Effect of Elevation

Perhaps the most obvious potential effect of island elevation (=altitude) on species occurrences is simple refuge from high water. Little documentation of such an effect has existed, but we now have a precise example. In 1999, a Category IV hurricane swept over small islands of the Great Abaco (Bahamas) region, completely inundating the lower ones with its storm surge. The islands were inhabited by a common lizard species, Anolis sagrei. Before the hurricane, island area was a better predictor (by logistic regression) of the occurrence of this species than was altitude. Immediately after, altitude was the better predictor. Apparently all lizards on islands lower than about 3 m maximum elevation had perished. After c. 1 year, area again became the better predictor: recovery occurred via overwater colonization (the islands were very close to the mainland) and propagation from eggs that survived inundation, mechanisms that were enhanced by a larger island area. While rapid recovery often follows catastrophic disturbance, as in this example and those given above, such is not always the case. Ricklefs and Bermingham postulated that a major change in the mean age of the Lesser Antilles avifauna occurred about 0.5 million years ago, perhaps caused by a catastrophic disturbance such as a tsunami; little postca-tastrophic recovery was evident.

The most commonly discussed effect of altitude on species diversity is more indirect than simple protection from high water: the greater the altitude, the more kinds of habitats, and the greater the number of habitat kinds, the more species can occur. Examples exist for the plants of the Galapagos and, to a lesser extent, the birds of the East Indies. This argument is a special case of the argument for the importance of habitat, and we turn to that topic now.

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