The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [22]
Berlins’s commonsense 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 rain forest or the Siberian tundra, now turn out to be more a product of human engineering than we once thought—and their vanished megafauna 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 percent 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 substantial 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