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.
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 traps and euthanized. of alternatives for conducting the eradication and was Nest boxes were also useful for squirrel population incorporated into a formal management plan for the species. monitoring, with their use by squirrels acting as an indicator The documented impacts on native vegetation from the of missed individuals during the eradication project. There squirrels strengthened the case for their eradication from is anecdotal evidence that squirrels will use nest boxes and Biscayne National Park. multiple nests that they have constructed in trees. In light The impact of the Mexican gray squirrel on South of this, we simultaneously removed squirrels from nest Florida ecosystems is poorly understood, although boxes and physically removed dreys and their inhabitants. introduced populations of other squirrels throughout the This proved to be an effective and humane method for world are known to have detrimental effects (Koprowski removing the entire population from the Biscayne National pers. comm.). Park islands. Primary concerns about the spread of squirrels within Aerial and ground surveys of the mixed-hardwood Biscayne National Park included: damage to native forest were conducted following eradication operations vegetation, such as the endangered P. sargentii , and state- to locate any remaining dreys in the canopy. Host trees threatened thatch palms (Thrinax radiata and T. morrisii) ; were fl agged and their coordinates recorded using a Global avian nest predation; competition with the state-threatened Positioning System to facilitate relocation. Aerial surveys white-crowned pigeon ( Columba leucocephala ); and were conducted by NPS staff in helicopters timed with feeding on the liguus tree snail, a species of special concern seasonal tropical hardwood hammock defoliation (typically in Florida. in the spring). Ground surveys are conducted by NPS staff The potential for further spread of the squirrels to with emphasis on previously identifi ed drey locations. other islands and mainland Florida is of environmental, During eradication, trained personnel returned at agricultural, and economic concern. Squirrels could sundown to any trees with dreys fl agged during the day. compete with the federally endangered Neotoma fl oridana Each drey was destroyed and its occupants euthanized smalli , the federally endangered Key Largo cotton mouse using 12-guage shotguns with non-lead ammunition at a (Peromyscus gossypinus allapaticola ), the state threatened safe, close distance. Weapons were fi red into dreys from Big Cypress fox squirrel ( Sciurus niger avicennia ), the grey directions that ensured areas utilised by visitors (such as squirrel (Sciurus caroliniensis ) and other native species. marina, buildings, campground) were not in the line of Potential damage to Florida’s agriculture and tropical fi re. Firearm use by non law enforcement NPS personnel
223 Island invasives: eradication and management for squirrel management in Biscayne National Park was The cryptic daytime habits of the squirrels, their ability conducted at the discretion and authorisation of the Park to move rapidly through the canopy, and effi ciency in Superintendent in accordance with a specifi c training building dreys meant that constant pressure was required syllabus developed for this project. Squirrel carcasses to achieve eradication. Additional funding and staff time were recovered, stored in freezers, and subsequently sent are still required to ensure that eradication is completed. to wildlife specialists for examination. However, given progress so far, we are now confi dent that Approximately 20 nest boxes were installed near the techniques used to eradicate the Mexican gray squirrels known squirrel populations on Elliott, Old Rhodes, and from Biscayne National Park will be successful and cost Sands keys. One nest box was also installed on each of effective. Porgy, Adams, and Totten keys where squirrels had not been observed. These islands are within Biscayne National ACKNOWLEDGEMENTS Park and between the squirrel populations and the Florida The authors thank Mark Lewis, Superintendent, and Keys to the south. This provided a means of detecting any Elsa Alvear, Supervisory Resource Management Specialist, squirrels migrating towards these keys as well as individuals of Biscayne National Park for project oversight, planning, previously undetected. Nest box construction and placement compliance and logistical support, Dr. John Koprowski was conducted in accordance with guidelines developed and Geoffrey Palmer of the University of Arizona for by the Oregon Department of Fish and Wildlife (http:// survey work 2005-2007, USDA-APHIS Wildlife Services www.dfw.state.or.us/conservationstrategy/naturescaping/ for eradication operations partnering, Judd Patterson and squirrel_nesting_box.asp). The boxes were constructed Brian Witcher, National Park Service, South Florida, and of rough-sawn cedar, installed before shooting operations Caribbean Network, for assistance with data management started, and were attached to trees using plastic