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Florida Grasshopper Sparrow Distribution, Abundance, and Habitat Availability
Michael F. Delany, Matthew B. Shumar, Molly E. McDermott, Paul S. Kubilis, James L. Hatchitt, and Rosanna G. Rivero

Southeastern Naturalist, Volume 6, Number 1 (2007): 15–26

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2007 SOUTHEASTERN NATURALIST 6(1):15–26 Florida Grasshopper Sparrow Distribution, Abundance, and Habitat Availability Michael F. Delany1,*, Matthew B. Shumar2, Molly E. McDermott3, Paul S. Kubilis1, James L. Hatchitt4, and Rosanna G. Rivero1 Abstract - Remote sensing, geographical information system applications, and ground and aerial assessments revealed a fragmented distribution of 44,933 ha of potential habitat for the endangered Ammodramus savannarum floridanus (Florida Grasshopper Sparrow) with most of this habitat (30,262 ha, 67%) located on conservation lands. A continued decrease in available habitat since 1996 was indicated. Searches of potential habitat and information from surveys at known locations found 278 male Florida Grasshopper Sparrows at seven sub-populations during 2004. No previously unknown breeding aggregations were found. The current distribution evinces a considerable contraction in range compared to historic distribution; however, other breeding aggregations may exist on private property (10,718 ha) where access was denied. Three formerly large sub-populations on Avon Park Air Force Range have declined and are now near extirpation. The low number of individuals and the paucity and fragmented distribution of suitable dry prairie will be limiting factors for recovery of this sedentary subspecies. Habitat expansion and management, and demographic improvements at existing locations may restore some Florida Grasshopper Sparrow sub-populations. Large areas (> 377 ha) of protected potential habitat in Manatee, DeSoto, and Glades counties offer the best opportunities for the establishment of additional sub-populations (> 50 pairs) to achieve recovery goals. The cooperative effort of public land managers from various agencies and of private landowners will be needed to prevent the extinction of this bird. Introduction Ammodramus savannarum floridanus Mearns (Florida Grasshopper Sparrow) is an endangered subspecies endemic to the south-central prairie region of Florida (USFWS 1999). Breeding aggregations (sub-populations) are known from only 6 locations, and fewer than 1000 individuals may exist (Delany et al. 1999). The recovery objective is to down-list the sparrow to threatened when > 10 protected locations contain stable, self-sustaining subpopulations of > 50 breeding pairs (USFWS 1999). However, only two extant sub-populations—Three Lakes Wildlife Management Area (WMA) and Kissimmee Prairie Preserve State Park—meet recovery criteria. One other protected sub-population occurs on the Prairie Lakes Unit of Three Lakes WMA, and 3 sub-populations occur on Avon Park Air Force Range. Florida Grasshopper Sparrows on protected lands are monitored with annual 1Florida Fish and Wildlife Conservation Commission, 4005 South Main Street, Gainesville, FL 32601. 2PO Box 32, Donegal, PA 15628. 343 Francis Street, Uniontown, PA 15401. 4Armasi Inc., 3966 Southwest 98th Drive, Gainesville, FL 32608. *Corresponding author - mike.delany@myFWC.com. 16 Southeastern Naturalist Vol. 6, No. 1 point-count surveys, and grassland habitat is maintained for sparrows with prescribed fire every 2 to 3 years (Delany et al. 1999). Basic information on population trends and habitat use is needed to develop and implement conservation strategies for the sparrow (USFWS 1999). Florida’s dry prairie is a variable and poorly known suite of plant communities (Bridges and Reese 1999). The habitat type is characterized as flat, open expanses dominated by fire-dependent grasses, Serenoa repens (Bart.) Small (saw palmetto), and low shrubs (Abrahamson and Hartnett 1990). Dry prairie occupied by Florida Grasshopper Sparrows (Fig. 1) is treeless and ranges from thick (34% shrub cover), low (57 cm) saw palmetto scrub to grass pastures with a sparse (< 10% shrub cover) or patchy cover of shrubs and saw palmetto (Delany et al. 1985). The area of native prairie in Florida has been greatly reduced by agriculture (Davis 1967), and this probably caused the extirpation of the sparrow from some former