One of the most disturbing current ideas in biology is that economic and technological developments are killing off biological species at a dangerous, perhaps disastrous rate. Some ecologists hold that such claims are greatly exaggerated.
The simple truth seems to be that our present cataloguing of the earth's biological species is far too incomplete to allow us confidently to gauge the rate at which we are losing biodiversity. The situation is described by W. Wayt Gibbs in last November's issue of Scientific American.
One expert estimates that the earth is losing biological species 1,000 times faster than it did before humans appeared on the scene, and that this rate will increase by a factor of 10 over the next century. The Harvard biologist Edward Wilson estimates that up to 10 per cent of species vanish every decade. If the gloomier estimates are true, we are losing species at the fastest rate since the extinction of the dinosaurs.
On the other hand, some scientists, such as Bjørn Lomborg, a professor of statistics and political science at the University of Aarhus, in Denmark, argues in his book, The Skeptical Environmentalist, that the best models project current extinction rates of 0.15 per cent of species per decade. Lomborg grants that such a rate is a problem, but it falls far short of a catastrophe.
In order to establish where we stand, we need to know three things: the natural background extinction rate, the current extinction rate and whether the current rate is increasing, remaining steady or declining. The natural background extinction rate has been studied by examining the fossil records, but such work is bedevilled by uncertainty.
Less than four per cent of all species are thought to have left fossil remains, for example. These were the most successful and hardiest species. Study of the fossil record would therefore miss the many species that became extinct quickly, before they could be fossilised. Estimates of the mean life of species in evolutionary history vary from one million to 10 million years.
It is also difficult to estimate how quickly species are being lost. A recent study that tried to correct for all known difficulties estimated that, over the past 200 years, the rate of loss of mammals was 120 times higher than the natural background rate.
It is one thing to study mammals, about which we know a lot, but we are unsure, by a factor of 10, how many species of life we share the planet with. Credible estimates of the total number of species (excluding micro-organisms) range from five million to 15 million. Taxonomists have named 1.8 million species, but we know almost nothing about most of them, particularly the insects, worms and crustaceans that dominate the animal kingdom. It is therefore possible for thousands of species to vanish without our knowledge.
ONE reason for predicting a disastrous loss of species in our time is the rate of habitat destruction, particularly the loss of tropical rainforests that are so rich in biodiversity. Wilson first worked out a mathematical relationship between habitat area and the number of species supported. According to his formula, if 90 per cent of a habitat is eliminated, the number of species falls by half.
Tropical rainforests were vanishing at a rate of 1 per cent per year in the recent past, and Wilson's equation predicts an early extinction of 0.25 per cent of species. If we assume a pool of 10 million species in the rainforests, we would expect 25,000 to vanish annually.
For several reasons, however, Wilson's species-habitat equation must be qualified, and measurements have shown that, in many cases, deforestation does not affect species numbers as severely as the equation predicts. Also, annual global forest loss is down to about 0.2 per cent, although it is 0.5 per cent in tropical regions.
Other studies have shown that species are lost much faster in newly developed areas than in areas that have been developed for a long time. In newly developed areas, those native species least compatible with modern agriculture are quickly lost; the species that survive are able to resist developmental insults.
The natural biological diversity on the planet is responsible for maintaining our environment. Green plants consume carbon dioxide and excrete oxygen; animals breathe oxygen and excrete carbon dioxide. Plants evaporate water into the atmosphere, which later falls as rain. The activity of animals such as worms creates our soil. And so on.
The effect of the millions of species in maintaining the environment has been likened to the thousands of rivets in an aircraft. You can remove some of them without causing much apparent damage. But remove too many and it falls apart.
The millions of uninvestigated biological species also represent a vast genetic reservoir that could be harnessed to produce new drugs, or new foods, should anything happen to the 30 crops or 14 animal species that feed the world. It would be folly and vandalism to pauperise the earth's biological diversity. Each species of life is a masterpiece of adaptation to its environment, produced by natural selection.
There is good reason to be worried about the loss of biodiversity. It may not be nearly as bad as some believe, but we cannot be sure. It would be irresponsible to take the matter lightly.
- William Reville is a senior lecturer in biochemistry and director of microscopy at UCC