Invasions and Competitive Displacement among House Geckos in the Tropical Pacific'

Since the 1 930s the common house gecko, Hemidactylus frenatus, a sexual species, has been inadvertently introduced to many tropical Pacific islands. Using markrecapture censuses and visual gecko searches we found that Lepidodactylus lugubris, an asexual gecko previously common on these islands, is nearly 800% more abundant on buildings in the urban/suburban environment on islands that lack H. frenatus than it is on islands where H. frenatus is present. On buildings in Hawaii and Fiji that have been surveyed in different years, the proportion of H. frenatus relative to L. lugubris has significantly increased over time. The degree of numerical dominance is also related to climate and habitat: L. lugubris is relatively more common on the more mesic, cooler, windward sides of large islands compared to the more arid sides of islands and in general H. frenatus does not penetrate forest habitats, where L. lugubris remains one of the most common geckos. Overall, buildings with external electric lights have more geckos than unlit buildings. Where the two species occur on the same buildings H. frenatus is closer, on average, to the prime feeding sites near lights that attract insects. In the absence of H. frenatus, L. lugubris is found closer to the lights. We hypothesize that the previously demonstrated agonistic dominance of H. frenatus over L. lugubris (Bolger and Case 1992) is likely to lead to competitive superiority when insect prey are concentrated into patches that are structurally simple (like flat building walls with lights), allowing easy detection of prey and intruders.