Potential impacts of intraguild predation by invasive Asian house geckos

The introduction of the Asian house gecko, Hemidactylus frenatus, to mainland Australia has sparked considerable debate in regard to its potential impacts on native lizard species. Their historically patchy distribution across Australia has gained attention as populations have spread into peri‐urban and native bushland. Hemidactylus frenatus potentially pose a threat to native species through competition and predation. I documented some activity of H. frenatus during the day, including foraging, which may broaden the potential impacts to diurnal species. Here, I highlight (i) the potential impact of predation by H. frenatus on native lizards and (ii) the need for empirical studies directly quantifying competitive encounters, with special interest in intraguild predation.     

Interactions between native and invasive gecko lizards in tropical Australia

Invasive geckos of the genus Hemidactylus (Gekkonidae) are spreading rapidly through urban environments in many tropical and subtropical parts of the world. The invaders have caused rapid declines in native gecko abundance in some areas, but their interactions with Australian native species remain unknown. In a small rural village near Darwin, we found that Hemidactylus frenatus is abundant around both lighted and unlighted buildings, but rarely found in surrounding bushland. It is sympatric with the larger Gehyra australis (Gekkonidae) in this disturbed site, and often forages on the same buildings, but is active mostly during the dry‐season (vs. wet‐season for G. australis) and is competitively subordinate to the larger native species. In laboratory encounters, H. frenatus fled from G. australis, and modified its refuge‐site use in the presence of the native lizard. In those same trials, the native gecko often attacked and rarely fled; and did not shift its refuge‐site selection. In field surveys, the two taxa frequently co‐occurred. However, substrate use of the invasive H. frenatus was modified by the presence of the native G. australis, consistent with competitive displacement. Our counts of H. frenatus were highest during the dry‐season, when G. australis (like most other small native reptiles) is relatively inactive. The invasive gecko thus appears to be exploiting a ‘vacant niche’ around buildings, rather than displacing the native gecko taxon. This outcome may reflect the size disparity between the native species and the invader; Hemidactylus frenatus may well have significant ecological impacts on smaller native lizards.     

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 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.     

Further observations of blunt-hooked pentastomids belonging to the genus Raillietiella Sambon, 1910 infecting Hemidactylus brookii (Sauria: Gekkonidae) in Africa and the Caribbean: comparison with closely related Raillietiella spp. from an African skink (Mabuya perrotetii)

Hemidactylus brookii, a gecko possessing great dispersal ability, has apparently spread from West Africa to the Caribbean and South America by natural rafting. The lungs of populations from West Africa and the Greater Antilles (hispaniola and Puerto Rico) are heavily infected by raillietiellied pentastomids. Nigerian populations harbour two distinct species, frequently as concurrent infections, the males of which can be separated principally by marked differences in the morphology of the copulatory spicule. However one of these species cannot be reliably distinguished from Raillietiella teagueselfi, a common parasite of another highly successful coloniser, the Mediterranean gecko H. turcicus in Texas. The other more closely resembles R. frenatus, first described from the lungs of yet another itinerant gecko (H. frenatus) from south east Asia. Raillietiella spp. from both the Hispaniolan and Puerto Rican H. brookii haitianus populations, possess bluntly-rounded spicules of the R. frenatus type, although comparative posterior hook dimensions indicate that they are not closely related to it, or indeed to each other. They clearly comprise recognisably different geographical populations, but it remains unclear as to whether these are endemic or introduced. Both of the Caribbean species are left unnamed because we do not know what proportion of the observed intra- and interspecific variation is genetically based. Analysis of another raillietiellid population from an African skink (Mabuya perrotetii) is also included because it harbours two more species which are not easily distinguished from R. frenatus and R. teagueselfi. More refined diagnostic techniques, ultimately supplemented by life-cycle studies, are required to resolve these taxonomic problems.     

Female reproductive cycles in the northernmost populations of the two gekkonid lizards,Hemidactylus frenatus andLepidodactylus lugubris

Lepidodactylus lugubris, a parthenogenetic gekkonid lizard of possible tropical origin, occurs on Kita-Daitojima Island of the Ryukyu Archipelago and lays eggs almost throughout the year. The oviposition season of the sympatric population of another primary tropical, but bisexual gecko,Hemidactylus frenatus, is confined to between April and September. Laboratory experiments indicate that the hatching ratio of the egg reduces with the decrease of incubation temperature in both species. Some eggs ofL. lugubris develop at 14°C of ambient temperature, wheras eggs ofH. frenatus always die at temperatures of 18°C or lower. Examinations of stomach contents and fat body mass in the monthly samples suggest that food availability is not severely low for the geckos even during the winter, rejecting the previous assumption that food stress suppresses vitellogenesis inH. frenatus. No other environmental factors that may induce reproductive seasonality as phenotypic physiological responses were detected either. Tropical populations ofH. frenatus andL. lugubris are known to lack distinct annual reproductive cyclicity. Thus, it is probable that the East AsianH. frenatus has evolved the winter quiescence of reproduction, presumably due to the poor embryonic tolerance of low temperatures. The absence of such seasonality in the sympatricL. lugubris seems to be attributable to its clonal nature which predicts the low genetic variability of the population from the colonization stage.     

