The black spiny-tailed iguana (Ctenosaura similis) on Gasparilla Island is an example of an exotic lizard species (Meshaka et al. 2004) where control was initiated at the behest of affected residents. Also known as ctenosaurs, these large lizards became established on this 11 km-long barrier island along Florida's west coast with an introduction of as few as three individuals around 30 - 35 years ago (Krysko et al. 2003). Since then, the ctenosaur population has saturated the terrestrial habitats on the island in high numbers, including all residential and commercial areas. The boundary line between two counties runs across Gasparilla Island, and the iguanas had become such a nuisance to property owners through damage to landscape plants and homes (especially attics) that residents of both counties voted to self-tax to secure funds for ctenosaur control programs. Moreover, as has been examined for green iguanas (Iguana iguana), ctenosaur burrows could undermine public works, such as seawalls and levees, weakening them for withstanding severe storm events (Sementelli et al. 2008).
Ctenosaurs conflict with a variety of ecological interests in addition to the economic interests on the island. While Gasparilla Island is largely developed, it also is the location for Gasparilla Island State Park, 49 ha of mostly natural area on the southern end of the island (FDEP 2002). Also despite the development, Gasparilla Island's beaches are home or potential nesting site for a variety of species federally or state-listed as threatened, endangered or of concern (FDEP 2002). The endemic listed species on Gasparilla Island for which this species may pose a threat include eggs and young of nesting shorebirds, beach mice, hatchling sea turtles and gopher tortoises (Gopherus polyphemus) (Krysko et al. 2003). It may also pose a threat to attack snakes on the island (Engeman et al. in press), including some size classes of eastern indigo snakes (Drymarchon corais couperi), a threatened species (Moler 1992). Further environmental impacts include a mutualistic association between ctenosaurs and Brazilian pepper (Schinus terebinthifolius), the most problematic invasive plant on Gasparilla Island (FDEP 2002, Jackson and Jackson 2007). Populations of both species are enhanced by ctenosaur foraging on Brazilian pepper (Jackson and Jackson 2007). Invasive plant control is time consuming and costly, and the ctenosaur serves to increase the problem and raise potential remediation costs.
Active iguana removal was implemented in both counties to reduce, and ultimately eradicate if possible, their populations, albeit differing approaches have been applied in the two counties. Lee County on the southern portion of the island applied a sole-source bounty system whereby a reward has been paid to a contractor for each lizard removed (by a variety of methods). Charlotte County on the northern portion of the island formed an agreement to remove ctenosaurs with the U.S. Department of Agriculture's Wildlife Services (WS), the federal agency authorized to resolve human-wildlife conflicts. Their multi-faceted approach includes population monitoring, iguana removal (also by a variety of methods), and research to develop and evaluate control methods (including toxicant screening tests). Over time, the approaches by the two counties will provide an interesting comparison in efficacies and economics. The cost-per-lizard to remove iguanas in Lee County remains constant, whereas the cost-per-lizard decreases with each subsequent iguana removed in Charlotte County. Once the number of ctenosaurs captured in Charlotte County exceeds the amount of the agreement divided by the amount of the Lee County bounty, then the Charlotte County approach becomes more cost-effective.
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