Extinction Of Species Research Paper

Earth’s stock of species generally is estimated to total a minimum of 10 million. Some scientists believe the true total could well be 30 million, possibly 50 million and conceivably 100 million (Ehrlich and Wilson 1991). Of the conservative estimate of 10 million species, around 90 percent usually are considered to be terrestrial; and of the terrestrial species, roughly 80 percent or more than seven million are believed to occur in the tropics, with roughly five million in tropical moist forests (Ricklefs and Schluter 1992). As we shall see, these tropical forests are not only the richest biome biotically but constitute the biome where habitat depletion is occurring fastest—‘depletion’ including not only outright destruction but gross disruption (fragmentation, etc.), with loss of many ecosystem attributes including food webs. So this is the prime locus of the mass extinction underway, and it is the main focus of the present analysis.

Consider again the higher estimates for the planetary spectrum of species, between 30 million and 100 million. Since many if not most of the additional species are believed to occur in tropical forests, the true planetary total is not only a matter for speculation. If the real total is 30 million species, the extinction rate will be much higher than the rate postulated on the basis of only 10 million species. But for the sake of being cautious and conservative, let us accept a total of 10 million species. As noted, at least half of these species live in tropical forests, even though remaining forests cover only 6 percent of Earth’s land surface.

Tropical forests are being destroyed at a rate of at least 150,000 square kilometres per year (Myers 1992). In addition to this outright destruction, an expanse as large again is being grossly disrupted through overheavy logging and slash-and-burn cultivation, with much degradation and impoverishment of ecosystems and species’ life-support systems. But in the interests of being cautious and conservative again, let us consider only outright destruction. The current annual loss represents 2 percent of remaining forests; and the amount increased by nearly 90 percent during the 1980s. If present patterns and trends of forest destruction persist, leading to still more acceleration in the annual rate, the current destruction of two percent of remaining forests may well have increased by another 50 percent during the 1990s. In a recent year more forest was burned in Amazonia and Borneo than ever before.

How shall we translate a 2-percent annual rate of forest destruction into an annual species extinction rate? An analytic approach is supplied by the theory of island biogeography, being a well-established theory with much empirical evidence from on-ground analyses around the world (MacArthur and Wilson 1967). The theory states that the number of species in an area rises or falls as approximately the fourth root of the increase or decline in the area, with actual values varying between the third and fifth roots. So when a habitat loses 90 percent of its original extent, it can generally support no more than 50 percent of its original species.

The calculation depends critically upon the status of the remaining 10 percent of habitat. If this relict expanse is split into many small pieces (as is often the case with remnant tracts of tropical forests), a further ‘islandizing effect’ comes into play, reducing the stock of surviving species still more. It is not clear how severe this additional depletion can be. Informed estimates suggest the 50 percent can readily be reduced to 40 percent, more generally to 30 percent, sometimes to 20 percent, and occasionally even to 10 percent. Similarly, if most species concerned occur in small, local endemic communities, the percentage loss of species can readily approach the percentage loss of area. Moreover, isolated remnants of forest become prone to additional depauperizing processes such as ‘edge effects’, also to dessicating effects due to local climate change (Myers 1992). So the 90 50 calculation for island-biogeography extinctions should be viewed as a minimum estimate.

The most broadscale application of island biogeography to tropical deforestation and species extinctions has been presented by one of the authors of the original theory, Edward Wilson of Harvard University. He estimates a late-1980s annual loss of 27,000 species in the forests (Wilson 1992). As he emphasizes repeatedly, this estimate is extremely optimistic. If we employ a more ‘realistic’ reckoning by qualitatively incorporating a number of other bioecological factors such as alien introductions, overhunting, and diseases, the annual total will become larger. (Indeed certain other analyses propose that 60,000–90,000 species are lost in tropical forests each year.) In addition, a good number of species, albeit undetermined in even preliminary terms, is presumably disappearing in other parts of the world. Suppose we accept a bare minimum estimate of 30,000 species eliminated worldwide per year. This means we are witnessing a rate at least 120,000 times higher than the ‘natural’ rate of extinctions before the advent of the human era, considered to be perhaps one species every four years (Raup 1991).

True, the tropical forest calculation is a broadbrush affair, viewing the forests as a single homogeneous expanse even though they are highly heterogeneous. So the island-biography approach needs to be complemented by a local-scale assessment, available in the form of a ‘hotspots’ analysis. Hotspots are areas that (a) feature exceptional concentrations of species with exceptional levels of endemism, and (b) face exceptional threats of imminent habitat destruction. There are 25 such hotspots identified which contain 44 percent of all Earth’s plant species and 35 percent of all species in four vertebrate groups confined to 1.4 percent of Earth’s land surface (Myers et al. 2000). So far as we can determine, they contain an even larger proportion of Earth’s animal species with highly localized distributions. Most of these hotspots have already lost the great bulk of their biodiversity habitats, and it is reasonable to suppose that many thousands of species are disappearing annually in hotspot areas alone.

