Bermuda consists of more than 120 named islands of various sizes, with a total land area of 54 km² located in the northwest Atlantic (lat 32°18′N, long 64°46′W), approximately 960 km ESE from Cape Hatteras, North Carolina. The pond turtle family Emydidae has 2 species represented in Bermuda: the native diamondback terrapin, Malaclemys terrapin, and the introduced red-eared slider, Trachemys scripta elegans (Bacon et al. 2006; Parham et al. 2008). The latter are native to the central United States, from Alabama through Indiana, westward to Kansas, New Mexico, and the northeastern states of Mexico (Conant and Collins 1991). This subspecies has been exported via the pet trade and biological supply industry to numerous countries and areas around the globe. The occurrence of feral populations, as a result of the escape and release of unwanted pet red-eared sliders, has been well documented (Newbery 1984; Platt and Fontenot 1992; Da Silva and Blasco 1995; Luiselli et al. 1997; Chen and Lue 1998; Cadi et al. 2004). Previous research on alien populations of T. s. elegans in other regions of the globe suggested that this subspecies may negatively impact some native wildlife, especially native chelonians, through competitive dominance and as possible vectors of disease (Hays et al. 1999; Spinks et al. 2003; Cadi and Joly 2003, 2004); however, many of the potential impacts of this introduced reptile still remain largely speculative (see Thomas 2006).

Red-eared sliders were first introduced to Bermuda via the pet trade. Although records do not exist that document when ornamental pet turtles were first imported, anecdotal evidence suggests that they were being sold in the early 1950s (J. Brady, pers. comm.). It is presently not illegal to import live red-eared sliders into Bermuda to be sold as pets; however, this subspecies is now rarely imported because there are a number of local breeders who are supplying the demand for these reptiles as pets.

Alien invasive species are considered the main threat to island biodiversity worldwide (Murphy 2002). Bermuda lacks the diverse suite of North American predators that naturally keep populations of this reptile in check (Dundee and Rossman 1989; Tucker and Janzen 1999). Trachemys scripta elegans has been nominated by the Invasive Species Specialist Group to be among the 100 “world's worst” alien invaders (www.issg.org/database) and is currently listed as exotic, naturalized, and invasive in Bermuda (Kairo et al. 2003; Sterrer et al. 2004). However, there is no published information regarding their ecological status in Bermuda. In 2005, the Bermuda Biodiversity Project launched a study to assess the abundance and country-wide distribution, as well as the potential environmental impact of these feral reptiles.

Methods. — All freshwater and brackish water ponds and drainage canals in Bermuda were visited during the summer months of 2005 and 2006 and visually checked for the presence of T. s. elegans. Turtles were then collected at 7 different sites by using 6 collapsible crab traps made from a half inch vinyl mesh. Traps measured 32 inches in length, 24 inches in width, 11 inches in height, and had a 22-inch-wide opening at both ends through which turtles could enter the trap. All were internally fitted with short segments of pool noodle that provided flotation to maintain a breathing space for the trapped turtles, baited with frozen herring, and anchored in random locations throughout each of the 7 study sites to prevent drifting. Traps were checked for the presence of turtles 3–5 days after baiting.

The maximum straight carapace length (tip to tip), straight plastron length (midline), and carapace width at the widest point of all trapped turtles were measured to the nearest 1 mm with vernier calipers. Animals were sexed, and juveniles were characterized, following Chen and Lue (1998). Food items were identified from the gastrointestinal tracts of 36 individuals (19 females and 17 males) caught with a long-handled dip net from 4 of the ponds. All of these individuals were euthanized immediately after capture, and stomach contents were examined under low microscopic magnification. The frequency of occurrence of each food item (based on the number of turtles that contained a given food item) was recorded and summarized in Table 1. Population densities (number of turtles per ha) were calculated in 6 locations solely based on the number of turtles trapped and, therefore, represent conservative estimates (Table 2). The total surface area of each pond was calculated in ArcGIS 9.0 by using a digitized aerial orthophotograph of the Bermuda islands.

Table 1. Food items obtained from 36 stomach samples of Trachemys scripta elegans collected from 4 sites (South Pond, Warwick Pond, David's Pond, Pitman's Pond) in Bermuda.

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Table 2. Minimum estimates of population density for Trachemys scripta elegans in 6 ponds across Bermuda.

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Results. — A total of 20 freshwater and slightly brackish ponds and canals (salinities ≤12%), mostly located in the central and western parishes of Bermuda, were found to contain feral populations of T. s. elegans (Fig. 1), including 9 ponds in lands designated as either nature reserves or national parks, 9 ponds on golf courses, and one 1.75-km drainage canal that runs through a significant part of the industrial area of the city of Hamilton. A total of 568 turtles were caught and permanently removed from 7 different ponds (all in nature reserves) over a 12-month period (Fig. 1; locations 1–7). Included in this total were 170 females (101–244 mm [carapace length (CL)], mean 172 mm), 304 males (100–217 mm CL, mean 136 mm), and 94 juveniles (30–99 mm CL, mean 79 mm). The majority of the turtles examined for this investigation were classified as adults (83.5%); however, juveniles (individuals < 100 mm CL and without male secondary sexual characteristics) were not uncommon (Fig. 2). Minimum estimates of population density for 6 ponds were as follows: 43 turtles/ha (Seymour's Pond), 81 turtles/ha (Warwick Pond), 226 turtles/ha (South Pond), 252 turtles/ha (Somerset Long Bay Pond), 543 turtles/ha (Edmund Gibbons Nature Reserve Pond), and 981 turtles/ha (Pitman's Pond).

