Part 3 of a series
This series about amphibians is in part an attempt to redress an imbalance; after all, I do write most often about birds. As I emphasized previously, however, we overlook amphibians to our peril. In concluding the series, let's look a little further into those declines, disappearances and deformities among the amphibians that I have mentioned, but not yet closely examined.
Amphibian declines are neither local nor isolated. They are being reported from around the world. In some cases the cause is clear; in others a number of causes are implicated. In some cases, though, we just don't know why, and that is perhaps most alarming of all.
The First World Congress of Herpetology was held in 1982 in Canterbury, England, and at that meeting herpetologists from around the world realized that they were, in their respective field study areas, all witnessing the same general phenomenon.
It took until 1990, though, before the Declining Amphibian Populations Task Force was established to track and trace the problems. The DAPTF's concerted efforts have revealed some obvious and some not so obvious correlations.
Clearly, habitat loss is a major problem. When cattle ranchers converted a cloud forest region in the mountains of Guatemala to pasture, seven out of eight local species of salamanders became extinct.
Other impacts on habitat are not quite so obvious. In some places, for example, increased activity in all-terrain vehicles is increasing the silt load in streams and rivers, reducing the viability of amphibian eggs and juveniles.
Impacts like these we might predict, in areas of large human populations, but more disturbing still is the evidence from relatively undisturbed habitats such as Australia. There, at least 14 frog species, once abundant, have either completely vanished or have been reduced to tiny populations.
The common toad in Europe is in decline primarily because of habitat destruction. Here again, though, declines and deaths that cannot be so simply explained have been noted. On the island of Jersey, for example, mass mortality of spawn has been observed, and even in some remote Scandinavian regions, far removed from direct human impact, toad populations have also declined.
Furthermore, back on the island of Jersey, the once-widespread agile frog has also declined to small numbers found at just one locality, even though other habitat still remains. Habitat loss cannot be the explanation for this dramatic decline.
In America's famous Yosemite National Park at least three amphibian species have vanished, and several more have declined drastically. Biologists have compared current data with surveys made between 1915 and 1919. During the early surveys seven species of amphibians were found at 70 locations, but 75 years later only four species were found, and only at 26 sites. Since Yosemite is a park, habitat loss cannot be blamed, and it is not subject, at least directly, to chemical pollution.
Perhaps the examples most widely discussed are the very beautiful golden toad of Costa Rica and a bizarre Australian frog that broods its young in its stomach. Both have survived through untold ages, yet in recent years they have mysteriously and inexplicably vanished without a trace.
In the southern Rocky Mountains, the American boreal toad has declined by about 80 percent. Why?
In Puerto Rico, a unique viviparous (giving birth to live young) frog has died out, yet its habitat still remains.
Further to the south, in Ecuador and Venezuela, eight species have gone missing from the Andean cloud forests; where researchers were, in the past, able to count hundreds of specimens in an hour, in 1990 a researcher from Venezuela was only able to locate three during more than 300 hours in the field. In southeast Brazil, at Boraceia, seven common amphibian species disappeared around 1979, and despite intensive study there, none of them has been found since.
As if the extinctions and declines were not worrisome enough, there is more. Even species that are still common, such as North American leopard frogs, are suddenly producing bizarre, alarming deformities.
The catalog of losses and declines is long and depressing, but clearly indicates that we are not dealing with simple habitat loss. Amphibians are disappearing from seemingly virgin habitats in remote locations as well as from well-known sites. There is something insidious going on in the world, and we need to learn more about it if we are to prevent ourselves being overwhelmed by the same fate.
Chemicals have been implicated either directly or indirectly. Agricultural chemicals may be a problem: herbicides and pesticides, and the wetting and dispersal agents that go with them. These are designed to permeate the environment, and they do so all too well.
Estrogens that result from the breakdown of other chemical pollutants disrupt the reproductive physiology of wild animals in unpredictable ways. Research has shown that such chemicals can masculinize female Japanese tree frogs and so effectively sterilize the population.
In some regions, acid rain is the most likely culprit, since it can lower the pH of ponds and streams to levels that are lethal to amphibian eggs.
Even less obvious in its influence is the depletion of the ozone layer. This thinning, once limited to the Southern Hemisphere, is increasingly affecting the north too, and we are likely to see many more changes in populations as a result. Particularly at higher altitudes, rising levels of UV-B, which results from reduced protection by the diminishing ozone layer, reduces the hatching success of some amphibian eggs.
Some pollutants operate at such minute quantities and are so easily carried on the wind that they instantly become a global threat even when released locally. Releasing multiple pollutants, a cocktail of chemicals, only increases the likely negative effects and makes tracing the cause of deformities or declines exceedingly difficult.
Yet, difficult though it may be, we ignore the problem at our own peril. We are changing the chemical composition of the water we drink and the air we breathe, and the amphibians are already warning us, by their deaths, disappearances and mutations, that it is a very bad idea.