Shifts in thermal preference of introduced Asian house geckos (Hemidactylus frenatus) in temperate regions of southeastern Australia

Despite its tropical origin, the Asian house gecko (Hemidactylus frenatus) is currently invading higher latitudes around the world. In this study, we investigated whether the introduced geckos in the subtropical/temperate region of southeastern Australia have shifted their thermal biology to cope with colder temperatures. In the lab, we measured the body temperatures of geckos from Thailand and Australia in a cost-free thermal gradient. Native H. frenatus from Thailand displayed a diel pattern of thermoregulation. Geckos maintained higher body temperatures during mid-afternoon and at dusk but selected cooler temperatures during the night. Introduced geckos showed a similar pattern of thermoregulation, but selected lower body temperatures in summer (mean = 28.9 °C) and winter (mean = 25.5 °C) than native geckos (mean = 31.5 °C). While the Asian house geckos from Thailand did not alter their body temperatures after feeding, their conspecifics from southeastern Australia selected body temperatures that were 1.6–3.1 °C higher after feeding. In conclusion, our study shows that invasive house geckos in Australia have shifted their preferred body temperatures downwards relative to their native conspecifics in Thailand, presumably as a result of plasticity or natural selection. Our findings suggest that these tropical geckos have adapted to colder regions, and thus, they may spread much further than expected for a tropical ectotherm.     

Invasive parasites in multiple invasive hosts: the arrival of a new host revives a stalled prior parasite invasion

The success of a biological invasion can depend upon other invasions; and in some cases, an earlier invader may fail to spread until facilitated by a second invader. Our study documents a case whereby an invasive parasite has remained patchily distributed for decades due to the fragmented nature of available hosts; but the recent arrival of a broadly distributed alternative invasive host species provides an opportunity for the parasite to expand its range considerably. At least 20 years ago, endoparasitic pentastomids (Raillietiella frenata) were brought with their native host, the invasive Asian house gecko Hemidactylus frenatus, to the port city of Darwin in tropical Australia. These geckos rarely disperse away from human habitation, restricting the transmission of their parasites to urban environments – and thus, their pentastomids have remained patchily distributed and have only been recorded in scant localities, primarily surrounding Darwin. The recent range expansion of the invasive cane toad Rhinella marina into the Darwin area has provided an alternative host for this pentastomid. Our results show that the cane toad is a competent host for Ra. frenata– toads shed fully embryonated pentastomid eggs in their faeces – and that pentastomids are now common in cane toads near Darwin. Likely reflecting the tendency for the parasite's traditional definitive host (the Asian house gecko) and only known intermediate host (the cockroach) to reside around buildings, we found the prevalence of this parasite follows an urban distribution. Because cane toads are widely distributed through urban and rural habitat and can shed viable pentastomid eggs, the toad invasion is likely to facilitate the parasite's spread across the tropics, into areas (and additional susceptible hosts) that were previously inaccessible to it.     

The distribution and prevalence of helminths,coccidia and blood parasites in two competing species of gecko: implications for apparent competition

Across the Pacific the invading gecko species Hemidactylus frenatus has competitively displaced the resident gecko species Lepidodactylus lugubris in urban/surburban habitats. Do parasites enhance, inhibit, orhave no effect on this invasion? Parasites can confer an advantage to an invading species when the invader (1) introduces a new parasite to a resident species that has a greater detrimental effect on the resident than the invader, (2) is less susceptible to endemic parasites than the resident, and/or (3) increases the susceptibility of the resident to parasites. Conversely, parasites may protect a resident against invasion when endemic parasites have a greater impact on the invader than the resident. We screened more than one thousand H. frenatus and L. lugubris in areas of sympatry and allopatry from 28 islands and 5 sites on mainland Asia for a broad array of blood parasites, coccidia and helminths in order to evaluate the potential for parasites to affect their interaction. We found that 1) There were no parasites which appear to protect L. lugubris against invasion by H. frenatus. 2) H. frenatus does not introduce the same parasite to L. lugubris in every location where the two come in to contact, but probably has introduced different parasites in different locations. L. lugubris also seems to have introduced at least one parasite to H. frenatus. 3) The prevalence of parasite species shared by the two hosts is generally higher in H. frenatus; however, prevalence is determined by many factors and cannot be directly translated as susceptibility. We discuss the implications of this difference in prevalence for the Red Queen hypothesis. 4) The prevalence of the cestode Cylindrotaenia sp. is significantly higher in L. lugubris that are sympatric with H. frenatus than those which are allopatric.     

