. 2006). Perkins and Conner (2004) found that season did not affect habitat use by southeastern fox squirrels; hence, surveys were conducted in both dry (November through April) and wet (May through October) seasons (Chen and Gerber 1990). Fox squirrels were surveyed along each transect at least once in both dry and wet seasons and at least once in the morning (0700–1100) and once in the afternoon (1300–1700). Occasionally, surveys were conducted mid-day (1100–1300) due to constraints in scheduling and access to private lands. Lacking quantitative studies of variation in fox squirrel detectability, we were not able to account for potential differences in fox squirrel behavior and detectability in response to temperature, weather conditions, time of day, or other factors. We measured understory variables at 100-m intervals along each transect. A 2-m cover pole was used to estimate understory height in nested 1-m2 subplots 5 m from the center of each plot in all 4 cardinal directions (Griffith and Youtie 1988). A total of 40 measurements were collected from each transect and then averaged to obtain a mean understory height value for each study site. Lee et al. (2009) classified understory height into two categories: short (≤1 m) and tall (>1 m). For better discrimination of short understory important to fox squirrels, we defined “minimal” understory as less than 50 cm tall, and each site was grouped into two mean understory height classes: less than 50 cm and ≥50 cm. Interviews and analysis One thousand interview questionnaires (60% to private land owners, 30% to golf course managers, and 10% to biologists and field personnel of public landmanaging agencies) were distributed to collect information about the historic and current trends in Big Cypress Fox Squirrel distribution and abundance. To enhance the reliability of participant observations, we included a photo of the 78 Southeastern Naturalist Vol. 10, No. 1 Figure 1. Locations of 1-km transect study sites. Map legend: 1 = Big Cypress National Preserve, 2 = Caloosahatchee Regional Park, 3 = Club at Olde Cypress, 4 = Club at Pelican Bay, 5 = Collier-Seminole State Park, 6 = Corkscrew Swamp Sanctuary, 7 = Fakahatchee Strand State Park, 8 = Flint Pen Strand, 9 = Florida Panther National Wildlife Refuge,10 = Hickey’s Creek Mitigation Park, 11 = Imperial Marsh Preserve, 12 = Picayune Strand State Forest, 13 = Pine Lake Preserve, 14 = Private Ranch 1, 15 = Private Ranch 2, 16 = Private Ranch 3, 17 = Royal Palm Golf Club, 18 = Royal Poinciana Golf Club, 19 = Royal Wood Golf Club, 20 = Six Mile Cypress Slough. 2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 79 species of interest and a comparison photo depicting the common Sciurus carolinensis Gmelin (Gray Squirrel). Big Cypress Fox Squirrels are distinguishable from Gray Squirrels because southeastern fox squirrels have characteristic black and white face markings and are polymorphic in pelage color ranging from buff to black (Humphrey and Jodice 1992), while Gray Squirrels are standard agouti. Participants were asked to indicate their personal sightings on the following scale: none (no sightings in at least ten years), rare (once or twice a year), and common (weekly or daily sightings). Land use was divided into three categories: urban greenspace, farm/ranch, and park/preserve. Data collected from questionnaires and field surveys were analyzed with contingency analyses to examine relationships between the presence/absence of fox squirrels and land use. Specifically, a G-test of independence, which is an alternative to the chi-square test in which only the observed frequencies are used to calculate the expected frequencies (Zar 1998), was used to test the hypotheses. G-tests of independence were used to examine if the Big Cypress Fox Squirrel is more widely distributed in urban greenspace and other managed landscapes where there is minimal understory growth than in natural areas. JMP (JMP 7, Statistical Analysis Software) was used to perform contingency analyses. Alpha (α) was set at 0.05. Results Of the 1000 questionnaires we distributed to document recent and historic sightings of Big Cypress Fox Squirrels, 145 were returned (52% from private land owners, 32% from golf course managers, 16% from biologists and field personnel of public land-managing agencies). Contingency analyses revealed a significant interaction between fox squirrel presence/absence and land use. The presence/absence of the Big Cypress Fox Squirrel in a site is not independent