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

Abstract

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 semiindependent 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 12 months on 10 transects running from the urban edge to 2 km into adjacent natural woodland. We recorded H. frenatus out to 2 km 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.

Antipredator behaviour of invasive geckos in response to chemical cues from snakes

“…The success of the Asian house gecko invasion is probably related to high propagule pressure (Barnett et al, ), and if their antipredator response is preadapted to Australian predators, this could undoubtedly offer them an advantage in natural environments (Chapple et al, ). Although we found only weak avoidance of certain snakes, even small differences in fitness can drive strong selection, so this behaviour could still offer a partial explanation for the success of Asian house geckos as invaders.…”

“…All geckos were captured by hand using spotlight surveys in two different substrate types, that is, from a building or man‐made structure or from vegetation ≥150 m from any building. Asian house geckos can occur in high density up to 2 km from the urban edge in Townsville (Barnett et al, ). We found Asian house geckos occurred in considerable density up to 150 m from anthropogenic structures (similar to reports by Keim () for Darwin), but at distances <150 m Asian house gecko numbers dropped dramatically at our particular site (pers.…”

“…Asian house geckos occur in urban environments, and although the reasons for their success are unknown, it is mainly attributed to their competitive superiority relative to many native gecko species, combined with a high tolerance for conspecifics (Canyon & Hii, ; Petren & Case, ). On the Australian mainland, Asian house geckos are typically associated with anthropogenic structures, but occur in natural areas close to human habitation (Hoskin, ) and have established populations in surrounding bushland (~2 km; Barnett, Phillips, & Hoskin, ). Introduced urban species may encounter favourable conditions in human‐altered environments, that allow for significant population growth adjacent to natural habitats, such as an abundance of warm microhabitats (Hoskin, ), areas under‐utilised by native species (Germaine & Wakeling, ) and an abundance of artificial lights that attract insect food (Zozaya, Alford, & Schwarzkopf, ).…”

“…One factor that could contribute to the expansion of Asian house geckos into natural areas may be their response to native predators (Barnett et al, ; Sih et al, ). Small lizards are exposed to novel environmental conditions, prey items, pathogens and possibly even predators, as “stowaways” during human‐mediated dispersal events; so possessing behaviourally flexible traits may increase successful colonization by introduced species (Chapple, Simmonds, & Wong, ).…”

The impact of parasites during range expansion of an invasive gecko

“…Despite documented impacts in some areas, H. frenatus is typically considered a benign invader due to the belief that it is restricted to built areas (Vanderduys and Kutt, 2013). In northern Australia, however, H. frenatus are spreading from urban areas into bushland (Hoskin, 2011; Barnett et al 2017). This range expansion is bringing H. frenatus into contact with a number of ecologically similar native species (Hoskin, 2011; Barnett et al 2017).…”