Going feral: Time and propagule pressure determine range expansion of Asian house geckos into natural environments

Upon establishment in a new area, invasive species may undergo a prolonged period of relatively slow population growth and spread, known as a lag period. Lag periods are, apparently, common in invasions, but studies of the factors that facilitate subsequent expansions are lacking in natural systems. We used 10 semi-independent invasions of the Asian house gecko (Hemidactylus frenatus) to investigate which factors facilitate expansion of this human-associated species across the urban-woodland interface. We conducted 590 surveys over 12months on 10 transects running from the urban edge to 2km into adjacent natural woodland. We recorded H.frenatus out to 2km from the urban edge on nine of 10 transects, and at high abundance at many woodland sites. Body size, body condition, sex ratio and proportion of gravid females did not vary with distance from the urban edge, suggesting viable, self-sustaining populations in natural habitats. The extent of expansion was, however, strongly dependent on propagule pressure (the abundance of H.frenatus at the urban edge), and time (time since H.frenatus established in the urban area). The size of the urban area and the structure of the surrounding environment did not impact invasion. Our results show that an invasive species that is deemed human-associated' over most of its range is invading natural habitats, and propagule pressure strongly controls the lag time in this system, a finding that echoes results for establishment probability at larger scales.