of land use (G = 10.24, df = 2, P < 0.01, n = 145). According to the surveys returned to us, Big Cypress Fox Squirrels were reported as present in 78% of sites in urban greenspace, but at only 51% of farm/ranch sites and 62% of park/preserve lands (Fig. 2). A significant interaction (G = 20.65, df = 2, P < 0.01, n = 96), also was found between fox squirrel sightings and land use. Big Cypress Fox Squirrel sightings were not independent of land use. Big Cypress Fox Squirrel sightings were reported as common in 63% of urban greenspace sites, but common sightings were only reported in 35% of farm/ranch sites (Fig. 3). No (0%) fox squirrel sightings in park/preserve lands were reported as common. Big Cypress Fox Squirrels were observed in only 7 (35%) of the 20 transect study sites, and 5 of those sites were urban greenspace lands. Fox squirrels were not observed along transects in 10 (83%) of the 12 sites located in natural areas. Gray Squirrels were observed at all study sites, including along transects in natural areas where fox squirrels were not observed. Contingency analyses revealed a significant relationship between fox squirrel presence/absence and understory height (G = 5.94, df = 1, P < 0.05, n = 20). Big Cypress Fox Squirrels are primarily found in areas with minimal understory growth, less than 50 cm in height (Fig. 4). Fox squirrels were not observed at sites where mean understory height was greater than 1 m. 80 Southeastern Naturalist Vol. 10, No. 1 Discussion Natural areas, private ranch lands, and urban greenspace all support Big Cypress Fox Squirrels, but individuals are widely distributed throughout southwestern Florida. In this study, Big Cypress Fox Squirrels were most commonly Figure 3. Percentages of responses from biologists, golf course managers, and private land owners for land use types characterized by Big Cypress Fox Squirrel sightings as rare and common (n = 96). Figure 2. Percentages of responses from biologists, golf course managers, and private land owners for land-use types characterized by the absence and presence of Big Cypress Fox Squirrels (n = 145). 2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 81 found in urban greenspace areas such as golf courses and other intensively managed lands, but were rarely observed in parks, preserves and other natural areas. These results are in accordance with findings from previous studies (Ditgen 1999, Jodice 1990) where, despite intensive searches in the Big Cypress National Preserve and other natural areas, very few fox squirrels were found. Thus, we accept H1 that Big Cypress Fox Squirrels occur more often in intensively managed landscapes than in natural areas. We also accept H2 that Big Cypress Fox Squirrels are found in areas with minimal understory growth because fox squirrels in our study primarily used sites characterized by less than 50 cm of growth and were not found in areas where the understory height averaged greater than 1 m. These results suggest that land uses that maintain open and low understory vegetation may provide suitable habitat for the Big Cypress Fox Squirrel. A caveat is that golf courses, pastures, and many residential areas may have large pines and oaks and support fox squirrels today, but may not support fox squirrels in the future due to low or nonexistent recruitment of appropriate trees (Wooding 1997). Fox squirrels historically existed in a landscape where lightning fires maintained a grassy, open forest floor (Kiltie 1989). Fire exclusion has altered the natural fire regime such that many areas within parks and preserves now have a dense and relatively tall understory. Although prescribed fire is commonly a component of habitat management plans in southwest Florida, budgetary and regulatory restrictions reduce the ability of land managers to burn as often or extensively as needed to replicate historic vegetation structure. Periodic fires are vital for maintaining habitat quality in many Florida ecosystems (Abrahamson and Harnett 1990). Pineoak forests thrive with frequent fires that release nutrients required by trees and their mycorrhizal fungi, and the open-canopied stands and patches of bare mineral soil Figure 4. Percentages of transect sites characterized by the absence and presence of Big Cypress Fox Squirrels in terms of understory height class (n = 20). 