Green Path doesn’t spend much time on lizards other than geckos or skinks, mainly because its author doesn’t see them terribly often, so a quick overview may be in order.
The lizard families we have in Queensland are Geckos, Skinks, Dragons, Monitors (Goannas) and Flap-footed Lizards (Legless Lizards). (Wikipedia has a beautiful taxonomic chart that places them in relationship to each other and all the other families worldwide, if you’re interested.)
Soon after I arrived in Townsville I learned that two species of gecko lived in the houses here, one native and the other a recent arrival from Asia. They looked almost exactly the same and had the same lifestyle, and no-one I knew could reliably tell the difference between them except that the Asian geckos chirped.
That’s where my knowledge stopped until I decided recently that it was time I sorted them out. Here goes:
Both of them are generally known simply as ‘gecko’ and I have also heard ‘jelly lizard’ as a common name from an older resident.
They are both about the same size, growing to approximately 15cm overall. Both of them change colour according to their surroundings and are usually dark with a distinct pattern by day but pale and almost patternless at night.
Both of them are primarily nocturnal and their hunting strategies are very similar: both are “ambush” hunters in that they sit and wait for prey rather than actively roam in search of it.
The most reliable way of distinguishing them is that the Asian gecko has a series of small spines along the top and edges of the tail (in its original state); the top row extends onto the lower back as small bumps. If you catch one, you can check its toes: all toes of the Asian gecko have claws, but the inner toes of the Dtella are clawless. Also, the native gecko generally has paired white spots along the spine, as in my second photo on this page, though they may not be at all obvious in darker colour modes.
The two species can also be distinguished, more easily though not so reliably, by their calls. The Asian gecko calls more often and more loudly than the native species; its loud “chuck-chuck-chuck” is obviously the origin of its Indonesian common name, Chichak or Cheechak. The native gecko’s call is a softer chattering.
There are slight behavioral differences between the species. It appears* that the Asian geckos are willing to tolerate brighter lights while hunting than the native species, which means that native geckos are more likely to lurk in the shade of curtains or picture frames and dart out at nearby prey whereas Asian geckos often rest in brightly lit open spaces, relying on immobility to remain unnoticed by their prey. It probably gives Asian geckos a competitive advantage in our houses, too.
The Department of Agriculture, Fisheries and Forestry information sheet on Asian geckos (pdf here) doesn’t give quite the same reasons but comes to similar conclusions:
H. frenatus is an aggressive and territorial species, features that allow it to successfully compete with native species. … Male and female H. frenatus are known to eat juveniles of other gecko species as well as their own progeny (Bolger and Case, 1992).
The ability of H. frenatus to replace locally native gecko species seems most pronounced in urban areas. Artificial lighting on buildings attracts large numbers of insects and H. frenatus is well adapted to utilise this food resource, perhaps more so than native gecko species, which may be better adapted to hunting more dispersed insect populations.
I walked around my house with a camera one evening to see whether I could identify the species we had here and the results lined up very neatly with that light tolerance. (For the record, I saw three or four geckos in each room and there are probably just as many in each bedroom. We don’t bother them, since they eat lots of insects and they don’t bother us at all except by leaving neat little black and white droppings.) I found only Asian geckos in the kitchen (most brightly lit and fewest shady hiding spots), only native geckos in the lounge/TV room (dimmest lighting) and both species in the study-cum-reading-room (mid-level lighting and plenty of hiding places).
*Information from Professor Lin Schwarzkopf of James Cook University’s School of Marine and Tropical Biology, which confirmed and explained a vague memory I had of being told, years ago, about a difference in hunting styles.