breeding locations (Delany and Linda 1994). The conversion of natural habitats on private lands in south-central Florida to agriculture and urban development is accelerating (Kautz 1998, Kautz et al. 1993), and continued habitat fragmentation will have detrimental effects on the Florida Grasshopper Sparrow (Perkins et al. 2003). Potential habitat needs to be identified and searched for additional Florida Grasshopper Sparrows (Delany et al. 1999, USFWS 1999). Our objectives were to ascertain the current location of potential habitat for Florida Grasshopper Sparrows in south-central Florida and determine the distribution and abundance of the subspecies. We used remote sensing and GIS applications to Figure 1. Dry prairie on Avon Park Air Force Range, Highlands County, FL. The location was dominated by saw palmetto and native grasses, and occupied by Florida Grasshopper Sparrows. Photograph © by Barry Mansell. 2007 M.F. Delany et al. 17 identify and map potential habitat. Areas of potential habitat were verified and searched for Florida Grasshopper Sparrows, and information was obtained on the status of sub-populations on protected lands. Methods Potential habitat for Florida Grasshopper Sparrows was estimated from a consensus of several data sources and confirmed by ground and aerial assessments. We obtained digital land-cover information for south-central Florida (30-m resolution, 1994 and 1995 Landsat Thematic Mapper data) processed into standard land-cover types, and a data file of Universal Transverse Mercator (UTM) coordinates for 1672 individual Florida Grasshopper Sparrow locations derived from point-count surveys and banding data from across all habitat occupied during 1996–1999 (Delany et al. 1999). Using ArcView (ESRI 1990) software, the Landsat land-cover file was overlain on the file of sparrow locations to determine the dominant land cover of occupied areas. Water Management District land-use land-cover (LULC) files from 1995 identifying pasture, shrub and brush land, and mixed rangeland categories also were projected to create a complementary profile to verify land-cover results from the Landsat analysis. Areas similar in composition to occupied locations (potential habitat) were identified in Florida south of 28o 30' latitude, and polygons surrounding each area were digitized using ArcView (ESRI 1990). Because A. savannarum Gmelin (Grasshopper Sparrows) avoid small grassland patches and forested edges (Vickery 1996), polygons < 100 ha or with a narrow configuration (< 1128 m wide, the diameter of a 100-ha circle) were excluded from the dataset. Because the Landsat dry prairie land-cover category included opencanopy pineland (10–15% cover) unsuitable for Grasshopper Sparrows, scanned aerial photographs (3-m resolution, USGS 1995 digital orthographic quarter quads) were geo-referenced to the polygon vector data to detect trees. Polygon outlines were viewed with aerial photograph backdrops and visually inspected. Polygons containing trees were eliminated from the dataset or redrawn to exclude tree cover. Distances between the edges of polygons were measured using ArcView (ESRI 1990). Landowners of potential habitat were determined using Land Use Mapping System (LUMS) property appraiser data files and county plat maps. The dry prairie land-cover category and Ilex glabra [L.] Gray (gallberry)/saw palmetto compositional group (a shrub and graminoid community associated with wet flatwoods) were portrayed when Florida Grasshopper Sparrow locations were viewed in context with the Landsat land-cover data. However, some occupied areas showed very little of either land cover. The LULC data generally coincided with the satellite data, but also indicated some potential habitat where there was no indication of the dry prairie or gallberry/saw palmetto land coverage based on the Landsat data. In this case, the LULC data were presumed to be something other than potential habitat and were not included. In cases where the amount of 18 Southeastern Naturalist Vol. 6, No. 1 relevant Landsat coverage was sparse (< 25%) but overlain by relevant LULC data, it was included in the delineation of potential habitat. Landowners and managers were mailed information about the Florida Grasshopper Sparrow and contacted by telephone for permission to verify habitat suitability and conduct sparrow surveys. We visited areas of potential habitat to