Translocation to Unoccupied Habitat
West Indian iguanas seem in general to be a resilient group of organisms able to survive in depauperate ecosystems and arid situations. These large lizards seem not to have been affected by Holocene climatic changes that arguably contributed to historical extinctions of rodents, monkeys, sloths, and tortoises. Even the arrival of Amerindians three or four thousand years ago seems not to have exterminated any taxa. European colonizations, however, and especially the exotic species accompanying them, have been devastating. Nevertheless, West Indian iguanas have been successfully introduced to many areas now dominated by human disturbance. The prospects for restoration of iguanas to previously occupied portions of their ranges seem quite sanguine. In selecting sites for translocation, the following issues should be considered.
Historical presence
Museum specimens, literature records, fossil and subfossil remains, and local testimony can all serve to indicate previous presence, and this is the best initial evidence for at least potential habitat suitability. It can probably be presumed that iguanas occurred throughout the coastal lowlands of any island bank that has an island on it where iguanas demonstrably occur or occurred in the recent past. Given glacial maximum low sea levels as recently as 10,000 years ago, there appears to be little that would have deterred iguana dispersal. Tiny islands, of course, might fail to sustain populations following post-glacial rise in sea levels. For example, rock islets like Monito might provide no suitable nesting soils or even sufficient vegetation to feed a viable iguana population. Still, it is expected that iguanas would have originally been present on most islands on occupied banks down to at least 100ha in area. The propensity of West Indian iguanas to survive, often in abundance, on very small cays is heartening.
Exotic predators and competitors
The different species of iguanas show a spectrum of abilities to deal with predators, especially mammalian carnivores. Some forms, like the rhinoceros iguana on Hispaniola or the Cuban iguana now introduced to Puerto Rico, seem able to survive and reproduce in sympatry with dogs, cats, mongooses, and even people who eat them. Other forms, like cornuta onchiopsis of Navassa and pinguis on Puerto Rico, went extinct so rapidly after European colonization that their existence has been very scantily recorded. Sites which are free of exotic predators, especially dogs, cats, and mongooses, hold the greatest promise for re-establishing populations.
Goats are the most efficient and detrimental of competitors, and it is often very difficult to extirpate them. Because goats are so versatile and opportunistic in their diets (to the extent of climbing trees and devouring plants often toxic to other species) iguanas would probably be unable to establish populations on islands with saturated goat populations. That iguana populations dwindle in the presence of introduced livestock is evidenced by the situation on Anegada in the British Virgin Islands. Although active reproduction is occurring and seemingly healthy hatchlings can be found, lack of available plant food may preclude juveniles from ever adopting a diverse and healthy herbaceous diet. The presence of young iguanas cannot be taken as evidence of a healthy population because exotic competitors may prevent them from ever reaching maturity.
Other iguana species, such as the now widely introduced common iguana, may be very effective competitors. Further, introgressive hybridization may occur between an introduced iguana species and the previously incumbent native, as seems to have happened in Les Iles des Saintes. In sympatry, it generally appears that one iguana taxon either out-competes or genetically swamps another. Therefore, attempts to establish sympatry should be avoided.
Microhabitat conditions
Iguanas have to be able to dig holes to lay eggs. Many species utilize beach berms, while others excavate inland soils. The requirements of each must be accommodated. Very small islands are especially likely to lack potential nesting sites.
Iguanas seem to do best in places where the sun reaches them early in the morning, particularly on flat islands, ridge tops, and east-facing slopes. On Guana Island, Goodyear and Lazell (1994) found that even large males climb trees in the evening, seemingly to be in good positions for early insolation. This may mean that densities on hilly islands with west-facing slopes will be lower (other factors being equal) than on flat islands. Circumstantial evidence on Guana Island indicates that hatchlings move from their nest sites principally into areas with early morning direct sun. This may limit dispersal on hilly islands to east-facing slopes that rise west of suitable nest sites. Evidence on these points is scanty at best, but their consideration can only improve restoration prospects, and suggest valuable lines of future research.
Potential for human interactions
Generally, human interactions with wild iguanas are negatively perceived. Traditionally, people have killed iguanas in the West Indies to eat them, or because they viewed them as agricultural pests, or because they fear any large wild animal, or for sport. All of these reasons are potential causes for concern over the fate of translocated iguanas, and should be among the most obvious factors considered in such planning. On the other hand, human interactions can sometimes also be very beneficial. Many people, including tourists, are enthusiastically fond of iguanas. They will go out of their way to see and photograph iguanas, and delight in the antics of semi-tame individuals. On Anegada, at least a few iguanas did well despite dogs, cats, goats, and usually hostile humans because one individual liked them enough to set up a feeding station.
While a dim view of subsidizing wild animals is usually taken because this is not regarded as being in their long-term interest, feeding stations may be appropriate during the early stages of translocation. Also, the popularity of iguanas at a particular site may be a factor in their protection (e.g., from dogs), and may help insure that new populations are monitored at least informally.
Genetic considerations
Much has been made of inbreeding depression and genetic problems associated with small numbers of founders. However, it is a certainty that many natural populations of West Indian iguanas were derived from one or a few founders, and subsequently did quite well. Different taxa vary in their reproductive capacity, but all are relatively fecund. Although a greater number of founders is preferable, it is probably the case that even a single healthy, gravid female can establish a successful population. From a dispersalist point of view, this is most likely the way all known taxa of West Indian iguanas originated.
Conclusions
In summary, iguanas are apt to do very well on small islands, provided these islands have been cleared of goats, mongooses, and other detrimental exotics, and that the vegetation either remains in or has recovered to a near-natural state. Islands with tourist presence may be especially suitable because feral livestock are often removed from them and interested people will tend to mitigate deleterious acts, such as killing by dogs or humans, and help to monitor iguana numbers.