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Plant material occurred in 86.1% of the stomach samples examined, animal material in 77.8%, and unidentifiable sediment in 69.4%. The most frequently ingested items were herbaceous in nature (mostly leaves and stalks of unknown species that occurred in 80.6% of the samples); however, terrestrial and aquatic insects (38.9%) were also frequently ingested. Small fish (13.9%), freshwater snails (11.1%), and bird feathers and bones (8.3%) occurred less frequently in the samples (Table 1).

Discussion. — During the last 400 years of human habitation, Bermuda has witnessed the naturalization of more than 1200 exotic species, the extinction of at least 25 endemic species, and the decline of an estimated 200 native species (Sterrer et al. 2004). Trachemys scripta elegans is the most abundant emydid turtle in Bermuda, and nearly half of known feral populations occur in nature reserves, some of which are critical habitat for rare endangered and threatened native biota. Furthermore, over 80% of freshwater wetland nature reserves and national parks contain feral Trachemys.

Of the 568 red-eared sliders caught for this investigation, 53.5% were considered male, 29.9% female, and 16.6% juvenile; with carapace lengths ranging from 30–244 mm (Fig. 2). The majority of turtles sampled in Bermuda were considered adult; however, the occurrence of 94 juveniles, 18 of which had plastron lengths that ranged from 28–56 mm, which indicated that they were in their first year of growth (Mitchell and Pague 1990), provides good evidence for local reproduction and recruitment that is contributing significantly to the feral populations in Bermuda.

Anecdotal evidence suggests that T. s. elegans has been successfully reproducing in Bermuda since the late 1990s, especially on golf courses, where ground staff have witnessed females laying eggs in sand bunkers and hatchlings emerging (April through August). The author has personally received hatchlings born in “back-yard gardens” from members of the public. In 2006, one local resident handed in 34 unwanted red-eared sliders, including 17 hatchlings that had emerged from the friable soil in a flower bed. Despite the fact that the Bermuda Aquarium, Natural History Museum and Zoo encourages members of the public to drop off unwanted pet sliders, these reptiles continue to be released into the wild (pers. obs.) even though the abandonment of unwanted pets in Bermuda is prohibited by law (Care and Protection of Animals Act 1975).

The omnivorous feeding habit of Trachemys is well documented (Newbery 1984; Ernst and Barbour 1989; Chen and Lue 1998, Prévot-Julliard et al. 2007). In northern Taiwan, 100% of the feral T. s. elegans sampled were found to have ingested animal materials (mostly snails, fishes, adult and larval flies, and unidentifiable terrestrial insects), and 76.5% plant materials (Chen and Lue 1998). Conversely, only 77.8% of the feral red-eared sliders examined in Bermuda had ingested animal materials; whereas, 86.1% had ingested plant materials, and 69.4% ingested organic pond sediment. Most of the vegetative matter comprised leaves, stalks, roots, seeds, and flowers; however, filamentous and blue-green algae were also occasionally ingested. Nearly half of the animal material ingested in Bermuda comprised aquatic and terrestrial insects. Small fishes, freshwater snails, and bird feathers and bones occurred less frequently in the samples. Although fish only occurred in 13.9% of the sliders examined, all came from a single pond location, Warwick Pond (Fig. 1). This site is presently 1 of only 9 small, isolated ponds in Bermuda that are inhabited by an endemic and endangered species of killifish, Fundulus bermudae. The only other fish species currently living in this pond is the introduced eastern mosquitofish, Gambusia holbrooki. An assessment of this Fundulus population in 2005 revealed a very low total population that was almost exclusively dominated by large individuals (Outerbridge et al. 2007). Two thirds of the red-eared sliders examined in 2006 from Warwick Pond (n = 5) were found to have ingested small fishes that ranged in size from 18–41 mm total length. A few could be confirmed as G. holbrooki; however, most were digested beyond species recognition.

Trachemys scripta elegans also coexists with a small population of diamondback terrapins, Malaclemys terrapin. The latter species is native to Bermuda and highly localized to one small area. The ecological impact that feral red-eared sliders have on native chelonians in other regions has been previously documented (Hays et al. 1999; Spinks et al. 2003; Cadi and Joly 2003, 2004); however, the impact on the Bermudian population of diamondback terrapins is currently unknown. Local conservationists fear that the feral sliders are competing with the diamondbacks for nesting sites in this location. All known nesting sites for M. terrapin in Bermuda are limited to 6 small sand bunkers on a neighboring golf course, of which at least 3 are also being used by sliders (pers. obs.). Further studies are needed to determine the extent to which M. terrapin are being negatively affected.

As long as red-eared sliders are kept as pets in Bermuda, and feral populations remain, the potential for dispersal to the remaining wetland nature reserves remains quite high. Some of these protected areas are critical habitats for a variety of rare endemic and native species, several of which are considered endangered. Conservative estimates suggest that there are still at least 1000 feral red-eared sliders in Bermuda. The number of private ponds that contain these reptiles is unknown. In keeping with global concerns, the Department of Conservation Services is giving serious consideration to the problem of invasive alien species, and the continued removal of feral turtles is planned for the future with emphasis placed on ponds that are important refuges for Bermuda's threatened native and endemic species.