The Loss of Species

Species extinctions have received the lion's share of the attention in debates regarding biodiversity and the need to sustain it. The loss of individual species in ecosystems, such as frogs in wetlands or ferns in a forest, can certainly affect the ways in which those systems work together to cycle essential nutrients and water and process energy. Since we have only limited ability to predict how ecosystems will respond in terms of replacement or built-in redundancy to the possible loss of a specific species, there is some reason to be concerned when any are threatened by extinction.

At the same time, the same degree of concern should apply to reductions in the populations of species, even if they are not reduced to disappearance altogether. The role that classes of organisms play in ecosystems depends not only on what they do in terms of material cycling and energy flow, but on how many are there to do it. Reductions in abundance of an essential species can clearly affect overall system functioning, and therefore the degree to which ecosystem services will continue to be provided.

Some, known as keystone species, play a role in ecosystems that seems out of proportion to their number, such that even small changes in their abundance may have great impacts on the ecosystems in which they live. A common example is the sea- otter, a marine mammal that lives along the coasts of the northern Pacific Ocean. They dive and prey on sea-urchins that principally feed, in turn, on large seaweed called kelp. When sea-otters are present, the number of urchins is kept sufficiently low that stands of kelp--which are of commercial value as a source of potash and iodine--can become established and thrive. When otters vanish from the scene, the resulting growth in urchin populations effectively prevents the plant's successful regeneration, and eventually leads to the loss of kelp forests.

In time, all classes of living things--like the dinosaurs, or, we must presume, our own species--must face extinction. But the disappearance of any of them is a critical endpoint, marking the end of 3.5 billion years of evolutionary development. In Nature it represents a permanent depletion of biodiversity and a loss of genetic information on which evolution is based. In terms of people and nations, it counts as a loss of potential economic value in terms of services or products. Each species is a reservoir of unique genetic information that cannot be reproduced once it is gone. In this broader sense, any extinction, however trivial it may seem, represents a permanent loss to the biosphere as a whole.

What we need to know for informed policy decisions are the ecosystem services that a threatened species provides; the degree to which it offers opportunities for direct economic benefit; how expected benefits weigh against costs of preservation; and on a more general level, how present or expected rates of extinction compare to what might be expected through natural changes. The telling questions are whether and by how much the present rate of species loss differs from the rate that Nature would exact, were we not here, and whether the species that are lost play important keystone roles. The challenge is that this sort of information is only rarely available. Nor do we have, as yet, a predictive theory of keystone species.

Rates of loss
The UNEP Global Biodiversity Assessment has recently reviewed the methods that have been used in the literature to calculate natural, or background extinction rates and have compared them against current trends. The results, which are intentionally conservative, are sobering. Best estimates are that current extinction rates for well- documented groups of primarily, but not exclusively, vertebrates and vascular (in general, seed-bearing and fern-like) plants are at least 50 to 100 times larger than the expected natural background. There is no good reason to expect these rates to differ very much for plant or animal groups that are less well-studied.

On the basis of recent estimates of land-use change, largely in the tropics, there is a reasonable expectation that extinction rates in the very near future could rise, worldwide, to as much as 10,000 times the natural level. Extinctions of this number and extent would approach, and possibly surpass, the major mass extinctions of the geologic past, as when dinosaurs and other life forms disappeared, about 65 million years ago.

The total number of species that inhabit the planet is unknown. The UNEP Global Biodiversity Assessment uses an estimate of about 13 million, but the range varies from 8 to 50 million or more. Only about 2 million species have been described scientifically, and they are distributed very unevenly among different taxonomic groups (Table 2). While important in its own right, the number need not be precisely known to be concerned about the rates at which the better documented species are now disappearing. In today's world, most extinctions will occur before the species have even been named and described, much less known ecologically.