Updated Distribution of the Lower Keys Marsh Rabbit

Transcription

1 Research Note Updated Distribution of the Lower Keys Marsh Rabbit CRAIG A. FAULHABER, 1 Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USA NEIL D. PERRY, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USA NOVA J. SILVY, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USA ROEL R. LOPEZ, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USA PHILIP A. FRANK, 2 United States Fish and Wildlife Service, Florida Keys Wildlife Refuges, Big Pine Key, FL 33043, USA PHILLIP T. HUGHES, Untied States Fish and Wildlife Service, South Florida Ecological Services Office, Vero Beach, FL 32960, USA MARKUS J. PETERSON, 3 Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USA (JOURNAL OF WILDLIFE MANAGEMENT 71(1): ; 2007) DOI: KEY WORDS biogeography, distribution, Florida Keys, habitat, lagomorph, Lower Keys marsh rabbit, metapopulation, Sylvilagus palustris hefneri. The Lower Keys marsh rabbit (LKMR; Sylvilagus palustris hefneri) is endemic to the Lower Keys of Florida and exists as a metapopulation in patches of salt-marsh buttonwood transition zone, freshwater wetlands, and coastal beach berm vegetation (Forys and Humphrey 1996). Local LKMR populations interact through dispersing individuals, and patches undergo periodic local extinctions and recolonization (Forys and Humphrey 1999a). Much of the LKMR s habitat has been lost or fragmented by human development over the past several decades, prompting the United States Fish and Wildlife Service (USFWS) to list the subspecies as endangered in 1990 (USFWS 1990). Knowledge of the distribution of occupied and potential LKMR habitat patches is critical to the design of recovery strategies for the subspecies. Historically, LKMRs occupied most, if not all, larger islands from Big Pine Key to Boca Chica Key, and may have occurred on Key West as well (DePourtales 1877, Layne 1974, Howe 1988, Lazell 1989; D. Stevenson, Felix Environmental Services, personal communication; Fig. 1). Although rapid habitat loss occurred in the 1970s and 1980s (USFWS 1997), little information is available on the LKMR s distribution prior to The oldest known written records are anonymous notes dated written on blue line photographs discovered at the National Key Deer Refuge in These records documented presence of LKMRs at 20 sites on Big Pine Key. Howe (1988) conducted the first published survey of the LKMR metapopulation by surveying 13 patches noted by J. Lazell (The Conservation Agency, unpublished data) and 3 additional patches. He reported that rabbits were absent from 4 of these patches, no longer occurred on Cudjoe Key, 1 Present address: Department of Wildland Resources, 5230 Old Main Hill, Utah State University, Logan, UT 84322, USA 2 Present address: Consulting, Engineering and Science, Florida Keys Field Office, Overseas Highway, Cudjoe Key, FL 33042, USA 3 mpeterson@tamu.edu and were threatened by human activities in remaining patches. Forys (1995) completed a more comprehensive survey from 1991 through She identified 59 patches of occupied and potential (i.e., unoccupied, but apparently suitable) LKMR habitat. This survey included all but 2 of the patches noted by Howe (1988). LKMRs consistently occupied 19 of these patches over the course of her study, occupied 23 other patches in 1 survey, and did not occupy 17 patches. In 1995, Forys et al. (1996) discovered an additional 19 and 47 occupied and potential patches, respectively. Forys (1995) and Forys et al. (1996) found no extant populations between Big Pine and Sugarloaf keys. Based on these surveys and demographic data collected by Forys (1995), a population viability analysis (PVA) model suggested the metapopulation had a 100% probability of extinction in the next 50 years (Forys and Humphrey 1999b). Researchers have not formally surveyed the range-wide status and distribution of the LKMR since the mid-1990s. Managers charged with the recovery of this species need current data. Therefore, we surveyed potential LKMR habitat throughout the Lower Keys from 2001 through Specifically, our objectives were 1) to update the distribution and occupancy status of LKMR habitat patches, 2) to document patterns in patch occupancy from 2001 through 2005, and 3) to draw comparisons to previous formal and informal surveys. STUDY AREA The Lower Keys formed the end of a string of limestone islands near the southern tip of peninsular Florida, USA, and extended.60 km in a southwesterly direction from Little Duck Key ( N; W) to Key West ( N; W; Fig. 1). The climate was tropical, with wet (May Nov) and dry (Dec Apr) seasons. Elevation rarely exceeded 2 m, but variations in elevation,0.25 m yielded distinct vegetative types that transitioned with increasing elevation from mangrove swamps, to salt- 208 The Journal of Wildlife Management 71(1)

