Rio Grande Chirping Frog from Atascosa County, Texas
The Rio Grande Chirping Frog (Syrrhophus cystignathoides) has recently been found in Mobile County, Alabama, where it is occurring sympatrically (syntopically ?) with a similar, related West Indian form, the Greenhouse Frog (Eleutherodactylus planirostris).
Both species are exotics to the area, with the Rio Grande Chirper having traveled approximately 822 miles northeastward along the Gulf Coast from its presumed native environment in the Lower Rio Grande Valley of extreme south Texas. If we take the date of the first known extra-limital record of this species as the beginning of its peregrinations (1969), it would indicate that this diminutive frog has been expanding its range along the Gulf Coast at a rate of almost 19 miles per year.
Conversely, the Greenhouse Frog, based on genetic evidence, seems to have originated with Cuban stock perhaps incidentally transported to southern Florida along with tropical vegetation from its homeland. This species was first reported from Key West in 1863; by 1943 it had reached Jacksonville (507 miles in 80 years = ~ 6 mi/yr).
There is some evidence, however, that the Greenhouse Frog had been established on Key West long before it was "officially" reported, and some authorities maintain that it may have arrived naturally via over-water waif dispersal considerably before 1863 (Lazell 1989). The interval between its arrival at Jacksonville (1943) and at its current western terminus in Galveston County, Texas (1999) is 56 years, covering 890 miles (~ 16 mi/yr), a rate much closer to that of the Rio Grande Chirper and perhaps more informative.
Such calculations would be more meaningful if these tiny frogs were accomplishing their movements on their own, but they're not. Although I don't think anyone has actually proven it beyond a reasonable doubt, the currently accepted explanation for these two frogs' substantial range expansions seems to be that they (and/or their eggs) are being transported incidentally by the potted plant trade.
At any rate, the present occurrence of these two very similar, equally exotic frogs together in an expanse of Gulf Coast real estate extending from Mobile, Alabama to Galveston, Texas, raises the question of whether they will be able to coexist or will one out-compete the other?
Lazell, James D., Jr. 1989. Wildlife of the Florida Keys: A natural history. Washington, D.C.: Island Press, xvi + 253 pp.