Differential colonization success and asymmetrical interactions between two lizard species

Small islands in the Bahamas often do not contain Anolis lizards. We took an experimental approach to investigate whether absence of A. carolinensis and A. sagrei from these islands results from the inability of populations to become established and whether the presence of the other species has a negative effect on population density and probability of successful colonization. We introduced propagules of five lizards of each species, either in allopatry or sympatry, on five blocks of three islands each. Populations were censused yearly using mark-recapture methods. Anolis sagrei populations thrived on almost all islands, whereas within three years, many A. carolinensis populations had gone extinct, and most others were very small. We conclude that an inability to successfully establish populations on small islands might explain the absence of A. carolinensis from these islands, but not the absence of A. sagrei. Anolis carolinensis was not more likely to become extinct on islands containing A. sagrei, but population densities of A. carolinensis were lower on islands with both species ("sympatric islands") compared to "allopatric islands." In addition, A. carolinensis tended to perch higher, relative to available vegetation, on sympatric islands than they did on allopatric islands. conversely, the presence of A. carolinensis had little impact on A. sagrei populations. However, inspection of population changes through time suggest that A. carolinensis may have initially had an effect on A. sagrei, but this effect decreased as A. carolinensis populations declined. The observed absence of A. sagrei from these islands may indicate that its dispersal abilities are limited relative to the frequency of devastating hurricanes.