2 Figure 1. Range-wide distribution of the endangered Lower Keys marsh rabbit in the Lower Keys of Florida, USA, Historically, rabbits probably occupied all of the larger islands within the Lower Keys. marsh buttonwood transition zones (brackish wetlands), to tropical hardwood hammocks or pinelands (McGarry MacAulay et al. 1994, Faulhaber 2003). The LKMR primarily occupied patchy brackish wetlands dominated by gulf cord grass (Spartina spartinae), marsh hay cord grass (S. patens), salt-marsh fringe-rush (Fimbristylis castanea), seashore dropseed (Sporobolus virginicus), and sea daisy (Borrichia frutescens), often under an open canopy of buttonwood trees (Conocarpus erectus). The LKMR also occupied patches of freshwater wetlands, typically dominated by sawgrass (Cladium jamaicense), which occurred only on the larger keys. Additionally, rabbits occurred on coastal beach berms covered by xerophytic vegetation and tropical hardwood trees. METHODS From May 2001 through June 2003, we delineated and surveyed occupied and potential (i.e., unoccupied, but apparently suitable) LKMR habitat patches. We identified historic habitat patches with the use of published surveys (Howe 1988, Forys 1995, Forys et al. 1996), unpublished data from A. Schuetz (Naval Air Station, Key West), M. Folk (The Nature Conservancy), and P. A. Frank (USFWS) made in the 1980s and 1990s, and notes on blue line photographs taken We identified new habitat patches by examining digital orthophoto quarter quads (DOQQs) and the Advanced Identification of Wetlands Geographic Information System (GIS) coverage developed by the Florida Marine Research Institute (McGarry MacAulay et al. 1994). To confirm the presence of LKMRs, we systematically searched each patch for LKMR fecal pellet groups along transects placed approximately 5 m apart set transverse to the long axis of the patch (Forys et al. 1996, Forys and Humphrey 1999a). We used this method to assess patch occupancy because it is the approach sanctioned by the LKMR recovery plan for monitoring species recovery (USFWS 1999). There were no other mammals producing similarly sized fecal pellets present within our study areas. We delineated patch boundaries in the field on printouts of DOQQs, 1999 United States Geological Survey aerial photographs, and 2001 satellite photographs of Boca Chica Key. We digitized patch boundaries with the use of ArcView (Version 3.2) GIS. For consistency, we adopted the definition of habitat patch used by Forys (1995), Forys et al. (1996), and the USFWS (1999), with areas divided by major roads, bodies of water, or separated by upland or mangrove vegetation considered separate patches. For this reason, a few of the smaller occupied patches undoubtedly were part of a larger patch biologically, and thus potentially could be smaller than the minimum range size for adult LKMRs (approx. 0.3 ha; USFWS 1999). Boundaries for occupied patches included all areas with fecal pellet groups and adjacent areas of similar vegetation composition and structure. Boundaries for potential habitat patches included brackish wetlands, freshwater marshes, and beach-berm vegetation types with thick herbaceous ground cover and vegetation structure suitable for daytime cover. We searched most patches at least twice, with the exception of patches on islands where we found no evidence of LKMRs anywhere on the island. From 2004 through 2005, we conducted additional surveys where observers searched for pellet groups in expanding concentric circles from a randomly selected point within the patches delineated during until they found pellet groups or until 15 minutes had elapsed. We classified patches in which we found fecal pellet groups at least once during as occupied, and further classified occupancy status as either consistent (fecal pellet groups found on each visit) or variable (fecal pellet groups observed on 1 visit but not on all). We calculated the total area of occupied and potential LKMR habitat throughout the subspecies historic range and examined patterns of patch occupancy from 2001 through We also compared patterns of patch occupancy during our study with notes on blue line Faulhaber et al. Distribution of Lower Keys Marsh Rabbit 209