Environmentally-induced shifts in behavior intensify indirect competition by an invasive gecko in Mauritius

Most recorded extinctions have occurred on oceanic islands, mainly as a result of introduced mammalian predators. The impact of introduced non-mammalian competitors, however, is poorly understood. The house gecko, Hemidactylus frenatus, is one of the most successful invasive reptiles and has been implicated in the decline of endemic geckos and other taxa on a number of tropical and subtropical islands. We investigated the patterns of niche utilization between the house gecko and endemic ornate day gecko, Phelsuma ornata, in Mauritius, two species which were not believed to compete because they had different diel activity periods. The dietary and temporal niche partitioning of the two species were examined in relation to seasonal invertebrate prey abundance for three seasons. Dietary overlap between the two species was least when prey abundance was lowest and temporal overlap in activity greatest. Exploitative competition was therefore inferred, whereby changes in dietary overlap were attributed to shifts in prey selection by the day, but not the house, gecko, which was hypothesized to deplete prey. The compensatory response of the day gecko may have been to increase its tendency for cannibalism, such that the smaller house gecko was indirectly responsible for population reduction of its larger competitor. This is the first study to show how an invasive nocturnal gecko may be affecting a predominantly diurnal species.     

Invasive house geckos are more willing to use artificial lights than are native geckos

There is increasing concern about the ecological effects of light pollution, as artificial lighting spreads with urban expansion. While artificial lighting can negatively affect some species, others use it in novel ways. In tropical and subtropical regions, artificial lighting has created a novel niche: the ‘night light’ niche. Geckos living as human commensals (house geckos) are apparently well adapted to occupy this niche. In an urban area in north‐eastern Australia, we found that the invasive Asian house gecko Hemidactylus frenatus (Gekkonidae) occupies a broader range of light environments in the field than does the native gecko Gehyra dubia (Gekkonidae). Experimental removal of the invasive species from a building indicated that it did not behaviourally influence the light environments chosen by the native species in the short term; they continued to use darker areas even after the invasive species was removed. In Y‐maze experiments, neither species showed a significant preference for light or dark areas; however, preliminary data suggest the invasive species was more willing to explore the Y‐maze than the native species. The willingness of H. frenatus to forage closer to lights, where insect abundance is typically higher, might account for its success as a global invader of human environments, even in areas where other gecko species are established.     

An experimental investigation of the competitive displacement of a native gecko by an invading gecko: no role for parasites

On islands across the Pacific the invasion of the gecko Hemidactylusfrenatus has caused a decline in the abundance of a resident gecko, Lepidodactyluslugubris. In a previous study we demonstrated that the prevalence of the cestode Cylindrotaenia sp. is higher in the resident gecko on islands where it is sympatric with the invader than on islands where it occurs alone. In the present study we experimentally test whether the presence of the invading gecko causes an increase in parasites, particularly Cylindrotaenia sp., in the resident. In addition, we test whether the effect of the invader on parasite prevalence in the resident is mediated through an increase in corticosterone in the resident. Corticosterone is the primary glucocorticoid, or “stress” hormone in lizards, and chronic elevation in corticosterone may suppress some types of immune responses. After experimental manipulations of interspecific interactions (single vs. mixed species treatments) and intraspecific densities (high vs. low), we detected no difference in parasite prevalence or circulating corticosterone among the experimental treatments in either L. lugubris or H. frenatus. Circulating levels of corticosterone were higher in geckos␣sampled at night than geckos sampled during the day, indicating a circadian cycle in corticosterone levels in these nocturnal animals. Circulating levels of corticosterone were higher in experimental geckos than in geckos that had not been used in the experiment, and, in some groups, higher in geckos infected with cestodes than in uninfected geckos. Circulating levels of corticosterone did not differ between non-experimental H. frenatus and L. lugubris, but when geckos used in the experiment were compared, circulating levels of corticosterone were significantly higher in H.␣frenatus than in L. lugubris. Received: 17 March 1997 / Accepted: 5 December 1997     

