By Peter Montague
New research on loss of species indicates that extinctions are occurring now at a rate 100 to 1000 times as fast as "natural background" rates of extinction. (Stuart Pimm, "Seeds of Our Own Destruction", New Scientist, April 8, 1995.) The "natural background" is the rate of extinction measurable in the fossil record of life before humans appeared on the scene. In some regions, the rate today is 10,000 times as fast as the natural rate.
Extinction is nothing new; it has been going on since life first emerged on Earth, perhaps 3 billion years ago. What is new is the speed with which extinctions are now occurring.
The most recent estimate of the "background" rate indicates that one species is lost per year out of each 1 million to 10 million species in existence. (No-one knows how many species exist. About 1.4 million species have been named and catalogued; Harvard biologist E.O. Wilson says his best guess is that somewhere between 5 and 30 million species exist.)
A recent conservative estimate of the present rate of species loss suggests that we are losing one species per year for every 10,000 species on Earth. Thus the present extinction rate is 100 to 1000 times as fast as the historical rate.
When a species is lost, all of the genetic information that it contained is lost as well. If a species has valuable characteristics — like the bread mould that gave us penicillin — its loss is particularly important to humans. But, in a crucial way, all species are important to biodiversity: the loss of component parts weakens the whole structure in ways that we cannot understand or appreciate. We just know that loss of species cannot be good for the system, or for us.
Why are we losing species so fast? A recent look at the loss of amphibians (frogs, toads and salamanders) reveals many complex reasons. (Andrew R. Blaustein and David B. Wake, "The Puzzle of Declining Amphibian Populations", Scientific American, April 1995.)
There are 5130 species of amphibians today. Amphibians have been on the Earth for well over 100 million years — 1000 times as long as Homo sapiens has been around. Amphibians managed to live through the stressful time when the dinosaurs went extinct, 65 million years ago. Clearly, amphibians are survivors.
Since about 1970, however, there has been accumulating evidence that amphibians are declining and disappearing in places as diverse as North America, Central and South America, Europe, Asia, Africa and Australia.
The rate of amphibian extinctions has been estimated as follows: there are about 4000 species of frogs and toads; only five are thought to have become extinct between the 19th century and the 1960s. But in the past 25 years, herpetologists have observed drastic population declines and disappearances of many more species. They believe that 89 amphibian species are now at risk of extinction. Other species have not been seen in recent years, but it is not known whether this is due to natural fluctuations of population or is a sign of trouble.
Why has the rate of amphibian extinctions increased so rapidly this century and especially within the last 30 years? Specialists writing in Scientific American and elsewhere list the following reasons:
1) Amphibians are born of eggs laid in water, eggs not protected by a leathery or hard shell. Thus amphibians start "sampling" the environment as soon as life begins.
2) During early life (egg and larval stages), they live in the water; during later stages, they live partly in water and partly on land. Thus they "sample" a spectrum of environments as their lives progress.
3) Amphibians have a permeable, exposed skin, not covered by thick scales, hair or feathers. They breathe through their thin, moist skin, again "sampling" the environment and whatever pollutants it may contain. Because they are "cold blooded" (having no internal thermostat), they move in and out of sunlight to avoid extremes of heat and cold.
4) Habitat loss: The US originally contained 60-75 million hectares of wetlands; during the 1950s to 1970s, about 10% of these wetlands were drained. Currently an estimated 43 million ha, or between 58% and 72% of the nation's original wetlands, remain in the continental US. Nearly 120,000 ha of wetlands are lost per year. Amphibians depend upon wetlands for life.
5) Habitat fragmentation. Amphibians live in local colonies. When one colony's habitat is damaged, it might migrate to a new location if conditions are right. But habitats have been fragmented — cut up by human developments — so that migration to new habitat is made difficult or impossible.
6) Introduction of exotic species for recreational fishing. Exotic species may outcompete indigenous species for food and breeding sites or may prey upon the indigenous species. In the US, introduced species such as rainbow trout, golden trout, brook charr, and largemouth bass eat eggs, larvae and adult amphibians.
7) Global climate change. Normally, the Earth's ozone shield filters out the sun's deadly ultraviolet radiation. In the last 30 years, humans have depleted the ozone shield by chemical contamination, thus increasing the amount of ultraviolet radiation striking the surface of the Earth. Field experiments by biologists in Oregon showed that the eggs of certain species of amphibians are killed by increased exposure to ultraviolet light. They concluded, "Clearly, amphibian eggs in wild populations were dying from exposure to ultraviolet-B radiation".
8) These same scientists note that certain bacteria and fungi have been known to decimate amphibian populations. One particular fungus is carried by fish raised in hatcheries; when the fish are released, the fungus may harm local amphibian populations. These scientists speculate that, because ultraviolet light is known to harm the immune system of many animals, ozone depletion may be harming the immune systems of amphibians, diminishing their defences against bacteria and fungi.
9) Acid rain, snow and fog: acidity itself seems to harm growth and development of amphibians; it also releases aluminium from the soil, which then harms growth and development of amphibians.
10) Agricultural contaminants: In the western US, agriculture uses 80-90% of available water. Recently, 31 of 50 states, plus the Virgin Islands and Puerto Rico, have reported concerns about ground water contamination by pesticides; there are few data on the effects of pesticides on amphibians. Recent studies at the Klamath Basin National Wildlife Refuge showed that irrigation drain water was either lethal to, or caused significant malformation of, developing frog embryos. The study concluded that poor water quality and/or pesticides may be contributing to the decline of indigenous frog populations.
11) Over-harvesting of frogs for human food: The importance of frog legs as a food item in France has apparently been linked to a marked decline in native frogs in Europe, India, and Bangladesh. The demand for frog legs in France is tremendous: the French eat 3000 to 4000 tonnes of them a year. Some 20,000 frogs must be sacrificed in order to supply a single tonne of legs.
12) Certain industrial chemicals can mimic the activity of naturally occurring hormones. Examination of birds, fish and reptiles indicates that these substances can have drastic consequences, such as a reduction in sperm count and the alteration of male genitalia.
At the most immediate, practical, monetary level, it makes no sense to ignore the decline and disappearance of species. From the perspective of humans, amphibians represent a storehouse of pharmaceutical products waiting to be tapped fully.
Hundreds of chemical secretions have been isolated from amphibian skin, and scientists are just beginning to learn how valuable these substances may be. Some of these compounds are already used as pain killers and in treatment of victims of traumas ranging from burns to heart attacks. It is clearly in our own interests to stop the destruction of amphibians, and of all other creatures great and small.
[From Rachel's Environment & Health Weekly (US).]