assess their suitability and determine if a search for Florida Grasshopper Sparrows was appropriate. Habitat evaluations were based on: 1) vegetative cover—the presence of species characteristic of the dry prairie plant community and the absence of trees; 2) vegetation structure indicative of recent fire—grasslands that were low in height (< 100 cm) with about 20% bare-ground; and 3) landscape context—large (> 100 ha) areas with a configuration that minimizes edge, and proximity to other patches of suitable habitat. Our criteria for designating potential habitat were similar to those used by Shriver and Vickery (1999) to describe “high quality habitat” for Florida Grasshopper Sparrows. From 7 April to 22 May 2004, surveys were conducted between 0650 and 0955 by two to four searchers who walked transects at 200-m intervals across potential habitat, stopping frequently to make visual and auditory observations. Transect start and end points were recorded with a GPS unit (Magellan GPS 300 and 315). Surveys were conducted during low-wind (< 10 km/hr) conditions and during mornings with no fog or precipitation. A tape recording of the territorial song of the male Grasshopper Sparrow was used to elicit responses from any males in the area. Information was obtained on the presence or absence of Florida Grasshopper Sparrows, survey location, and general habitat description. Information on the status of populations on public lands during 2004 was obtained from land managers who conducted pointcount surveys according to Walsh et al. (1995). Point-count survey methods were the same for all years and locations (three 5-minute unlimited-distance observations during the breeding season); however, the number of points sampled differed by year at some locations. Because of concerns of land-use restrictions if an endangered species was found, most private landowners denied access to verify habitat (40 polygons), and owners of four polygons could not be contacted. For these areas, visual assessments were conducted aerially from a Bell Jet Ranger helicopter during 16.1 hours of flight time at an altitude of < 122 m from 7 February to 28 March 2006. Flights were first made over areas of Three Lakes WMA and Kissimmee Prairie Preserve State Park currently occupied by Florida Grasshopper Sparrows, to develop an aerial search image for suitable habitat. A GPS receiver and topographical maps were used to navigate to the center point of each polygon of potential habitat where access was denied. Large polygons were circled until a complete view of the area was obtained. “High quality” habitat identified during 1996 aerial assessments by Shriver and Vickery (1999) also were visited to determine their current suitability, and we searched for additional potential habitat during flights. 2007 M.F. Delany et al. 19 Results Remote-sensing and GIS applications, and ground and aerial verification revealed a fragmented distribution of potential habitat (44,933 ha) for Florida Grasshopper Sparrows (Fig. 2), with most of this habitat (30,262 ha, 67%) on protected lands. Mean polygon size was 1248 ha (SD = 1248, N = 36) and ranged from 233 to 5355 ha. Mean distance from the edge of extant sub-populations to the edge of the nearest unoccupied polygon was 10.4 km (SD = 9.01, N = 7) and ranged from 2.3 to 29.3 km. Large-scale maps and landowner or manager contact information are available from the senior author. We searched for Florida Grasshopper Sparrows in 12 polygons, obtained data for seven polygons on conservation areas that were sampled during existing point-count surveys, and acquired information on a subpopulation on private property (USFWS 2002) (Figs. 2 and 3). A total of 278 male Florida Grasshopper Sparrows were detected at seven disjunct Figure 2. Areas (36 polygons) of potential habitat for Florida Grasshopper Sparrows derived from 1994 and 1995 Landsat Thematic Mapper data, land-use/landcover files, aerial photographs, and 2004 ground and 2006 aerial assessments. Black polygons were occupied by Florida Grasshopper Sparrows, darkgray polygons were searched but no Florida Grasshopper Sparrows were detected, and light gray polygons were unsearched because access was denied. 