The Lynx became the top predator in Doñana after the last wolf was shot in 1951. That is how it goes with predators and large animals. The bigger they are, the sooner they tend to vanish. Among mammals, the risk of extinction rises sharply for species that weigh more than three kilograms—about as much as a small pet cat. Big creatures need more food and more space to find it in than small ones; they are slower to reproduce, and are apt to get on the wrong side of humans. "The species that tend to go extinct first tend to be the big-bodied things, and the tasty things," says Rob Ewers of Imperial College London. He is talking about the Amazon forests, but it’s a general truth.
Big animals, particularly those at the top of food chains, "are really fundamentally important to holding ecosystems together," says Jim Estes, a biologist based at the University of California, Santa Cruz. When they go, ecosystems unravel and reorganise, removing more species in the process. "Apex consumers" can take whole habitats with them. Wolves may protect forests by preying on the deer that browse saplings. If the wolves are wiped out, the deer multiply at the expense of the trees, preventing the forest from renewing itself: the end-point, as on the once-forested Scottish island of Rùm, is a treeless landscape. Globally, the result is the "downgrading of Planet Earth", as Estes put it in an article for the journal Science in 2011.
The exits began long before roads or rifles were devised. Nearly three-quarters of North American and a third of Eurasian megafauna disappeared between 50,000 and 10,000 years ago. Woolly mammoths, woolly rhinoceroses, giant sloths and sabre-toothed cats were among the species that vanished from the face of the Earth. While climate change was one part of the story, human expansion was another. The selective disappearance of large animals marks this period out from other extinction episodes, and was the start of what Estes and his fellow authors suggested "is arguably humankind’s most pervasive influence on the natural world". For Estes, it was the beginning of the sixth mass extinction.
If that was the opening phase, the second distinctive spike was the wave of extinctions in historic times that took place on islands colonised by humans and the animals that came with them on their voyages. Species after species ended up as dead as the dodo, which succumbed a few decades after people began to settle the Indian Ocean island of Mauritius in the 17th century.
Today Mauritius is the humanised world in a nutshell. Smaller than many English counties, it has been cleared for cane fields, strung with towns, dotted with resorts, factories and call centres. A mountainous outcrop endures as a national park, and the fragility of island nature is given poignant expression on the Ile aux Aigrettes, an offshore microdot the size of a large town park from which the rats and non-native plants have been cleared. Its 26 hectares are now covered in a low net of native vegetation that offers shelter to threatened native creatures. A pink pigeon huddles beneath a bush: it’s one of fewer than a hundred on the Ile, and of fewer than 500 in the world. A little bird called a Mauritius olive white-eye darts among the leaves: there are a couple of dozen here, and a couple of hundred in all.
The pink pigeon demonstrates even more dramatically than the Iberian lynx how emergency intervention can throw a species a lifeline—in 1990, there were just ten left in the wild. Turnarounds like these affirm the value of conservation efforts. But they may also induce complacency about the broader sweep of extinction. As a child I was distressed by the thought of species disappearing for ever, thanks largely to a small album entitled "Wildlife in Danger", filled with cards given away in cartons of Brooke Bond tea. Written by the ornithologist Peter Scott, it was published in 1963 and reissued ten years later. None of the 50 animals featured on the cards is yet officially classed as extinct—though one of them, the North American ivory-billed woodpecker, very probably is—and three or four are now out of danger. In the case of the Javan rhino, it is almost as though extinction has been put on pause. The album gives the population figure as just 40. Today’s estimates are pretty much the same—and the album has been superseded by a website showing 35 of them in individual video clips. The last of the species have become something like reality-TV stars. And my generation, the first to grow up with a background sense of ecological crisis, has reached middle age without having to read many obituaries of species.
That isn’t just because special efforts have been made for charismatic creatures, or because naturalists prefer to regard a species as missing until they are quite sure that it must be dead. About 800 extinctions have been recorded since 1500, a low figure even allowing for the likelihood that there are several unknown species for each one that has been given a Latin name, and most of them were on islands. Now that the island phase has largely run its course, the big question is what will happen on the land masses, where species are more vulnerable than in the oceans.
Estimates of future extinction levels are usually based on the relationship between the area of a habitat and the number of species in it. A rough rule of thumb is that if the area shrinks by 90%, 50% of its species will be lost. They may not go straight away, though. The difference between the number of species remaining and the number predicted is regarded as an "extinction debt" that will be paid in the long run. Rob Ewers took part in a study that found 80% of local extinctions in Amazon forests were still to come.
The accuracy of such predictions was challenged in 2011 by Fangliang He and Stephen Hubbell, who argued in a theoretical paper for the journal Nature that they always over-estimate extinction rates. Nevertheless, the researchers agreed that "the sixth mass extinction might already be upon us or imminent." And reports from remaining fragments of Brazil’s Atlantic-coast forests show that the situation on the ground may be much worse than it would appear from the graphs. "These habitat patches are essentially sitting ducks," says Carlos Peres of the University of East Anglia.
He and his colleagues surveyed 200 forest fragments across an area the size of Britain, wrecking four 4x4 vehicles in the process. They found what they called a "staggering" rate of extinctions among medium-sized and large mammals. Four-fifths of the populations had gone, although species-area calculations predicted that up to four-fifths would still be there. Fragmentation had left patches exposed to hunters and fire; their effects interacted "in a very perverse way", as Peres puts it, with those of area loss and isolation. Similar perverse synergies can be expected elsewhere. "I think that the processes we describe are actually quite ubiquitous," he says.
Peres regards the death of the last member of a species as "relatively trivial". What matters is the decline in population that leads to it. "People only care about those very terminal patients once the very last of a species dies out," he observes. "They hardly ever care about the long march towards global-scale extinction."
That view of nature, as an assembly of examples, does have a powerful hold on the imagination. We seem prone to a kind of Noah delusion: as long as we save a pair of each kind, we have fulfilled our responsibilities. But although a species that is down to its last few members is not extinct, it is not fully alive either, because it is no longer part of society. It is no longer contributing, competing, or helping to shape a larger living system. "Nature is not like a museum collection of the world’s species," says Georgina Mace of University College London. "It’s not just a matter of naming and keeping every one of those things. We should care about keeping the parts of the system. Can they still interact with each other? Can they still migrate, disperse, adapt, evolve?"
Mass extinction is thus more than the loss of kinds. It is the loss of abundance, of range, of populations. It is local extinction, the attrition of diversity as ranges shrink to enclaves, as well as global extinction. And, as the ecologist Daniel Janzen recognised in the 1970s, it is "what escapes the eye…a much more insidious kind of extinction: the extinction of ecological interactions."
Extinctions of this kind are surely happening all around us. Will they develop into a sixth mass extinction on the scale of the "Big Five", in which three-quarters of living kinds vanish? Extinctions among mammals, birds and amphibians are already running at higher rates than those which led to mass extinctions in the past, according to a study led by Anthony Barnosky, of the University of California, Berkeley. His team calculated that if all currently threatened species were to disappear within a century—the likelihood of which "would be quite high if we continue doing business as usual," Barnosky says—and extinctions carried on at the same rate, they could reach "Big Five" levels around 300 years from now.
by Marek Kohn, Intelligent Life | Read more:
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