3 Table 1. Number and area (ha) of occupied and potential Lower Keys marsh rabbit habitat patches documented during and distribution surveys in the Lower Keys of Florida, USA a,b Occupied habitat patches Potential habitat patches Occupied habitat patches Potential habitat patches Key No. Area No. Area No. Area No. Area Big Pine Boca Chica Cudjoe Saddlebunch Sugarloaf Summerland The Torch Keys Other Total a Data from Howe (1988), Forys (1995), Forys et al. (1996) as well as unpublished data from P. A. Frank (United States Fish and Wildlife Service), A. Schuetz (Naval Air Station, Key West), and M. Folk (The Nature Conservancy). b The surveys covered less total search area and thus included fewer patches and than surveys. photographs from 1968 through 1987 and with published and unpublished data from 1988 through 1995 to compare the amount of useable space (Guthery 1997) available to LKMRs through time. RESULTS We delineated 112 patches of occupied LKMR habitat (547.1 ha) on 13 keys during (Table 1). The median size of occupied patches was 2.1 ha, with an interquartile range of ha (range ¼ ha). Big Pine, Sugarloaf, Saddlebunch, and Boca Chica keys contained 86% of both the number of occupied patches and the total occupied patch area (Table 1). Big Pine Key and Boca Chica Key together encompassed 58% of all occupied patches and 67% of the total occupied land area. LKMRs occupied Little Pine Key (3 patches, 12.3 ha) and Water Key (one patch, 1.9 ha) because of reintroductions in 2002 and 2004, respectively (Faulhaber et al. 2006). By 2004, translocated rabbits had colonized an additional patch on both Little Pine Key (0.7 ha) and neighboring East Water Key (0.9 ha). Other islands occupied by LKMRs included Annette, Mayo, Howe, Geiger, East Rockland, and No Name keys. We discovered 45 patches (93.4 ha) of occupied LKMR habitat that previous surveys had not documented; most of these patches were on Big Pine and Boca Chica keys. On Boca Chica Key, some patches occupied by LKMRs included vegetation features not previously described for this subspecies, including areas dominated by wire bluestem (Schizachyrium gracile) and lead tree (Leucaena leucocephala) and areas dominated by gulf coast spike rush (Eleocharis cellulosa) with clusters of buttonwood and mangrove trees. Moreover, we consistently found pellet groups in mangrove vegetation with closely spaced white (Laguncularia racemosa) and black mangroves (Avicennia gerimans) that were seasonally dry because of artificial impoundment. Because we observed this situation only on Boca Chica and East Rockland keys, we did not include mangroves as potential LKMR habitat elsewhere. For patches visited more than once on Big Pine, Boca Chica, and Sugarloaf keys, LKMRs consistently occupied 22, 29, and 5 patches, and variably occupied 6, 7, and 7 patches, respectively. By 2005, LKMRs no longer occurred on East Rockland and Geiger keys, 2 smaller islands adjacent to Boca Chica Key. Moreover, 6 of the 16 occupied patches in the Saddlebunch Keys, though classified as variable by our criteria, exhibited no sign of LKMRs by We found a fecal pellet group in only one patch during one survey on No Name Key conducted in We delineated 108 unoccupied potential habitat patches totaling ha (Table 1). This included 41 patches (59.4 ha) of potential LKMR habitat not previously documented by surveys. The median patch size was 1.4 ha with an interquartile range of ha (range ¼ ha). We found no sign of LKMRs north of United States Highway 1 on Sugarloaf Key or south of United States Highway 1 on Big Pine Key, and there was a large gap in the LKMR s distribution between Big Pine Key and Sugarloaf Key (Fig. 1). This gap contained both half the potential habitat patches and half the total area of potential habitat (Table 1). We were unable to visit 8 patches identified in previous surveys. LKMRs did not occupy any of these patches during the surveys; all but 2 were on small, isolated islands, and the probability of colonization by LKMRs was unlikely because of their isolation from occupied habitat. Fifty-two of the 71 patches occupied by LKMRs during the surveys remained occupied during (Table 1). Additionally, excluding reintroduced populations, LKMRs had colonized 13 of the 71 patches identified as potential habitat during the surveys. Thus, there was a net loss ( 6) in patch occupancy between survey periods. Big Pine Key exhibited the greatest net loss ( 3) of inhabited patches, with 9 of 17 occupied patches still inhabited and 5 of 6 potential habitat patches colonized by The 8 extirpated patches on Big Pine Key included all 6 south of United States Highway 1 (Fig. 1). We found no sign of LKMRs on 12 of 20 sites on Big Pine Key indicated on the blue-line photographs taken in The Journal of Wildlife Management 71(1)

