evolution

On my journey home I am sometimes accosted by people with collecting tins exhorting me to 'Save The Tiger'. The question that springs to mind is -- "why?" Surely, the extinction of the tiger will affect virtually no-one, and might even have benefits. Fewer villagers in India will be attacked; the illegal trade in poached tigers will collapse; and irritating people with collecting tins would have to think of something else to save.

Taking a broader view, evolution is all about change. A species of mammal, such as a tiger, will exist, on average, for a few million years before it becomes extinct anyway -- or evolves into something different. And as species become extinct, new ones evolve to take their place. Death, it is said, is simply nature's way of telling you to take it easy. Why should it matter that human beings, rather than natural processes such as disease or climate change, are the agents of death?

Sometimes death cuts down enormous numbers of species in a relatively short time -- far more than can be replenished as part of the natural turnover of 'background' evolution and extinction. In the past 530 million years, there have been five of these 'mass extinctions'. The largest, at the end of the Permian period (251 million years ago), destroyed more than 96 per cent of all species. The extinction at the end of the Cretaceous period (65 million years ago) came a poor second, despite the celebrity of the dinosaurs, its most prominent victims. It is possible that we are currently living through, and causing, another mass extinction.

What happens after the wave of extinction has passed? How long does it take for biodiversity to return to its pre-extinction richness? You might think that recovery would take longer after more destructive pulses of extinction. If so, you would be wrong -- as two American scientists show in Nature¹.

James Kirchner of the University of California, Berkeley, and Anne Weil of Duke University in North Carolina have been looking at what the fossil record can tell us about the relationship between extinction and replenishment. Their results are clear: re-stocking the ecosystem after an extinction always takes around ten million years, irrespective of the scale of the preceding destruction.

Kirchner and Wier's study indicates there is a minimum recovery period between mass-extinctions (middle) and the return of biodiversity (top) -- of around ten million years..

This has important implications for present-day conservation, because it illustrates the interdependence of the ecosystem. If tigers were all there was to it, this would not be a problem. But no species is an island: tigers are 'top predators' that affect the entire balance of nature. They feed on other animals, which in turn feed on plants. And each tiger provides a home for parasites, and acts as a carrier of disease. The disappearance of any one species may have far-reaching consequences.

The model in which an ecosystem takes longer to recover from a larger extinction is, therefore, based on a false assumption -- that each species is somehow isolated from every other, and recovery is a simple matter of filling the vacant ecological niches. The wider the destruction, the more vacant niches there will be, and the longer filling them will take. But if species are interdependent, species themselves are niches, so the destruction of one species removes opportunities for many others. To destroy many species simultaneously therefore makes it very hard for new, pioneer species to get started.

Once they do get started, life is easier for other newcomers, as the ecological 'infrastructure' is in place. This explains why extinctions are always followed by an interval in which the flora and fauna are very dull -- like the weeds found on the sites of recently demolished buildings, but on a global scale. An interval of a few hundred thousand years in which there was little but ferns, for example, followed the end-Cretaceous extinction.

As an analogy, think how hard life was for the first European colonists of new territories, such as America or Australia. Without an established infrastructure, the colonists had to do everything for themselves: life was hard and it took a long time to establish viable populations that could stand alone, without help from home. But the efforts of these pioneers made the lives of subsequent arrivals much easier. Today's immigrants do not have to be agriculturally self-sufficient, for instance, to survive.

As Weil and Kirchner show, the interdependence of species is the primary determinant of recovery -- so much so, in fact, that the scale of the preceding extinction is unimportant. When nature is replenishing the planet, it follows its own intrinsic timetable. Using the record of Earth's history, the researchers have discovered something fundamental about the ecosystem, and this has an important message for present-day conservationists coping with what might be a mass extinction caused by humans.

If we destroy the last remaining tigers, we will not see their like again. To be sure, other top predators will evolve. But if tigers are destroyed along with the jungles in which they live -- an interdependent resource of biodiversity -- then it may take ten million years for the future equivalent of the tiger to appear. This appearance will depend on the evolution of suitable prey animals, and the existence of suitable habitat. This process takes time -- so much time, in fact, that humanity will almost certainly be extinct by the time the ecosystem has fully recovered. For humans, like tigers, are mammals, and our species can be expected to last for only a few million years before becoming extinct itself, or evolving into something else. The successor to the tiger will not be hunted by human beings.

Saving the tiger, therefore, is about more than tigers. Tigers are simply the 'headline' species -- attractive animals whose demise might prompt passing commuters to part with their loose change. Conservationists have long understood that for all the world's biodiversity, the public recognizes only two animals, namely 'aaaah! animals' and 'ughhh! animals'. If the campaign was about saving the tapeworm, or saving the parasitic wasp, campaigners would collect less money -- but these species are as important as tigers in maintaining the overall health of the ecosystem.

Removing species from an ecosystem reduces its ability to cope with environmental change. David Tilman of the University of Minnesota and his colleagues showed, for example, that experimental plots of grassland with fewer species were less able to resist drought than were plots richer in species. Conservation is about ecosystems -- about habitats.