Pernas, A.J.; D.W. Clark. A summary of the current progress toward eradication of the Mexican gray squirrel ( Sciurus aureogaster F. Cuvier, 1829) from Biscayne National Park, Florida, USA Island invasives: eradication and management A summary of the current progress toward eradication of the Mexican gray squirrel ( Sciurus aureogaster F. Cuvier, 1829) from Biscayne National Park, Florida, USA A. J. Pernas 1 and D. W. Clark 2 1US National Park Service, Florida/Caribbean Exotic Plant Management Team, 18001 Old Cutler Road, Suite 419, Palmetto Bay, Florida 33157, USA. <[email protected]>. 2US Fish and Wildlife Service, Pacifi c Islands Fish and Wildlife Offi ce, 300 Ala Moana Blvd., Room 3-122, Honolulu, HI 96850, USA. ABSTRACT In 2007, the US National Park Service (NPS) began an eradication programme against Mexican gray squirrels ( Sciurus aureogaster ) on islands within Biscayne National Park. Planning included the 2007 development of a management plan, effi cacy monitoring, a resource management weapons training programme, and ground and aerial surveys to locate dreys (squirrel nests) for follow-on removal. Aerial survey for dreys was incorporated in 2008 and included the use of helicopters and a digital aerial sketch mapping technique. During eradication operations, marked dreys were systematically visited after dusk by trained technicians on foot. Each drey was precisely destroyed by a shotgun using non-lead ammunition and humanely euthanasing any occupants. Project monitoring consisted of regularly scheduled aerial and ground drey surveys, camera traps and nest boxes. Since eradication operations began in 2007, 1410 dreys have been located, marked, and removed from 1360 trees. A total of 33 squirrels have been removed (15 male and 18 female) from Elliott and Sands keys. The eradication project has been a collateral duty of NPS biologists and has been conducted as funding and staff time permit. In light of this limitation, the project is ongoing with complete eradication expected in early 2011. Current project cost is approximately $US70,000 and the fi nal cost is estimated to be $US80,000. Keywords: Squirrel invasions, rodent eradications, restoration, dreys, nest boxes INTRODUCTION Rodents have been eradicated from over 332 islands the islands (Ogden 1992; Davis et al . 1993). However, the around the world (Howald et al . 2007), often with signifi cant species was subsequently found on Elliott Key indicating benefi ts to native biodiversity (e.g., Rauzon 2007). Of the that a population had survived (Koprowski et al . 2005). mammalian invaders, rodents present formidable ecological A survey of Elliot Key in 2005-2007 by Geoffrey and economic threats, which are exemplifi ed by tree squirrels Palmer of the University of Arizona focussed on the (Palmer et al . 2007). Biological characteristics that have conspicuous leaf and stick nests (dreys) built by squirrels enabled tree squirrels to become invasive include: high near the tops of trees as a refuge from weather, predators, reproductive potential, high vagility, diverse food habits, ability to construct nests, and plasticity in human-impacted landscapes. Islands are particularly vulnerable to these invasions because tree squirrels are also able to establish viable populations with very small propagules (Palmer et al . 2007). At least two large-scale squirrel eradication attempts in Europe have failed. In Great Britain during the 1940s and 1950s, attempts to eradicate Scuirus carolinensis included private citizen hunting efforts (Sheail 1999) and poisoning (Dagnall et al . 1998; Sheail 1999), which led to dissent from animal rights groups. This case did give rise to new ideas about squirrel control including manipulation of the physical environment and sterilisation (Dagnall et al . 1998). The second attempt against S. carolinensis was in Italy but was halted because of protests from animal rights groups (Bertolino and Genovesi 2003). In this paper we outline an eradication campaign against the Mexican gray or red-bellied squirrel ( Scuirus aureogaster ) from islands in Biscayne National Park in Florida, USA. We describe the invasion, effects of squirrels on native species, methods used to delimit the populations, and their removal. SQUIRREL INVASION The Mexican gray squirrel is an arboreal species native to southern Mexico. Two pairs of squirrels were purposefully introduced from eastern Mexico to Elliott Key in Biscayne National Park in 1938 (Fig. 1), where they established and became widespread by the 1960s. Squirrels were also reported on the adjacent Adams Key and Sands Key and one was captured swimming across Caesar’s Creek toward Old Rhodes Key (Layne 1997). The squirrels were considered extirpated in 1992 (Layne 1997), Fig. 1 Biscayne National Park and the keys named in the when the tidal surge from Hurricane Andrew submersed text. Pages 222-224 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN,222 Gland, Switzerland. Pernas & Clark: Mexican gray squirrel eradication Biscayne NP and as a safe place to rest and sleep (Brown and McGuire fruit production was also of concern, since Mexican gray 1975). The survey revealed squirrels throughout the squirrels are known to damage agricultural crops such as hardwood hammock forest habitat, with 115 nests (dreys) corn in their native range (Romero-Balderas et al . 