In the early 1980s I visited northwest South America several times and was always amazed at the thousands of beautiful, jet-black toads that lived in open meadows above the tree line in the high Andes. By 1997, however, frog experts reported that this species had become extinct. Millions had totally disappeared from a physical environment that seemingly had not changed at all. Similar reports have come from many other parts of the world-including, lately, from right here in Southwest Florida.
Just 20 years ago, about the same time I was visiting the Andes, frogs and toads were thriving in Southwest Florida. On rainy summer evenings, the streets in many wooded neighborhoods were hopping with these delightful creatures, and the air was filled with their pleasant calls. In undeveloped areas, lusty choruses of male frogs of a half-dozen species called for mates from many small pools of standing water. Even in the cooler months, frogs seemed to make frequent nocturnal appearances around the windows of our houses, while toads hunted for edible insects around our doorsteps. (Frogs tend to be lean, smooth-skinned high leapers, while toads tend to be plump, rough-skinned low hoppers.)
But frogs and toads have started to disappear, especially in suburban neighborhoods. Some experts estimate that as many as 90 percent of Southwest Florida's frog population has vanished; others pin the number at a lower level. But all agree that frogs, which have been on earth for more than 150 million years, are no longer a ubiquitous part of many landscapes.
When such a massive, worldwide event occurs, we seem to look for simple explanations that fix the blame on some uncontrollable natural causes, such as prolonged droughts. But the total picture is more complex. Some studies indicate that the loss of frogs is related in part to an increase in ultraviolet radiation resulting from a thinning of the earth's ozone layer. Another factor could be widespread acid rain, produced when sulfur dioxide and nitrogen oxides are released into the atmosphere, mainly from fuel combustion in automobiles and the burning of coal to generate electricity. Frogs' ability to reproduce can be affected by exposure to intense UV radiation and by the acidification of their watery habitats. New viruses have also become a major threat, particularly to frogs in the tropics.
Fortunately, none of our Florida species has totally vanished in a sudden die-off so far. In fact, when we receive abundant rain, as during most summers, frog populations soar in areas that still have the right kinds of natural breeding ponds.
Most of our frogs and toads lay their eggs in shallow, temporary ponds and roadside ditches, where they can't be eaten by fish and other hungry creatures that live in deeper, permanent ponds. That's why Florida's frogs are disappearing fastest around areas that are being bulldozed for development. Replacing a natural breeding pond with a large artificial one, such as on a golf course, appeals more to developers than it does to frogs. In addition, as we pump more and more water for human use, we lower the water table. Many ditches and shallow depressions that once held water throughout the frogs' breeding period now dry up more quickly, sometimes within days after a heavy rain, thus preventing the delicate transition from egg to tadpole to baby frog.
While most frogs depend on water, a curious exception is the little greenhouse frog, a species from the tropics that's becoming more abundant here. Unlike our native frogs, the females of this species lay their eggs on land rather than water. Baby greenhouse frogs hatch directly from their eggs, bypassing the vulnerable tadpole stage.
Large-scale development pollutes nearby waters with the residues of pesticides, fungicides, herbicides and agricultural chemicals, which are readily carried by rainwater runoff into ponds and streams. Some of these products, like the weed-killer Atrazine, have been proven to be highly toxic to frogs. In many parts of the United States, frogs are being discovered with abnormalities such as missing legs, extra legs, blindness and, most important, an inability to reproduce-all the results of genetic disturbances caused by toxic substances in their breeding waters.
Probably the most conspicuous frog nowadays in Southwest Florida is the hardy Cuban treefrog. Until the early 1970s, this species was confined mostly to the Keys and lower Dade and Collier counties. Its range has recently expanded much farther northward, even above Tampa Bay. You can often see Cuban treefrogs at night around lighted patios or clinging to outside walls and windows. Although it appears inoffensive, the species is highly predaceous and eats many of our smaller native frogs. Consequently, we are less frequently visited by our formerly abundant buddies, such as the green and squirrel treefrogs.
How can we bolster populations of our native frogs? The answers are simple-and difficult. We could preserve more wetlands, limit or eliminate toxic chemicals in our air and water, stop pumping so much groundwater, and set limits on development. Otherwise, we may find ourselves in a world in which native frogs will be rare indeed, found only in remote parks or preserves. And then we may ask ourselves a question we should be raising now: Is the current plight of frogs and toads a warning about our own future.