20 Southeastern Naturalist Vol. 6, No. 1 sub-populations: Three Lakes WMA (2), Avon Park Air Force Range (3), Kissimmee Prairie Preserve State Park (1), and the privately owned Beaty Ranch (1; USFWS 2002). Sparrow occurrence on Kissimmee Prairie Preserve State Park was considered to be one sub-population, and distribution reflects the location of point-count survey sampling grids. Survey results from 2004 are in Table 1, and specific locations of extant sub-populations are shown in Figure 3. No previously unknown breeding aggregations were Figure 3. Conservation areas (light gray), polygons of potential habitat (dark gray), and locations of Florida Grasshopper Sparrows (black), 2004. For Avon Park Air Force Range, locations are from top to bottom: Bravo Range, OQ Range, and Echo Range. Florida Grasshopper Sparrows on Kissimmee Prairie Preserve State Park were considered one sub-population. 2007 M.F. Delany et al. 21 found. For all potential habitat detected, Florida Grasshopper Sparrow subpopulations occupied 10 polygons (20,852 ha), 11 polygons (13,364 ha) were surveyed without detecting additional birds, and 15 polygons (10,718 ha) were unsearched. Discussion Most potential habitat for Florida Grasshopper Sparrows occurred in disjunct patches within the former dry prairie land cover mapped by Davis (1967). Our delineation of potential habitat using remote sensing and GIS applications detected four of the seven locations known to be occupied. Bravo Range was not selected because the amount of relevant Landsat coverage was sparse (< 25%) and not supported by LULC data. Though the site was occupied, a high percentage of bare-ground cover at this military target classified it as disturbed land marginally suitable for Grasshopper Sparrows. The Prairie Lakes Unit of Three Lakes WMA was not selected because of changes in vegetation since Landsat data were obtained. Rollerchopping and prescribed fire since 1999 improved habitat for the sparrow, and translocation of 18 juveniles to this location during 2001 and 2002 (Florida Fish and Wildlife Conservation Commission [FWC], Tallahassee, FL, unpubl. data) may have established a new sub-population. The location of an estimated 12–20 male Florida Grasshopper Sparrows in a 371-ha Paspalum sp. L. (bahia grass) pasture and 3 males in an adjacent 17-ha pasture on the Beaty Ranch in Okeechobee County (USFWS 2002) were not detected in our search for potential habitat or during aerial surveys by Table 1. Number of male Florida Grasshopper Sparrows detected during distribution and point count surveys at all known locations, 1996–1998 and 2004. The number of point count stations is in parentheses. Males Males Prairie Location County 1996–1998A 2004 (ha) Source Three Lakes WMA S. Glass, FWCB Main population Osceola 94 (161) 124 (169) 4000 Prairie Lakes Unit Osceola – 6 841 Avon Park Air Force Range J. Tucker, ABSC Bravo Range Polk 21 (39) 1 (39) 581 OQ Range Highlands 49 (67) 4 (82) 700 Echo Range Highlands 67 (119) 10 (118) 1195 Kissimmee Prairie PreserveD Okeechobee 111 (137) 107 (188) 8470 P. Miller, DEPE Adams RanchF Osceola – 18 171 This study Beaty Ranch Okeechobee – 8 388 USFWS, unpubl. data AFrom Delany et al. (1999). Point count survey methods were the same as in 2004. Each point sampled about 12 ha. BFlorida Fish and Wildlife Conservation Commission. CArchbold Biological Station. DNot all available habitat was sampled. This population is probably much larger. EDepartment of Environmental Protection. FContiguous with main population at Three Lakes WMA. 22 Southeastern Naturalist Vol. 6, No. 1 Shriver and Vickery (1999). Improved pastures (plowed and planted with non-native grasses) were not considered potential habitat for Florida Grasshopper Sparrows (see Delany and Linda 1994). A breeding location in an improved pasture on Kissimmee Prairie Preserve State Park was similarly overlooked. Advantages and caveats associated with remote sensing and GIS applications in ornithological research were reviewed by Shaw and Atkinson (1990) and Glen and Ripple (2004). Despite some errors, Landsat coverage supported by other relevant data sources were useful not only to perform initial assessment of occupied and potential habitats, but also to optimize the process of selection and delineation of these areas. However, after this initial stage, ground and aerial assessments were the best methods to verify habiat suitability and improved our results. Searches of large areas of potential habitat on Myakka River State Park (718 ha, Manatee County) and conservation easements