4 1987. Lower Keys marsh rabbits occupied 8 of these 12 sites during the surveys. Excluding reintroduction sites, the only island with a net gain in patch occupancy (þ4) was Sugarloaf Key, with 7 of the 8 patches occupied by LKMRs during the surveys still inhabited and 5 of 15 potential habitat patches colonized by Of 19 consistently occupied patches observed by Forys (1995), 3 exhibited no sign of rabbits over the course of our study, and we failed to find signs of LKMRs on 3 others by DISCUSSION Most occupied LKMR habitat was on Big Pine, Sugarloaf, Saddlebunch, and Boca Chica keys, with.50% on Big Pine and Boca Chica keys. We did not find LKMRs between Big Pine Key and Sugarloaf Key (Fig. 1). This pattern of occupied and unoccupied keys corresponds with the results of earlier surveys (Forys 1995, Forys et al. 1996). We attribute our discovery of undocumented occupied and potential patches to increased search area rather than increased rabbit numbers. Within some patches, we observed LKMRs using habitat features previously undocumented for this subspecies, although all these areas were in close proximity to habitat features commonly used by LKMRs. Other subspecies of marsh rabbits have shown variability in their habitat use. For example, the marsh rabbit of mainland Florida (S. p. paludicola) uses hammocks, agricultural fields, roadsides, and disturbed areas along canals and greenways (Carr 1939, Schwartz 1952; C. A. Faulhaber, Texas A&M University, unpublished data). At any rate, achieving a better understanding of LKMR habitat use would help to refine our understanding of LKMR useable space (Guthery 1997) and may change our delineation of patch boundaries or reveal additional areas of potential habitat. We observed a net reduction in the number of occupied patches over the past decade when we only considered patches found during historical surveys. Reasons for extirpation of LKMR populations likely vary, and causes may include both stochastic and deterministic factors. In 1998, Hurricane Georges created storm surges on Big Pine Key as high as 1.75 m above the mean high tide (Lopez et al. 2003), which may explain the extirpation of rabbits from patches south of United States Highway 1 on this key. Although LKMRs have persisted in other portions of Big Pine Key, development along the United States Highway 1 corridor or habitat alteration due to the storm may have hindered recolonization of the extirpated patches. In other areas, succession of open salt marsh to a closed canopy of buttonwood forest or habitat degradation due to sea-level rise may be reducing the useable space within historically occupied patches. Additionally, predation by feral and freeroaming cats is a significant cause of LKMR mortality (Forys 1995) and may have played a role in initial patch extirpations and subsequent lack of recolonization. We frequently observed cats on Geiger and East Rockland keys, where the extirpation of LKMRs occurred by Although we discovered previously undocumented patches of occupied and potential LKMR habitat, extirpations during and comparisons with historic surveys reveal a reduction in LKMR patch occupancy over the past decade. This downward trend may increase the risk of extinction for the LKMR because metapopulations with fewer occupied patches and smaller total occupied area are less likely to persist over time (Hanski 1998). The decline in LKMR patch occupancy is consistent with the PVA results of Forys and Humphrey (1999b), which predicted a gradual decline in LKMR abundance and extinction within 50 years (of 1995). Between 2001 and 2005, the number of occupied patches on Big Pine, Sugarloaf, and Boca Chica keys remained relatively stable. Conversely, we recorded a large net reduction of occupied patches on the Saddlebunch Keys during this time. This reduction could represent fluctuations typical of metapopulations (Levins 1970, Hanski and Gilpin 1991) because the extirpated patches were relatively distant from development and were interspersed with occupied patches. However, a reduction in the number of occupied patches and total occupied area puts remaining patches at greater risk of extirpation due to demographic and environmental stochasticity (Hanski 1998). Further, it is possible that global warming and rising sea level resulted in progressive degradation of habitat quality on low-elevation keys such as the Saddlebunch Keys. Thus, the situation warrants monitoring, and managers should seek to identify potential deterministic threats. MANAGEMENT IMPLICATIONS Although LKMR abundance appears stable on keys that provide the greatest amount of useable space throughout the year (Guthery 1997), the range-wide downward trend in patch occupancy reinforces the need for managers to increase useable space. To accomplish this, managers must continue to acquire and protect LKMR habitat, promote