Where did all the frogs GO?
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Disappearing Amphibians
The facts are that amphibian species have been disappearing world-wide at an alarming rate. The decline of the amphibian population began in the late 1980’s. This trend continues today.
Environmental changes brought on by man are categorized as either global or local. Global effects include the usual suspects, such as global warming and the greenhouse effect, weakening of the ozone layer, air pollution (particularly exhaust fumes), and expanding deserts. Local effects are also the usual culprits and include deforestation, the induction of predatory fish in natural rivers and streams, and the pollution of water by pesticides, fertilizers, and detergents. Local effects take place in many places and can have a lasting impact on the environment, but the amphibian disappearance is a global problem, so most research is focusing on threats to the environment that have more of worldwide impact. Topping the list of possible reasons for the demise of amphibians is the depletion of the ozone layer.
Studies have shown that more ultraviolet radiation reaches the earth in areas where the ozone layer has been adversely affected than where the ozone layer is relatively unchanged. This kind of radiation damages the DNA in amphibian eggs. But in a recent study it was shown that a reduction in the ozone layer doesn’t necessarily affect all amphibian species in a specific area to the same degree. Many amphibian eggs possess an enzyme with the capability of repairing DNA damaged by excess radiation. Needless to say, some amphibian eggs are better off than others. Perhaps this theory explains the disappearance of many species, but doesn’t provide the complete answer.
Nowhere in the world is the disappearance of the amphibian population more apparent than in the Monteverde Cloud Preserve in the Tilarán mountain range of Costa Rica. There, 40% of the frogs and toads (some 20 species) in Monteverde have disappeared since 1987. It should be noted that the amphibians that were affected, but survived the aftermath of 1987, have greatly reduced number, and still 21 years later have still not recovered. Unfortunately, the radiation theory doesn’t seem to explain the decline in Monteverde; case in point, the plight of the golden toad. When the breeding season began in 1988, only one golden toad appeared at the most important known breeding pond, instead of the more than 1,500 seen just a year before. The last golden toads were spotted in Monteverde Cloud forest in 1989, and none have been spotted since. The problem is, the golden toad spends 95% of the time in its burrow, protected by harmful effects ultraviolet radiation.
Acid rain has also been mentioned as a reason for the massive disappearance, since it is known to kill amphibian embryos. Yet, it doesn’t kill adults, so it doesn’t really explain the decline of the golden toad. Three hypotheses have been proposed for the extinction and decline of the frogs and toads in the Monteverde Cloud Preserve. Extreme dry weather brought on by the 1986-1987 El Niño weather patterns was thought to be a possibility. The drought like conditions may have dried out breeding pools, everyday habitats, and even subterranean retreats so severely that the frogs died from dehydration. A related explanation is that the frogs, weakened by dehydration may have fallen victim to bacterial, parasitic, fungal or viral infections.
The second hypothesis is also linked to El Niño. It is thought that high concentrations of pesticides were deposited on the reserve through the ever present mist and cloud coverage. Because of the relative lack of rainfall, insecticides, herbicides, and fungicides, which were used abundantly and unchecked in surrounding areas, may have evaporated from exposed soils, only to be deposited, practically undiluted by the rainwater, onto the reserve.
The third hypothesis came about when weather patterns from 1977-1998 in the Monteverde Cloud Forest Preserve were analyzed. The studies show that three major crashes in the size of amphibian populations- in 1987, 1994, and 1998- corresponded with unusually high frequency of dry days. It is thought that sea surface temperatures, which have risen as a consequence of global warming have caused an increase in the number of dry, cloudy days in the Monteverde area. These periods of dryness have not only affected the amphibian population, but some lizard and bird populations as well. It’s somewhat normal for toad and frog populations to fluctuate with changes in climate. Often entire populations disappear from a region only to recolonize the same region at a later time.
The jury is still out whether or not the golden toad and others from the Monteverde Cloud Forest Preserve are actually gone forever. Recent studies have shown that fungal infections may also explain the demise of large number of frogs worldwide, and not only in developed areas, but untouched rainforest environments as well.
Globally, habitat destruction alone does not explain the entire picture, but it does seem to be one of the most destructive factors on a local scale. However, what should be understood, amphibians are twice as susceptible to pollution as animals that strictly live on land or in the water. These sensitive animals are the bench mark of environmental health in any given area and their mass extinction should serve as a wake-up call to human kind.