in DeSoto County (782 ha) (this study) and Glades County (2490 ha) (Delany et al. 2000) failed to locate Florida Grasshopper Sparrows. Areas of seemingly potential habitat are often unoccupied by the sparrow (Howell 1932, Walsh et al. 1995). Some patches of dry prairie may be unused because of their isolation from extant sub-populations. Historically, most dry prairie was found within the Kissimmee River Basin and west of Lake Okeechobee (Davis 1967). Because of variation in methods and the inclusion of different plant communities, estimates of the extent of historical dry prairie and changes in coverage are difficult to interpret. Kautz et al. (1993) calculated 830,000 ha of pre-settlement dry prairie from Davis (1967), and used 1985–1989 Landsat imagery to estimate 561,114 ha of remaining dry prairie statewide in the 1980s. Shriver and Vickery (1999) estimated that only 19% (156,000 ha) of the original dry prairie remained in central peninsular Florida, and their aerial assessment of eight counties identified 63,968 ha of “high-quality” habitat for Florida Grasshopper Sparrows in 1996. Our estimate of 44,933 ha of suitable dry prairie indicates a continued loss of potential habitat for Florida Grasshopper Sparrows. Compared to results from Shriver and Vickery (1999), we identified smaller patches of dry prairie and found potential habitat outside their search area. Previous information indicated a more widespread distribution of Florida Grasshopper Sparrows (reviewed in Stevenson and Anderson 1994) and may reflect a formerly greater coverage of dry prairie. The current distribution of Florida Grasshopper Sparrows at only seven locations within about 900 km2 evinces a considerable contraction in range compared to their historic occurrence. The six males found on the Prairie Lakes Unit of Three Lakes WMA (3.2 km from the main population) may be a recently formed breeding aggregation due to habitat improvements and translocation of sparrows. Not all potential habitat was sampled at Kissimmee Prairie Preserve State Park, so this sub-population is probably larger than our data indicate. All three sub-populations on Avon Park Air Force Range have declined in size and are 2007 M.F. Delany et al. 23 near extirpation. Intermittent extirpations of small populations (< 10 individuals) of grassland birds are to be expected (Curnutt et al. 1996); however, the synchronous decrease of formerly large sub-populations (see Delany et al. 1999, and Shriver and Vickery 1999) at Avon Park Air Force Range is cause for concern. The low number of Florida Grasshopper Sparrows and the paucity and fragmented distribution of suitable dry prairie will be limiting factors for recovery. Current protected areas of dry prairie may not provide adequate habitat to meet recovery goals. In addition to Three Lakes WMA and Kissimmee Prairie Preserve State Park, eight additional sub-populations of > 50 breeding pairs are needed to down-list the sparrow to threatened (USFWS 1999). A sub-population of > 50 breeding pairs of Florida Grasshopper Sparrows may require 1348 ha of contiguous prairie habitat (Delany et al. 1995). We found 12 polygons that met this size criterion, with 11 located entirely or partly on protected lands, including 6 polygons with extant sub-populations. One 1392-ha polygon of potential habitat was located entirely on private property in southwest Highlands County. Perkins et al. (2003) recommended 4000 ha of prairie to support a subpopulation, but only three polygons of potential habitat met their size criterion. Habitat expansion and management and demographic improvements may restore Florida Grasshopper Sparrows on Avon Park Air Force Range. With further land-management and translocation efforts, the Prairie Lakes Unit of Three Lakes WMA also may support a sub-population of > 50 pairs. Because of its large size and protection, potential habitat on Myakka River State Park (Manatee County) and conservation easements in DeSoto and Glades counties offer the best opportunities for the establishment of additional sub-populations to achieve recovery goals. However, these locations were > 47.4 km from extant Florida Grasshopper Sparrows. Banding studies indicated that dispersal limitations imposed by habitat fragmentation may be problematic for the formation of new sub-populations (Delany et al. 1995, Perkins and Vickery 2001, but see Miller 