Distributional Patterns in St. Croix Sphaerodactylus Lizards: The Taxon Cycle in Action

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Abstract
Two species of small gekkonid lizards, genus Sphaerodactylus, occur on St. Croix. S. macrolepis occupies most of the island and S. beattyi is restricted to 3 disjunct areas in the eastern part of St. Croix. There is little or no overlap of the 2 species'' ranges. At several points the 2 are very sharply parapatric. The current distributions are compared with records from the late 1930''s and from 1965. Borders between the 2 species may have oscillated back and forth during the last 40 yr. This distributional pattern is probably due to the competitive exclusion of S. beattyi by S. macrolepis from the more mesic portions of the islands, which may be the most favorable habitat for both species. Evaporative water loss is significantly lower for S. beattyi, supporting this interpretation. A model of interspecific competition along an environmental gradient is discussed which could explain these observations.

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