management designed to increase useable space within protected areas, and reintroduce LKMRs to apparently suitable habitat. Moreover, our results indicate that LKMRs use vegetation types in addition to those previously considered suitable, illustrating a need to revise the working definition of spaces usable by LKMRs. Finally, determining whether habitat management practices and reintroduction efforts actually result in increased space usable by LKMRs throughout the year will require consistent monitoring of LKMR habitat patch occupancy. ACKNOWLEDGMENTS We thank B. Edwards, J. A. Harris, K. B. Melton, J. Mott, B. A. Porter, P. J. Stevko, and L. A. Witter for helping collect field data. The United States Fish and Wildlife Service, Florida Department of Transportation, National Fish and Wildlife Foundation, and Texas A&M University provided funding. Special thanks go to the staff of the National Key Deer Refuge, who provided critical logistical support. We thank Ecology and Environment, Inc. (Miami Lakes, FL) and Naval Air Station, Key West, for access to invaluable DOQQ photos. Faulhaber et al. Distribution of Lower Keys Marsh Rabbit 211

5 LITERATURE CITED Carr, A. F., Jr Notes on escape behavior in the Florida marsh rabbit. Journal of Mammalogy 20: DePourtales, L. F Hints on the origin of the flora and fauna of the Florida Keys. American Naturalist 11: Faulhaber, C. A Updated distribution and reintroduction of the Lower Keys marsh rabbit. Thesis, Texas A&M University, College Station, Texas, USA. Faulhaber, C. A., N. D. Perry, N. J. Silvy, R. R. Lopez, P. A. Frank, and M. J. Peterson Reintroduction of Lower Keys marsh rabbits. Wildlife Society Bulletin 34: Forys, E. A Metapopulations of marsh rabbits: a population viability analysis for the Lower Keys marsh rabbit (Sylvilagus palustris hefneri). Dissertation, University of Florida, Gainesville, USA. Forys, E. A., P. A. Frank, and R. S. Kautz Recovery actions for the Lower Keys marsh rabbit, silver rice rat, and Stock Island tree snail. Florida Game and Fresh Water Fish Commission, Office of Environmental Services, Tallahassee, Florida, USA. Forys, E. A., and S. R. Humphrey Home range and movement of the Lower Keys marsh rabbit in a highly fragmented environment. Journal of Mammalogy 77: Forys, E. A., and S. R. Humphrey. 1999a. The importance of patch attributes and context to the management and recovery of an endangered lagomorph. Landscape Ecology 14: Forys, E. A., and S. R. Humphrey. 1999b. Use of population viability analysis to evaluate management options for the endangered Lower Keys marsh rabbit. Journal of Wildlife Management 63: Guthery, F. S A philosophy of management for northern bobwhites. Journal of Wildlife Management 61: Hanski, I Metapopulation dynamics. Nature 396: Hanski, I., and M. Gilpin Metapopulation dynamics: brief history and conceptual domain. Biological Journal of the Linnean Society 42:3 16. Howe, S. E Lower Keys marsh rabbit status survey. U.S. Fish and Wildlife Service, Jacksonville Field Station, Jacksonville, Florida, USA. Layne, J. N The land mammals of South Florida. Pages in P. J. Gleason, editor. Environments of South Florida: present and past. Miami Geological Society, Miami, Florida, USA. Lazell, J. D., Jr Wildlife of the Florida Keys: a natural history. Island Press, Washington, D.C., USA. Levins, R Extinction. Pages in M. Gesternhaber, editor. Some mathematical problems in biology. American Mathematical Society, Providence, Rhode Island, USA. Lopez, R. L., N. J. Silvy, R. F. Labisky, and P. A. Frank Hurricane impacts on Key deer in the Florida Keys. Journal of Wildlife Management 67: McGarry MacAulay, G., T. J. Leary, F. J. Sargent, M. M. Colby, E. J. Prouty, and C. A. Friel Advanced identification of wetlands in the Florida Keys, final report. Florida Department of Environmental Protection, Division of Marine Resources, Florida Marine Research Institute, Marathon, Florida, USA. Schwartz, A The land mammals of southern Florida and the Upper Florida Keys. Dissertation, University of Michigan, Ann Arbor, USA. United States Fish and Wildlife Service Endangered and threatened wildlife and plants; endangered status for the Lower Keys marsh rabbit and threatened status for the Squirrel Chimney cave shrimp. Federal Register 55: United States Fish and Wildlife Service Biological opinion on FEMA s administration of the National Flood Insurance Program in Monroe County, Florida. U.S. Fish and Wildlife Service, Atlanta, Georgia, USA. United States Fish and Wildlife Service South Florida multi-species recovery plan. U.S. Fish and Wildlife Service, Atlanta, Georgia, USA. Associate Editor: Chambers. 212 The Journal of Wildlife Management 71(1)