On the 14th of July, 2002, I wanted to test the possibility of using a modified funnel-trap to collect Anolis sagrei. The first lizard to respond to my trap, though, was an adult male of the agamid, Japalura swinhonis, that was attracted by the movements of the crickets in the trap. The J. swinhonis attempted to prey on the prey items for about 30 seconds. When an adult male A. sagrei approached, the J. swinhonis moved up the trunk of the betelnut palm onto which the trap was secured. No further observations were made after the A. sagrei lost interest after about one minute and moved off.
This was to date the only instance I observed in which a J. swinhonis gave way to an A. sagrei, and I am quite convinced that the J. swinhonis actually just lost interest in the possible prey in the trap, and as it moved away the A. sagrei thought he could try his luck. And this is my point concerning A. sagrei in Taiwan.
In my study area in Santzepu, Chiayi County, southwestern Taiwan, J. swinhonis males (mean ± SD = 70.5 ± 8.4 mm) and females (mean ± SD = 58.2 ± 13.9 mm) are substantially larger than A. sagrei (males; mean ± SD = 46.2 ± 9.1 mm; females; mean ± SD = 38.2 ± 5.5 mm). In most other aspects, both species are quite similar; both are diurnal trunk-ground ambush foragers and are very territorial. In a paper I am currently preparing, I compared the diet of these species and found that A. sagrei has a much wider dietary niche breadth than J. swinhonis, and that in areas where J. swinhonis and A. sagrei are sympatric, there is a substantial dietary niche overlap, and competition for prey is very likely.
Although both species are human commensals, J. swinhonis is more shade tolerant, while A. sagrei reaches higher densities in open disturbed habitats. So, my view of A. sagrei in Taiwan is that this species is here to stay, and we have to accept that it is becoming part of local ecosystems. I believe it is occupying spaces that are created when local species are displaced due to human activities. Eradicating it is an unrealistic option, so we should rather manage it by restoring more forested habitats, which are more suited for local species and will thus increase the populations and genetic diversity of native species. Such habitats would be reservoirs for species like J. swinhonis, which can compete with A. sagrei in the trunk-ground niche, while species, like Eutropis longicaudata and Plestiodon elegans, will not only compete with it on the ground in disturbed areas and forest margins, but large species like the former can also act as predators. Such competition and predation pressures can mitigate the impacts that A. sagrei can have on local ecosystems.
And for those who want to know, the funnel-traps were completely ineffective for the collection of these lizards. The best way to collect them remains noosing and hand collection at night.