Has the Green Tree Frog (Hyla cinerea) Leap-Frogged North?
The Green Tree Frog
In the fall of 2015 my wife encountered a small, green, tree frog on a raspberry bush while picking berries. Then, last year, there was one on our deck in the morning. I identified both as Hyla cinerea. However, this is far out of their reported range. The latitude and longitude here in upstate New York Is 43.41 x 73.26! All the literature I have or have seen gives the green tree frog’s northernmost limits as approximately northern Maryland.
We’re organic growers, and we collect rainwater from the barn roof in stock tanks that are then cascaded downhill into a large holding tank on a slab near where we grow our crops. In early June this year (2017) my wife said that for the first time in 35 years we seemed to have mosquito larvae in that tank. She said she dropped in a mosquito dunk (an organically approved biological control for mosquitoes) to control them. I thought no more of it, until I took a look myself a week later.
A Journal of Tadpoles
What was assumed to be mosquito larvae earlier were now obviously tadpoles, which struck us as strange, for no egg masses were seen before. Nor was there aquatic vegetation for the eggs to be laid on. The eggs must have been laid singly.
At that stage the tadpoles were almost black and resembled those of Bufo americanus, the common toad. Curious as to what they would mature into, we decided to let them be. We began leaving scraps of lettuce leaves from harvest in the water for them, as there was no vegetation or algae for them to feed on.
Surprisingly, we noticed there were no depredations by Backswimmers (family Notonectidae). In the past, I have seen them virtually exterminate toad egg cluster hatch-outs, attacking one after another, and sucking them dry in no time.
By July 3, the tadpoles had begun to come to the surface to gulp air, so their lungs were now functioning. Their growth was rapid, going from comma-sized to an inch long. Their bellies have begun to swell noticeably, becoming opalescent (from the internal growth of legs?).
The females seem to be laying eggs at intervals, because by July 17 there were several broods of differing sizes. The oldest have now sprouted legs.
As soon as the legs became visible, the body shrunk, so that the half frog/half tadpole was slimmer and shorter than the tadpoles. The first frogs (13) were noted and they were definitely Hyla cinerea. They are approximately an inch long and can sit comfortably on a penny. Yet the pattern of egg-laying seemed to fit the squirrel tree frog, Hyla squirrella, more than Hyla Cinerea, which I’ve read lays masses of eggs at once. Yet the squirrel tree is also a southeastern species.
It was noticed on July 23 that while the younger tadpoles seem to be exclusively vegetarian, the ones with legs and tails were observed nibbling at drowned insects.
Once their legs are approaching full size, they hauled themselves out of the water, or into very shallow water, while they waited for their tail to be re-absorbed. That happened very rapidly. Frogs seen in the morning with a tail by nightfall had no tail.
On August 27, the largest emergence of adult frogs occurred. Fifty-one were tallied. At first, some are a dark grey-green while others are a brighter green. After a day or so, all are that same bright green, with a small pale streak from the jaw heading towards the side. No other colors or blotches of other color, which points to the green tree frog again.
By October 10, only 26 tadpoles without legs were left as far as we could count. Luckily, we were experiencing yet another unusually warm autumn here in the North Country, which may allow the remaining tadpoles to mature.
A week later, 13 more frogs had emerged. Then on October 19, a 100 more frogs were seen. And still a handful of tadpoles.
By November there still had been no severe cold or even any days below freezing. No ice yet, which is unusual to say the least up here. Or; it used to be unusual. Today’s count is seven frogs, eleven tadpoles with legs, and one tadpole without legs.
November 8: Last night it got down to 20 degrees F and the water tank is iced over. No sign of life, but below the skim of ice I can see a half-frog/ half-tadpole resting on a piece of wood near the surface. I think it’s safe to say the tadpoles’ time has run out for this year.
Without a doubt, this is another example of species’ ranges moving northward, and the only way that can be possible is because the climate has become gentler. This amphibian, whether the green tree Frog as I believe, or the squirrel tree frog, was indigenous to the southeast U.S. until now. Inland locales like here have had far too severe a winter presumably before now. It is possible this is a micro-population from the release of pet frogs, but I doubt it. The local population are not Herpetologists, but farmers. If that possibility is excluded, then northward migration seems to be the only likely explanation. But then there should be other populations noted contiguously from here south to their “normal” range.
Anybody else seen these?