Behavioural mechanisms of invasion and displacement in Pacific island geckos (Hemidactylus)

The house gecko Hemidactylus frenatus is currently displacing an ecologically similar, all-female parthenogenetic gecko H. garnotii on a global scale. We tested the previously proposed hypotheses that resource competition is responsible for this displacement, and that competitive interference occurs through male aggression. We quantified consumption of insect resources and behavioural interactions in dyads and triads composed of different combinations of sexes and species. Geckos consumed fewer insects when in pairs than when they were alone, indicating general evidence of resource competition. However, resource consumption of parthenogenetic H. garnotii was not negatively affected by the presence of invasive H. frenatus, suggesting that resource competition is not the primary mechanism of displacement. Males were aggressive towards conspecific males, but showed little aggression towards conspecific females and parthenogenetic females; females and parthenogens displayed little aggression. Male H. frenatus courted and copulated with both conspecific and heterospecific females in dyads and in triads and, by some measures, showed a preference for larger H. garnotii females. When paired with females or parthenogens, males consumed fewer resources because they turned their attention to courtship. Evidence of interspecific courtship suggests that sexual interference may be a more likely mechanism of displacement. Our results exclude the possibility that courtship decreases resource consumption in H. garnotii, but further study is required to determine whether interspecific mating disrupts parthenogenetic reproduction. This study demonstrates the importance of testing commonly invoked mechanisms of invasion and displacement by quantifying individual behaviour in an experimental setting.