The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [23]
However, it is not only predators that count. Bottom-up interference can also dramatically destabilize 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 percent 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 percent 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 plowed 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 kilometers 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, while 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, while 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, ecologists now understand a fundamental principle of biodiversity: that the greater the diversity of species, the more resilient and stable an ecosystem can be. The same, of course, applies to the biosphere as a whole. We are only just beginning to realize all the myriad ways that different species act unconsciously together to keep this planet habitable and its climate tolerable. Might there be some kind of global “tipping point”—like the ones that were passed in the Newfoundland cod fishery and the Caribbean coral reefs—where some kind of irreversible global ecosystem shift takes place? This is the possibility that the planetary boundary on biodiversity is intended to prevent: It is now absolutely clear that the Earth’s living biosphere depends fundamentally on the maintenance of a broad level of species diversity. If the Sixth Mass Extinction is allowed to continue