2006). documented initially and more than 200 dreys documented The invasive potential of these squirrels was over the course of the study (Palmer 2010). This survey demonstrated from the aerial and ground surveys of dreys was also conducted on other islands within the previous on Elliott Key. However, the isolation of these populations range of the squirrels, including those they had attempted from mainland Florida suggested that the species could to reach. Monthly surveys on Adams Key from December be eradicated. Primary goals for the eradication from 2005 to July 2006 failed to fi nd any sign of squirrels but Biscayne National Park included: 1) eliminate potential squirrels were found on Sands Key and Old Rhodes Key in effects of the squirrels on natural resources within the March 2007. This discovery raised further concern about Park; 2) remove any possibility for squirrel populations to the likely effects of this invasive species within and outside expand their range outside of the Park; and, 3) increase Biscayne National Park. public and agency awareness of the signifi cant threats from In Biscayne National Park, male squirrels had a home invasive species. range of 2.3 ha, and females a home range of 0.9 ha (Brown and McGuire 1975). The squirrels breed year-round and MATERIALS AND METHODS are opportunistic feeders (Koprowski pers. comm.), relying heavily on introduced plants for food including Eradication operations coconut palm ( Cocos nucifera ), sapodilla ( Manikara The National Park Service (NPS) implemented a zapota ), Australian pine ( Casuarina equisetifolia ), and management project for squirrels throughout Biscayne papaya ( Papaya carica ) (Brown and McGuire 1975). National Park in September 2007 through trapping and Subsequent control of these and other non-native plants on humanely eliminating squirrels on National Park islands, Elliott Key has forced squirrels to rely on native plants for follow-up population monitoring, survey, and retreatment. food, including the fruits of sea grape ( Coccoloba uvifera ), The eradication effort began on Old Rhodes Key and Sands mastic ( Mastichodendron foetidissimum ), gumbo limbo Key, in order to eliminate outlying squirrels that likely (Bursera simaruba ), keys thatch palm ( Thrinax morrissii ), originated from Elliott Key. This minimised the risk of Florida thatch palm ( Thrinax radiata ), and the endangered squirrels spreading to additional islands and/or to mainland Sergeant’s Buccaneer palm ( Pseudophoenix sargentii ). Florida. Efforts were then focused on the main population The squirrels also feed on birds’ eggs and invertebrates. on Elliott Key. National Park Service assessments of effects of the squirrel Biscayne National Park is the largest marine park on Elliott Key before Hurricane Andrew (Tilmant 1980) in the US National Park system, with 95% of its 70,000 suggested that they preyed on the declining liguus tree hectares covered by water and few terrestrial resource snail ( Liguus fasciatus) and collected palm leaves from the management staff. The squirrel eradication was conducted Thrina x spp. to line their nests. In 2006, Palmer (2010) and coordinated by staff of the NPS Florida and Caribbean found squirrels using parts of Thrinax to line nest cups, but Exotic Plant Management Team. Biscayne National Park failed to document any nests that utilised parts of the state staff provided project oversight, planning and logistical endangered Pseudophoenix sargentii. support, and assistance. Other damage to native trees from squirrels included Mexican gray squirrels use cavity nests in addition clipped branches and feeding on the plants’ phloem, fruits to constructing dreys. However, there are few trees with and seeds. Nest trees were damaged during the construction cavities on Biscayne National Park islands. Because and maintenance of nests as these trees were the primary cavities are a limited resource, nest boxes were an effective source of nesting material. These data on range, foraging, attraction as nest sites for squirrels. Squirrels in the nest and nest building by squirrels helped with the development boxes were then fl ushed into cage