tie straps to and GIS and Henrik Moller, University of Otago, New prevent damage to host trees. Locally obtained leaf litter Zealand, and an anonymous reviewer for their insightful was used in each nest box to eliminate introduction of non- comments. native species and to stimulate use by the squirrels. Monitoring REFERENCES The removal of all squirrel nests and their inhabitants Bertolino S. and Genovesi, P. 2003. Spread and attempted eradication of the grey squirrel (Sciuros carolinensis) in Italy, and consequences for from each island should have eliminated all squirrels. The the red squirrel (Sciuros vulgaris) in Eurasia. Biological Conservation nest boxes installed following nest removal provide an 109 : 351-358. immediate place for staff to check for any squirrels missed Brown, L.N. and McGuire, R.J. 1975. Field ecology of the exotic Mexican during nest removal. Monthly visual monitoring of the red-bellied squirrel in Florida. Journal of Mammalogy 56 : 405-419. nest boxes commenced in the summer of 2007. To date, no Mexican gray squirrels have been detected. Inspections of Dagnall, J.; Gurnell, L. and Pepper, H. 1998. Barkstripping damage by gray squirrels in state forests of the United Kingdom: a review. the next boxes will continue monthly for one year. In: Steele, M.A.; Merritt, J.F. and Zegers, D.A. (eds.). Ecology and Twelve camera traps have also been placed evolutionary biology of tree squirrels , pp. 249-261. Special Publication, systematically throughout Elliott Key and Sands Key in Virginia Museum of Natural History, Martinsville, U.S.A. trees at bait stations baited with corn and/or sunfl ower Davis, G.E.; Flora, M.; Loope, L.L.; Mitchell, B.; Roman, C.T.; Smith, seeds. Bait stations and cameras are monitored at monthly G.; Soukup, M. and Tilmant, J.T. 1993. Assessment of Hurricane Andrew’s immediate impacts on natural and archaeological resources of intervals and will remain in place for a year. Big Cypress National Preserve, Biscayne National Park, and Everglades National Park. George Wright Forum 10 : 30-40. RESULTS AND DISCUSSION Howald G.; Donlan C.J.; Galvan J.P.; Russell J.C.; Parkes J.; Samaniego From September 2007 to February 2010, 1410 dreys A.; Wang Y.; Veitch D.; Genovesi P.; Pascal M.; Saunders A. and Tershy B. 2007. Invasive rodent eradication on islands. Conservation Biology and 33 Mexican gray squirrels (15 males and 18 females) 21 : 1258-1268. were removed with an average of 43 dreys per squirrel. Koprowski, J.L.; Kellison, G.T. and Moneysmith, S.L. 2005. Status of As drey removal progressed, the number of dreys and red-bellied squirrels ( Sciurus aureogaster ) introduced to Elliott Key, subsequently the number of squirrels declined per unit Florida. Florida Field Naturalist 33 : 128-129. effort. We anticipate that eradication will be completed in Layne, J. 1997. Non-indigenous mammals. In: Simberloff, D.; Schmitz, 2011 at a total cost of about US$ 80,000 (Table 1). D. and Brown, T. (eds.). Strangers in paradise: impact and management This project is the fi rst attempt to eradicate a squirrel of nonindigenous species in Florida , pp. 157-186. Island Press, population in the State of Florida for conservation purposes. Washington, D.C., U.S.A. We found no examples in the literature where this had been Ogden, J.C. 1992. The impact of Hurricane Andrew on the ecosystems of achieved elsewhere for the conservation of native species. South Florida. Conservation Biology 6 : 488-490. NPS biologists continue to be concerned about the Palmer, G.H. 2010. Ecological assessment of Mexican red-bellied potential ecological effect Mexican gray squirrels on the squirrels ( Sciurus aureogaster) introduced to Elliott Key, Florida. MS habitats and listed species found within Biscayne National Thesis, University of Arizona, Tucson, AZ, U.S.A. Park. It is particularly important to keep the species from Palmer, G.H; Koprowski, J.L. and Pernas, A.J. 2007. Tree squirrels reaching the mainland of Florida and the United States. as invasive species: conservation and management implications. In: Witmer, G.W.; Pitt, W.C. and Fagerstone, K.A. (eds.). Managing vertebrate invasive species: proceedings of an international symposium , pp. 273-282. USDAIAPHISIWS, National Wildlife Research Center, Table 1 Costs of the Mexican Gray squirrel eradication Fort Collins, CO., U.S.A. from Biscayne National Park for the period: September 2007- February 2010 Rauzon M.J. 2007. Island restoration: exploring the past, anticipating the future. Marine Ornithology 35 : 91-107. Action Cost US$ Romero-Balderas,K.G.; Naranjo, E.J.; Morales, H.H.; and Nigh R.B. Initial Population Research/Assessment $18,187 2006. Interciencia 1: 276-283. Nest/inhabitant Removal $27,688 Sheail, J. 1999. The grey squirrel (Sciurus carolinensis) - a UK historical perspective on a vertebrate pest species. Journal of Environmental Monitoring $12,090 Management 55 : 145-156. Transportation $6100 Tilmant, J.T. 1980. Investigations of rodent damage to the thatch palms Equipment/Supplies $6200 Thrinax morrisii and Thrinax radiata on Elliot Key, Biscayne National Total $70,725 Park, Florida. South Florida Research Center, Everglades National Park, U.S.A.
224