So much for the current extinction rate. What of the future? Through detailed analysis backed by abundant documentation, Wilson (1992) considers we face the prospect of losing 20 percent of all species within 30 years and 50 percent or more thereafter. Another expert, Peter Raven of the Missouri Botanical Garden, calculates that half of all species exist in those particular tropical forests that, in the absence of adequate conservation measures, will be reduced to less than one tenth of their present expanse within the next three decades. In accord with island biogeography, Raven (1990) concludes—and he stresses this is a conservative prognosis—that one quarter of all species are likely to be eliminated during the next 30 years. Worse, ‘fully half of total species may disappear before the close of the 21st century.’ Another two biodiversity analysts, Paul and Anne Ehrlich (1992), assert that ‘If the current accelerating trends (of habitat destruction) continue, half of Earth’s species might easily disappear by 2050.’ These estimates are in line with those of several other biodiversity specialists. If we accept, then, that half of all species are likely to be eliminated within the foreseeable future, this means in turn that perhaps one third of the problem is confined to the 25 hotspots.

The loss of large numbers of species will be far from the only outcome of the present biotic debacle, supposing it proceeds unchecked. There is likely to be a significant disruption of certain basic processes of evolution (Myers 1996). The forces of natural selection and speciation can work only with the ‘resource base’ of species and subunits available—and as we have seen, this crucial base is being grossly reduced. To cite the graphic phrasing of Michael Soule, ‘Death is one thing; an end to birth is something else.’ Given what we can discern from the geologic record, the recovery period, i.e., the interval until speciation capacities generate a stock of species to match today’s in abundance and variety, will be protracted. After the late Cretaceous crash, between 5 and 10 million years elapsed before there were bats in the skies and whales in the seas. Following the mass extinction of the late Permian when marine invertebrates lost roughly half their families, as many as 20 million years were needed before the survivors could establish even half as many families as they had lost.

The evolutionary outcome this time around could prove yet more drastic. The critical factor lies with the likely loss of key environments. Not only do we appear set to lose most if not virtually all tropical forests. There is progressive depletion of tropical coral reefs, wetlands, estuaries, and other biotopes with exceptional abundance and diversity of species and with unusual complexity of ecological workings. These environments have served in the past as pre-eminent ‘powerhouses’ of evolution, meaning they have thrown up more species than other environments. Virtually every major group of vertebrates and many other large categories of animals have originated in spacious zones with warm, equable climates, notably the Old World tropics and especially their forests. The rate of evolutionary diversification—whether through proliferation of species or through emergence of major new adaptations—has been greatest in the tropics, especially in tropical forests. Tropical species, notably tropical forest species, appear to persist for only brief periods of geologic time, which implies a high rate of evolution.

As extensive environments are eliminated wholesale, moreover, the current mass extinction applies across most if not all major categories of species. The outcome will contrast sharply with the end of the Cretaceous, when not only placental mammals survived (leading to the adaptive radiation of mammals, eventually including man), but also birds, amphibians, and crocodiles among many other nondinosaurian reptiles. In addition, the present extinction spasm is eliminating a large portion of terrestrial plant species, by contrast with mass-extinction episodes of the prehistoric past when terrestrial plants have survived with relatively few losses—and have thus supplied a resource base on which evolutionary processes could start to generate replacement animal species forthwith. If this biotic substrate is markedly depleted within the foreseeable future, the restorative capacities of evolution will be the more diminished.

All this will carry severe implications for human societies extending throughout the recovery period estimated to last at least five million years, possibly several times longer. Just five million years would be twenty times longer than humankind itself has been a species. The present generation is effectively imposing a decision on the unconsulted behalf of at least 200,000 follow-on generations. It must rank as the most far-reaching decision ever taken on behalf of such a large number of people in the course of human history. Suppose that Earth’s population maintains an average of 2.5 billion people during the next five million years (rather than the six billion we have today), and that the generation time remains 25 years. The total affected will be 500 trillion people. Just 1 trillion is a large number: check the length of time made up by 1 trillion seconds.

A final thought, this one as regards our policy responses. The Global Environment Facility under the World Bank has provided half a billion dollars over 3 years to assist biodiversity. This is quite the largest such dispensation ever made. But compare it with what is at stake. Every year, just the commercial value to just the rich nations of just the present array of plant-derived pharmaceuticals is 100 times greater. More significant still, the worldwide amount spent annually on ‘perverse’ subsidies, i.e., subsidies that inadvertently foster overloading of croplands, overgrazing of rangelands, profligate burning of fossil fuels, wasteful use of water, overcutting of forests and overharvesting of fisheries (to cite but a few examples of activities that also reduce biodiversity) is 40 times greater again (Myers and Kent 2000).

Biodiversity loss is indeed a profound problem. It is also a glorious opportunity. As is obvious, species extinction is irreversible. And we face the prospect of eliminating species in their millions, perhaps half of all that share this planet with us, by virtue of what we do (and don’t do) during the immediate future. Time is of the super-essence. Suppose we allow ourselves until, say, the year 2010 to take the vital decisions that will affect our planetary home for hundreds, thousands, and even millions of years. After the year 2010, the processes of habitat destruction will surely have worked up so much momentum that they will be hard to slow down, let alone to halt or even reverse. Till 2010 there are roughly 3,500 days. We lose one percent of our maneuvering room every five weeks!

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