2006). The distance between extant sub-populations and the nearest potential habitat (> 2.3 km) may inhibit colonization. Although even low dispersal rates can promote the persistence of meta-populations (Brown and Kodric-Brown 1977), recovery of this sparrow may require active translocation to facilitate the formation of new sub-populations. The efficacy of experimental translocations at Three Lakes WMA need to be evaluated. Methods and results of translocations need to be thoroughly documented (Fischer and Lindenmayer 2000, Scott and Carpenter 1987), and pending evaluation, other locations assessed for possible translocations. Without adequate habitat acquisition and management to improve connectivity, translocation may be an important alternative for saving endangered species (Lublow 1996). Increasing the area of dry prairie and improving the connectivity of subpopulations were the most effective management options for improving the viability of the Florida Grasshopper Sparrow (Vickery and Perkins 2003). 24 Southeastern Naturalist Vol. 6, No. 1 Larger prairies (> 4000 ha) would support a greater number of sparrows, and a concomitant increase in core area (> 400 m from edge) may improve reproduction (Perkins et al. 2003). Because Grasshopper Sparrow density and reproduction is usually negatively correlated with edge (Vickery 1996), the shape of grasslands (perimeter to area ratio) should also be a factor in land acquisition, restoration, and management. Immediate and intensive efforts are needed to restore Florida Grasshopper Sparrows on Avon Park Air Force Range. The sparrow appears to be responsive to habitat improvements (Delany 1996, Perkins and Vickery 2005), and management actions (e.g., adequate prescribed fire to reduce woody vegetation and promote grass coverage, and removal of intervening and encroaching woody vegetation) may promote optimal breeding conditions (see Delany and Linda 1998a,b) and expand potential habitat. The feasibility of restoring improved pastures to more prairie-like conditions should be investigated to increase potential habitat in former dry prairie once occupied by Florida Grasshopper Sparrows. Although plant species composition may differ from native prairie, structural features of restored grasslands may provide suitable breeding habitat for Grasshopper Sparrows (Fletcher and Koford 2002). The removal of pine plantations to improve the connectivity of sub-populations should be considered. Other breeding aggregations may exist on unsearched private lands where access was denied, and landowners should be contacted periodically for permission to conduct surveys. Search efforts should be expanded to include improved pastures with some remaining native vegetation that may provide suitable breeding habitat, and Florida Grasshopper Sparrow reproductive success in these improved pastures needs to be determined. The cooperative effort of public land managers from various agencies as well as private landowners will be needed to prevent the loss of this sparrow. Acknowledgments This study was funded by the Florida Nongame Wildlife Trust Fund and US Air Force through US Army medical research acquisition activity award DAMD 17-00-2- 0023, and USFWS grant agreement 401815G180. We thank P. Ebersbach, P.B. Walsh, S.M. Cumberbatch, T.F. Dean, K.B. Donnelly, and L.M. Torres, who effectively promoted this effort. This publication does not necessarily reflect the position or policy of the US Government, and no official endorsement should be inferred. We thank L. Adams (Adams Ranch, Inc.), R. Bateman (Bright Hour Ranch), M. Chanen (Cargill Fertilizer), G. Paul (Bob Paul, Inc.), R. Overstreet (Overstreet Ranch), C. Wilson (Latt Maxcy Corp.), and M. Folk and S. Woiak (The Nature Conservancy’s Disney Wilderness Preserve) for access to private property. M.H. Friedman and J.W. Tucker, Jr. participated in surveys. T. Macklin piloted the helicopter and helped search for dry prairie. J. Bridges, D. Donaghy, P. Miller, D. Myers, D. Smith, and W. VanGelder provided logistical support. S. Glass, J. W. Tucker, Jr., and P. Miller provided information about Florida Grasshopper Sparrow populations on public lands. K.E. Miller, T.E. O’Meara, D.W. Perkins, J.F. Quinn, Jr., J.A. Rodgers, Jr., and two anonymous reviewers commented on previous drafts of this paper. 2007 M.F. 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