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

Antipredator behaviours and the ability to appropriately assess predation risk contribute to increased fitness. Predator avoidance can be costly; however, so we expect prey to most strongly avoid predators that pose the greatest risk (i.e., prey should show threat sensitivity). For invasive species, effectively assessing the relative risk posed by predators in the new environment may help them establish in new environments. We examined the antipredator behaviour of introduced Asian house geckos, Hemidactylus frenatus (Schlegel), by determining if they avoided shelters scented with the chemical cues of native predatory snakes (spotted pythons, Antaresia maculosa [Peters]; brown tree snakes, Boiga irregularis [Merrem]; common tree snakes, Dendrelaphis punctulata [Grey]; and carpet pythons, Morelia spilota [Lacépède]). We also tested if Asian house geckos collected from vegetation vs. anthropogenic substrates (buildings) responded differently to the chemical cues of predatory snakes. Asian house geckos did not show a generalised antipredator response, that is, they did not respond to the chemical cues of all snakes in the same way. Asian house geckos avoided the chemical cues of carpet pythons more strongly than those of other snake species, providing some support for the threat‐sensitivity hypothesis. There was no difference in the antipredator behaviour of Asian house geckos collected from buildings vs. natural vegetation, suggesting that individuals that have invaded natural habitats have not changed their antipredator behaviour compared to urban individuals. Overall, we found some evidence indicating Asian house geckos are threat‐sensitive to some Australian predacious snakes.     

Effect of competition on habitat utilization in two temperate climate gecko species

Competition over spatial niche utilisation is one of most common competitive interactions between species in sympatry. Moreover, competitive interactions may involve age classes, and can fluctuate temporally. Consequently, evasive strategies that enable co-existence are likely to be important in the evolution of species assemblages. Here we investigate a system of two co-existing species of temperate geckos with similar ecologies (the house gecko, Hemidactylus turcicus and the wall gecko, Tarentola mauritanica), providing an opportunity to study the effect of species interactions. Juveniles and adults of both species were investigated throughout their daily and annual cycle to explore the effect of inter- and intra-specific interactions on microhabitat use. The two species showed differences in habitat use for both age classes in sympatry. In sympatry, T. mauritanica uses more open habitats and is more active. In contrast, H. turcicus is found in more closed habitats, closer to the ground and to vegetation cover. In allopatry, H. turcicus was observed in more open habitats, closer to the ground, and to vegetation cover, when compared to the population in sympatry with T. mauritanica. These differences in habitat usage were significant for both age classes. Moreover, there were differences, both in sympatry and in allopatry, between age classes that were dependent on season. In conclusion, the presence of a competitor induces a spatial shift in individuals of both age classes of H. turcicus. Observed plasticity in habitat utilisation in both age classes of H. turcicus is used to argue for the invasive potential of this species.     

The role of introduced species in shaping the distribution and abundance of island reptiles

Summary Species interactions, as revealed by historical introductions of predators and competitors, affect population densities and sometimes result in extinctions of island reptiles. Mongoose introductions to Pacific islands have diminished the abundance of diurnal lizards and in some cases have led to extinctions. Through these population level effects, biogeographic patterns are produced, such as the reciprocal co-occurrence pattern seen with the tuatara and its predator, the Polynesian rat, and with the tropical gecko competitorsHemidactylus frenatus andLepidodactylus lugubris in urban habitats in the Pacific. Although competition has led to changes in abundance and has caused habitat displacement and reduced colonization success, extinctions of established reptile populations usually occur only as a result of predation.These introductions, along with many manipulative experiments, demonstrate that present day competition and predation are potent forces shaping community structure and geographic distributions. The human introduction of species to islands can be viewed as an acceleration of the natural processes of range expansion and colonization. The immediate biotic consequences of these natural processes should be of the same intensity as those of the human introductions. Coevolution may subsequently act to ameliorate these interactions and reduce the dynamical response of one species to the other. The role played by coevolution in mediating interactions between competitors and predator and prey is highlighted by the susceptibility of predator-naive endemic species to introduced predators and the invalidity of species-poor communities.     

The relationship between habitat use and body shape in geckos

Geckos are a highly diverse group of lizards, with more than 1,700 species that exhibit a wide range of behaviors, ecologies, and sizes. However, no study has examined links between habitat use and body shape in pad-bearing geckos. We set out to answer a basic question using a data set of pad-bearing geckos (112 species, 103 pad-bearing, 9 padless, 42 genera): Do geckos that occur in different habitats also differ in body shape? Overall, we found that body shape was surprisingly similar among our sample of pad-bearing species, with the exception of the genus Uroplatus, which was clearly distinct from other geckos due to its depressed body and long limbs. However, the padless geckos differed in body shape from the pad-bearing geckos by having longer arms and legs and less rotund bodies. We found that about half of the pad-bearing species primarily inhabit trees, with the other half, divided approximately equally among rocks, the ground, and mixed habitats. We found no significant links between habitat use and body shape, nor any propensity for larger species to occupy different habitats than smaller species. Padless species tend to inhabit rock and ground substrates. Our results indicate that pad-bearing geckos have a relatively uniform body form, which contrasts with to their diversity in color, size, and behavior. Indeed, our data show that the general gecko body shape is suitable for a wide range of habitats, ranging from arboreal to terrestrial. This pattern is a departure from other ecomorphological studies and suggests that geckos may not easily fit into the mold of adaptive radiation, as suggested by prior studies.     

