How Human Beings Are Downgrading Life on Planet Earth

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We live in the Anthropocene, as some scientists have come to call our new geologic era. The term acknowledges the fact that human beings—nearly 7 billion strong and growing—have so much influence over the life, geography and even chemistry of planet Earth that we’re now essentially responsible for the whole show. For good and for ill, the planet from now on will be what we make of it.

But just because our actions are shaping the future of the planet—and everyone and everything else that rides on it—doesn’t mean that we always know the effects of what we’re doing. Case in point: in a new review in the July 14 Science, a group of researchers persuasively argue that one of the biggest impacts that human beings have had on the planet isn’t necessarily deforestation or climate change or pollution. It’s the extirpation—through hunting, habitat loss or disease—of apex predators, species like tigers or wolves or sharks that skit high on the food chain. When those species go, there can be drastic knock-on effects for animals and plants below them, as the paper argues:

The loss of apex consumers is arguably humankind’s most pervasive influence on the natural world. This is true in part because it has occurred globally and in part because extinctions are by their very nature perpetual, whereas most other environmental impacts are potentially reversible on decadal to millennial time scales. Recent research suggests that the disappearance of these animals reverberates further than previously anticipated (6–8), with far-reaching effects on processes as diverse as the dynamics of disease; fire; carbon sequestration; invasive species; and biogeochemical exchanges among Earth’s soil, water, and atmosphere.

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Removing those apex consumers produces what’s called “trophic downgrading,” which refers to the cascade of damage that can occur when an intact ecosystem is disrupted in such a significant way. It’s a relatively new concept for ecologists and conservationists, who long studied nature one plant or animal at a time. But as we now know, everything is connected. Actually, revise that—everything is connected, but some species are more connected than others, and when they’re taken out of the picture, ecosystems can change utterly or even collapse altogether. A few examples from the Science paper:

  • The reduction of cougars in Utah has led to a sharp increase in the number of deer, a loss of vegetation and an overall decline in biodiversity.
  • Whaling in the 20th century—when it reached industrial levels—actually changed the diet of killer whales, which then led to a decline in sea lions, seals and sea otters.
  • The hunting of lions and other big cats in Africa led to a population explosion in olive baboons, which then came into closer contact with human beings—and spreading disease along the way.
  • A rinderpest epidemic decimated the populations of wildebeest and other ungulates in the Serengeti, resulting in more woody vegetation—which then led to stronger and more frequent wildfires, until the disease was eliminated.
  • The collapse of sharks—remember them?—has been followed by a collapse in shellfish populations.

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The lesson here is that in nature, almost everything comes with a price. We can’t harvest an entire species—or “manage” them, as we long did with predators like wolves—without blowback, to use the defense parlance. The trouble, of course, is that these changes are very hard to model, because the web of species interactions is invisible—until we come along in our bumbling fashion and perturb it, like a hiker walking through a spiderweb. What the Science authors are suggesting is that the burden of proof has shifted—we should assume that top-line predators are vital to their ecosystems, and think twice before disturbing them.

Ideally, we’d disturb nothing, and leave nature to its perfect balance. Except—as Emma Maris writes in this lovely post—the idea of untouched nature is a myth, and has been ever since the first human being started using fires and tools. We’re not going back to Eden, however much we may long for it, because we never lived there. In his new book, the writer Mark Lynas describes us—accurately, I think—as the “god species.” We’re nearly all-powerful—but we’re not all-knowing, unfortunately. We’ve seized the keys to the kingdom, and it’s our responsibility to safeguard the planet—for ourselves, as much as for everything and everyone else.

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Bryan Walsh is a reporter at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.