82 Southeastern Naturalist Vol. 10, No. 1 produced by fire result in better pine reproduction (Weigl et al. 1989). Fire suppression, and also probably a greater use of winter as opposed to growing-season burns (Tanner and Mullahey 1999), has led to increased cover of Saw Palmetto (Serenoa repens) and other shrubs. These practices have created a dense understory unsuitable for fox squirrels, which travel and forage extensively on the ground (Ditgen et al. 2007, Jodice and Humphrey 1992). Loeb (1999) found that Gray Squirrels may outcompete fox squirrels in areas with dense understory growth. Big Cypress Fox Squirrels were found to coexist with Gray Squirrels in urban greenspace areas like golf courses and private ranch lands. Big Cypress Fox Squirrels probably occur more often in urban greenspace areas than natural areas because understory vegetation in suburban areas is typically intensively managed. The Big Cypress Fox Squirrel was recently denied federal listing, in part because of its opportunistic use of golf courses and other suburban lands (Ditgen et al. 2007). Although fox squirrels are attracted to suburban areas because of an open understory and year-round food supply (Ditgen et al. 2007), populations may not be prospering at such locations and may not persist in the long term due to a deficiency of large trees (Wooding 1997). Ditgen (1999) documented Big Cypress Fox Squirrel population levels in a variety of golf courses and developed a landscape evaluation index that ranked golf courses in terms of habitat suitability for fox squirrels, concluding that fewer than 5 of 48 golf courses were capable of maintaining populations during the period of intensive development that is expected through the year 2020. Suburban areas may have attractive habitat cues for fox squirrels such as a minimal understory. Nevertheless, although the habitat structure in these urban greenspace areas may be currently suitable for fox squirrels, the presence of roads, pets, and other risks may make these areas ecological traps or “attractive sinks” (Delibes et al. 2001). A species has source-sink dynamics if births exceed deaths in some habitats (sources), while deaths exceed births in other habitats (sinks). Source-sink dynamics often are related to habitat fragmentation (Pulliam 1988). Although large, unfragmented tracts of forest are not a prerequisite for successful management of southeastern fox squirrels, the effects of various land-use practices on population dynamics can differ greatly among areas, even within subspecies (Lee et al. 2009). Additional study is needed to determine whether fox squirrel recruitment on golf courses and other suburban areas represents local reproduction or dispersal from other areas. Such data would help illuminate source-sink or other metapopulation dynamics and clarify whether these populations are likely to remain viable in the long term. Long-term studies of tree recruitment in habitats that appear suitable today also are needed. Big Cypress Fox Squirrels were found to be more common in urban greenspace and other suburban areas than in natural areas. Although the ability to detect fox squirrels in natural areas (with greater than minimal cover) was likely lower than in suburban areas (with clear open landscapes and habituation of squirrels to human presence), results from our surveys of natural area biologists and managers corroborated these findings. Understory height may be the key determinant in fox squirrel avoidance versus preference of habitats, whereas other factors, such as road density, traffic volume and fire frequency may separate source from sink populations. Understory growth due to fire exclusion has resulted in less suitable 2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 83 habitat for fox squirrels, which require open-canopied, mature pine forests with minimal understory (Wooding 1997). Therefore, habitat management practices such as more extensive and frequent prescribed burning in parks, preserves, and other conservation lands may be a critical tool to enhance habitat quality in natural areas for Big Cypress Fox Squirrels. Spatially explicit population viability analyses would aid future decision-making and the development of policies to best conserve the Big Cypress Fox Squirrel. Acknowledgments The US Fish and Wildlife Service provided the funding for this project. We thank J.F. Weishampel and J.D. Roth for their input and guidance throughout this research project. We also thank C. Knickerbocker for generating the map showing study site locations and J.E. Fauth and P.F. Quintana-Ascencio for statistical advice. 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