Biogeographic Considerations
Despite, or perhaps because of, the enormous amount of factual information developed since 1492, the West Indies remain a region of extreme controversy (Williams 1989). Probably no other tropical realm has been so extensively studied, but fundamental disagreements over the origin of the islands and their biota remain. A critical but neglected factor in West Indian biogeography has been the enormous change in land areas and potential dispersal distances wrought by sea level fluctuations dependent on glacial and interglacial climatic cycles. Dramatic sea level changes in the region have greatly modified total land area and in many cases altered dispersal distances by orders of magnitude (Lazell 1996).
As a rough generalization, the West Indian islands today surmount banks of rather shallow water which fall off abruptly at their edges. Not surprisingly, the biogeographic significance of the banks is largely reflected in the degree of difference in biotas: islands on the same bank have rather similar, slightly differentiated faunas, while islands on separate banks show high degrees of endemism (Williams 1969). The West Indian iguanas form a suite of obviously very closely allied forms. Hispaniola, the Great Bahama bank, the Greater Puerto Rico bank (Virgin Islands), and the Guadeloupéen archipelago have two forms each, but otherwise there is only one form on any island bank. In general, West Indian iguanas present an overall picture of weak differentiation in which geographically intermediate forms are apt to be morphologically intermediate (Lazell, 1989). Hybridization between forms is known to occur.
Climate change has combined with human disturbances over the last 4,000 years to alter the West Indian fauna dramatically. Many mammals, large birds, and reptiles once occupied a broad array of islands but are now extinct. It seems that iguanas, however, tolerated these changes well until European invasion. However, in the last century, and especially in the last three decades, changes in iguana populations and ranges have been catastrophic. Teeming populations have simply disappeared and animals once abundant are now critically endangered.
Iguanas tend to be most common in coastal lowlands, and these areas tend to be covered with oceanic limestone formed during times of high sea levels dating back at least to the Miocene. Limestone, being porous and often cavernous, drains quickly and provides poor soil compared to volcanic terrain. Iguanas may be associated with limestone terrains because they find security from exotic predators like dogs and cats in the limestone cavities, and as a result of their ability to survive on soils of very low agricultural potential. In the Lesser Antilles, however, Iguana delicatissima was once abundant on volcanic strata directly adjacent to the sea. It is possible that limestone habitats are suboptimal for iguanas, and iguana presence there today represents fortuitous survival rather than adaptation.
