Species Numbers, Density Compensation, and Colonizing Ability of Lizards on Islands in the Gulf of California

The distribution of lizard species and numbers on islands in the Gulf of California was examined in light of island biogeographic theory. Using linear multiple regression techniques with the percent completeness of the island's lizard fauna as the dependent variable (LS), the relationship and predictive power of several island independent variables (in original and transformed forms) were explored. The results of this analysis differed sharply for different subsets of islands. The independent variable accounting for the most variance in LS for oceanic islands was island area; their distance from the mainland explained most of the residual variance. For land—bridge islands, maximum island elevation was the most predictive single variable, and island distance was least important. In both these island groups and considering all the islands together, island area, elevation, and the number of perennial plant species were highly correlated and explained little of each others residual variance. On 12 islands, diversity of shrub volume was also measured and was found to have less predictive power than the other variables considered. Life history attributes of each lizard species were gleaned from the literature and compared with the respective colonizing abilities of the species. In general species occurring on many Gulf islands (e.g., Uta stansburiana) may reach high mainland population densities, have potentially high birth rates and high death rates, and may be habitat generalists. Species absent from most islands and most likely to go extinct on land—bridge islands (e.g., Xantusia vigilis) have the converse of these attributes. The reasons for the divergence of these results and those expected from predictions is discussed. Lizard fauna relaxation times were calculated for land—bridge islands and were found to be inversely related to island area as predicted from island biogeographic theory. Extinction rates for lizards are about one half as high as those calculated for birds and mammals. Timed searches for lizards were also performed on most of the islands and the adjacent Baja California peninsula. These revealed that the numbers of Uta and total lizard numbers were both inversely related to the number of lizard species occupying an island. The probable reasons for such excess density compensation were examined, and it was concluded that differences in pedation intensity, insect productivity, and possibly competitive interference account for the extremely high number of lizards on some small isolated islands.