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For over-water dispersal to be considered a realistic explanation for the distribution of species in the Caribbean, it must be demonstrated that a viable population could be established. This can be accomplished by the invasion of either a pregnant individual, an asexually reproducing individual or several individuals of both sexes. Animals have been observed on rafting flotsam3, but most of these are small organisms, such as insects4,5. The few accounts of vertebrates found on rafts6,7 have reported only single individuals (but see ref. 8), and do not provide convincing evidence that a population can become established once landfall is reached.

On 4 October 1995, at least 15 individuals of the green iguana, Iguana iguana, appeared on the eastern beaches of Anguilla in the Caribbean. This species did not previously occur on the island. They arrived on a mat of logs and uprooted trees, some of which were more than 30 feet long and had large root masses. Local fishermen say the mat was extensive and took two days to pile up on shore. They reported seeing iguanas on both the beach and on logs in the bay.

Dispersal events are often assumed to be caused by large storms, such as hurricanes9. The 1995 hurricane season had above normal activity, with 11 hurricanes and 8 tropical storms10. On 4 and 5 September 1995, hurricane Luis moved through the eastern Caribbean (Fig. 1). The storm was rated category 4 on the Saffir/Simpson Hurricane Scale (SSHS). Hurricane-force winds were reported as far south as Guadeloupe. A week and half later, on 14-17 September, hurricane Marilyn (SSHS category 2) followed a parallel path slightly south of hurricane Luis, and many of the same islands once again experienced hurricane-force winds (Fig. 1). Approximately a month after the first of these hurricanes, iguanas reached the shores of Anguilla.

Figure 1: Tracks of hurricane Luis and hurricane Marilyn through the islands of the Lesser Antilles in the Caribbean.
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Dates with arrows indicate the first sightings of iguanas.

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A survey was established within a month of the invasion. We preserved one individual (CM 145848) and marked seven others. Another iguana escaped before processing, and three more were caught by the fishermen but died shortly afterwards. A further three iguanas were sighted on Scrub Island, 0.5 km northeast of Anguilla, and green iguanas were also reported on the northeast coast of Barbuda, 150 km southeast of Anguilla (D. V. Nicholson, personal communication).

Captured individuals (three males and five females) ranged in snout-vent length from 276 to 400 mm. Iguanas were captured between December 1995 and March 1998. Our most recent sighting of I. iguana on Anguilla, on 11 March 1998 (29 months after the invasion), was of a female with enlarged ovarian follicles (possibly oviductal eggs). Because both males and females invaded the island, survived and appear to be healthy, with a female in reproductive condition, the likelihood of reproduction is high.

There are two species of Iguanain the Lesser Antilles: I. iguana and I. delicatissima (Fig. 1). I. iguana is widespread, occurring from Mexico through Central America into South America as far south as northern Paraguay, and is also found on many of the southern islands of the Lesser Antilles. I. delicatissima occurs on most islands in the Lesser Antilles where I. iguana is absent. The track of hurricanes Luis and Marilyn (Fig. 1), the current distribution of I. iguana in the Lesser Antilles (Fig. 1), and the general west-northwest ocean currents in the region suggest that these iguanas originated on the island of Guadeloupe.

The probability that a species will successfully colonize an island depends on both the probability that it will reach the island and the probability that it will survive once it arrives. Our observations confirm that raft dispersal can occur successfully, and document the over-water dispersal of a group of large vertebrates and their persistence and possible reproduction after landfall.