The microscopic anatomy of epidermal glands in two species of gekkonine lizards, with some observations on testicular activity.

The gross and microscopic anatomy of epidermal glands has been studied in laboratory maintained tokays (Gekko gecko), and house geckos (Hemidactylus bowringii) captured from the wild throughout the year. Annual testicular activity in the house gecko has also been studied. While no significant differences in glandular development at various times have been observed in G. gecko, there are clear-cut annual cycles in H. bowringii. The evolution of epidermal glands in gekkonid lizards is reviewed; the cellular dynamics of beta-glands are compared with those of unspecialized epidermis; the possibility that gekkonine epidermal glands respond to quantitative variation in circulating testosterone titers is discussed.     

Polyclony in a triploid gecko,Hemidactylus stejnegeri,from Taiwan,with notes on its bearing on the chromosomal diversity of the H. garnotii-vietnamensis complex (Sauria: Gekkonidae)

Karyological survey for populations of Hemidactylus stejnegeri, a triploid parthenogenetic gecko recently described from Taiwan, revealed the occurrence of chromosomal variation in this species within Taiwan. The significance of the present findings to the process of divergence in the Hemidactylus garnotiivietnamensis complex are discussed.     

Into the light: diurnality has evolved multiple times in geckos

Geckos are the only major lizard group consisting mostly of nocturnal species. Nocturnality is presumed to have evolved early in gecko evolution and geckos possess numerous adaptations to functioning in low light and at low temperatures. However, not all gecko species are nocturnal and most diurnal geckos have their own distinct adaptations to living in warmer, sunlit environments. We reconstructed the evolution of gecko activity patterns using a newly generated time‐calibrated phylogeny. Our results provide the first phylogenetic analysis of temporal activity patterns in geckos and confirm an ancient origin of nocturnality at the root of the gecko tree. We identify multiple transitions to diurnality at a variety of evolutionary time scales and transitions back to nocturnality occur in several predominantly diurnal clades. The scenario presented here will be useful in reinterpreting existing hypotheses of how geckos have adapted to varying thermal and light environments. These results can also inform future research of gecko ecology, physiology, morphology and vision as it relates to changes in temporal activity patterns. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, ●●, ●●–●●.     

A new large species of gecko of the genus Hemidactylus Oken, 1817 (Reptilia: Sauria: Gekkonidae) from the Eastern Ghats,India

A new large species of gecko superficially resembling Hemidactylus maculatus Duméril & Bibron and its allied large Hemidactylus is described from Eastern Ghats. The new species, Hemidactylus kangerensis sp. nov. can be easily distinguished from members of the H. maculatus complex based on the number of femoral pores, i.e. 18–21 on each side, separated medially by 4 non-pored scales. A phylogenetic analysis based on partial sequence of mitochondrial gene cytochrome b shows that the gecko is a member of the “H. prashadi” clade and is sister to H. maculatus, from which it differs in an uncorrected pairwise sequence divergence of 10%. The discovery of a new large gecko from Eastern Ghats advocates the need for biodiversity assessment across the neglected Eastern Ghats.     

Patterns of plant visitation by nectar-feeding lizards

Geckos in the genus Hoplodactylus visit flowers to feed on nectar. I examined the patterns of flower visitation exhibited by two gecko species (H. maculatus and H. duvauceli) having access to two plant species: pohutukawa (Metrosideros excelsa: Myrtaceae) and flax (Phormium tenax: Agavaceae). Individual geckos were not observed to visit both plant species; individuals visiting flax tended to revisit the same plant. Geckos visiting pohutukawa were larger than those visiting flax and exhibited an early night peak in plant visitation, while lizards on flax displayed a more even pattern of activity throughout the night. On flax, geckos were more likely to be found on plants with a greater number of male flowers. Male flax flowers were of greater diameter than female flowers and produced nectar at higher rates and with greater concentrations of sugars. Experimental manipulation of pohutukawa nectar volumes suggested that the distribution of geckos is influenced by the pattern of nectar availability.