Friday, August 7, 2015

It's a jungle right here (if you're a frog)

Cheryl and I are addicted to the detective programs we get on Netflix and Google. The British ones are usually best but we also like the Swedish and Danish and Australian and New Zealand and Canadian ones. The same formulas run through all of them. Some woman is walking her dog along a beach, the dog digs in the sand and a bloody hand is revealed. Cut to the police there in their crime-scene suits, joking over their last bite of breakfast while the pathologist examines the cadaver. Special make-up people on the different programs compete with each other for the most gruesomely realistic split-open heads or maggot-crawling torsos.

Something like that is going on in our garden. We are out innocently looking for bugs and suddenly discover gory, half-eaten, and very human looking remains.

These are green treefrogs but we're not quite sure who the murderer is. Green treefrogs are murderers themselves, sticking to our windows at night to feed on moths attracted to the lights, or waiting under flowers for butterflies to approach. But they are also the main small vertebrate prey for just about everything else. Ribbon snakes for instance hunt them mercilessly and are probably their chief predator. But ribbon snakes swallow their prey whole, leaving no bodies behind.

Green treefrogs can hide or hop away, but they don't otherwise have many defenses against ambush or trap-setting hunters, no teeth or claws or poisonous skin . And, they are just small enough to occasionally become prey to invertebrates, which seems almost against the laws of nature.

For example, our backyard ponds are so dense with aquatic vegetation the surface of the water is not visible, and that vegetation is densely populated with day-roosting green treefrogs, but also with enormous black-and-yellow garden spiders, Argiope aurantia. I was observing their interactions last summer, and the spiders, after they were nearly full-grown, were feeding almost entirely on treefrogs.

But the spiders did not leave behind half-chewed remains either.

However, we had a clue. The very wet spring this year, which had led to an unusual number of treefrogs, and an unusual number of Argiopes, had also led to, we noticed earlier in the summer, an unusual number of Chinese mantises, and they had steadily been going through their instars, getting bigger and bigger. Here's one from the beginning of July, still a long way from adulthood, but already an impressive size.

This one looks like it is feeding on a roadkill skunk but it's actually a Milkweed Tussock Moth caterpillar, a creature that came out in the hundreds this year and chomped all our milkweed plants to the ground (after the Monarchs failed to come).

Now the mantises are molting into adulthood, four-inch-long giants that look fully capable of catching a treefrog.

Take a closer look at the spines in the powerful grasping arms.

So there was our prime suspect, an animal with a ferocious reputation and the tools to back it up. Finally we caught one in the act. We were eating breakfast and Cheryl looked out the window and said, "There's a mantis with a frog." We rushed out with our cameras. I suppose it would have been more in keeping with the crime programs if we had rushed out with our cell phones.

So, with the snakes and spiders and mantises after them, what is happening to treefrog populations in our backyard? Well, so far as I can see, it's not even making a dent.

1 comment:

  1. This is a really fascinating essay, Norm, with superb photos. While parts of it are sad for a frog-watcher, it does at least have a happy ending!