The God Species: How Humans Really Can Save the Planet.... Mark Lynas

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СКАЧАТЬ discovered. Scientists working on a 2009 update for a global biodiversity report first issued in 2006 had to add 48 new reptiles, 200 new fish and 1,184 flowering plants, all identified for the first time in the intervening three years.³⁵ Recently ecologists working in the crater of a single extinct Papua New Guinean volcano found 16 new frogs, three new fish, a giant bat and giant rat; luckily a BBC camera crew was on hand to record each unique moment of discovery.³⁶

      But who cares anyway? Here’s Marcel Berlins, columnist on the Guardian: ‘I passionately believe in saving the whale, the tiger, the orang-utan, the sea turtle and many other specifically identified species. What I do not accept is the general principle that all species alive today should carry on existing forever. We have become so attuned to treating every diminution of animals, insects, birds or fish with concern that we have forgotten to explain why we think it so terrible.’ Warming to his argument, Berlins concludes: ‘How many mammal species can you think of? Can the remainder be that important? Can their loss matter that much, to you or to the world? Of course we must fight hard to retain as many species as we can; but it isn’t a tragedy if we lose quite a few along the way.’

      Berlins’s common-sense argument is a reasonable one, and its answer not as obvious as one might expect. After all, the biosphere has lost woolly mammoths, Tasmanian tigers and countless other charismatic species already, and yet the world goes on turning. Environments we previously assumed were pristine, like the Amazonian rainforest or the Siberian tundra, now turn out to be more of a product of human engineering than we once thought – and their vanished mega-fauna have left little identifiable trace, and certainly not one that affects our current lives from day to day. Indeed, most people are unaware that the Quaternary Megafaunal Extinction even happened, and view the disappearance of the mammoth as an interesting but still unsolved mystery, if they think about it at all. Does it really matter if the thinning-out process accelerates a little more?

      There are some good utilitarian arguments to show why destroying biodiversity is not a good idea. The biologist E. O. Wilson tells a story of how a small tree in a remote swamp forest in Borneo yielded an effective drug against HIV – except that when collectors returned to the same spot a second time they found the tree had been cut down, and no more could be found.³⁷ (Happily for AIDS sufferers, a few remaining specimens were eventually located in the Singapore Botanic Garden.) Who knows which tangled Amazonian vine might one day deliver a cure for cancer? But this is only part of the story, for it is ecosystems in their entirety that are valuable and irreplaceable as much as the individual species they contain. Biodiversity loss is a planetary boundary of the utmost importance not because killing off species is morally wrong, but because a healthy diversity of living organisms is essential for ecosystems to function properly.

      Living systems keep the air breathable and water drinkable for themselves and us, but to continue to perform these vital services they need to retain their complexity, diversity and resilience. Once humans start to pick off component parts, an ecosystem may appear to function as normal for a while – until some unpredictable tipping point is reached, and collapse occurs. Conceptually this is a bit like the game of Jenga, where wooden blocks are built together in a tower and pieces removed from underneath one by one by each player. Needless to say, whoever removes the crucial ‘keystone’ piece that topples the tower loses. The lesson of Jenga is an important one, because it shows that there is no single keystone: each removed block makes the tower less and less stable, but no one knows in advance which piece will lead the tower to collapse.

      Keystone predators are particularly important to ecosystems. In the marine realm, great sharks – like tiger, hammerhead, bull and thresher sharks – have in recent years been mercilessly targeted worldwide: their numbers have plunged by up to 99.99 per cent in some seas.³⁸ On the eastern North American coast, rays are no longer being eaten by the vanished sharks, and have increased their numbers as a result. They in turn eat scallops and oysters, destroying the formerly productive scallop fishery.³⁹ The process is known as a ‘trophic cascade’ and is now understood to be a fundamental part of ecological dynamics. An ecosystem shift can be irreversible: the Newfoundland cod, whose numbers collapsed because of overfishing in 1992, are unlikely ever to return in substancial numbers. Cod larvae are eaten by smaller fish and crustaceans like lobsters (once kept in check by more numerous adult cod), which dominate the ecosystem instead.⁴⁰

      For land-based ecosystems apex predators are just as important. In Yellowstone, the reintroduction of wolves in 1995 has allowed the regrowth of native aspen trees for the first time in half a century. This is because elk populations are now being controlled by wolf predation, preventing overgrazing and allowing trees to recover.⁴¹ In nearby Grand Teton National Park in Wyoming small birds like the gray catbird and MacGillivray’s warblers may depend for their survival on wolves, recently reintroduced to the area after an absence of 75 years. Both birds flourish in riverside willows: but the willows, like Yellowstone’s aspens, were being overgrazed by hungry moose. In places where predators are still absent, expensive management schemes have to artificially keep down the populations of deer and other grazing herbivores – a service that wolves perform for free.

      However, it is not only predators that count. Bottom-up interference can also dramatically destabilise an ecosystem. In the early 1980s a new pathogen appeared in the Caribbean near the mouth of the Panama Canal, wiping out sea urchin populations with extraordinary virulence: within a year 98 per cent of the urchin population was gone, in what is still the worst recorded die-off of any marine animal in history. Because urchins are herbivorous grazers they perform an important function on reefs, keeping the corals clear of algae and seaweed that would otherwise choke the reef systems. Without them, the corals lacked protection, and within a year reefs from Jamaica to the coast of Venezuela disappeared under a thick layer of green slime.⁴² After a decade, just 5–10 per cent of the original coral cover was left,⁴³ and little more remains to this day.⁴⁴ A whole marine ecosystem had irreversibly collapsed because of the removal of one of its key components.

      Functioning ecosystems need not just a varied number of species, but also – just as crucially – habitat. Humans have disturbed, fragmented or ploughed up huge areas of the planet’s terrestrial surface. But there is a direct correlation between biodiversity and land area: the smaller the remaining fragment, the fewer species it can support. This so-called ‘species–area relationship’ was illustrated by a massive – though unintentional – field experiment beginning in 1986, when a gigantic hydroelectric dam was built in the jungles of Venezuela. When the lake behind the dam began to fill, the rising tide turned a hilly area of four thousand square kilometres into isolated islands, each with its tropical forest plant and animal species cut off by the surrounding waters. Some of the new islands were very small, just an acre or two in size, whilst others were relatively large, with areas of 150 hectares or more. As you might expect, the smallest islands lost the most biodiversity – three quarters of their original complement – due to their small areas. All islands, large and small, lost their top predators: the jaguar, puma and harpy eagle. But the species that did survive quickly became more abundant as both competition for food and predation ceased abruptly. Some islands were overrun by leaf-cutting ants. One, having housed a large herd of capybaras as the waters rose, ended up as little more than bare ground covered by capybara dung. On some islands, monkeys decimated bird populations, whilst on others rodent populations increased 35-fold.⁴⁵ In all cases, complex and formerly diverse ecosystems were torn apart and thrown into chaos.

      From these and many other examples, СКАЧАТЬ