Here's hoping the human species likes its own company, because at the rate Earth is going, we might be the only ones we've got left.
Nobody can say with certainty how many species there are on Earth, but the number runs well into the millions. Many of them, of course, are on the order of bacteria and spores. The other ones, the ones we can see and count and interact with—to say nothing of the ones we like—are far fewer. And, according to a new and alarming series of papers in Science, their numbers are falling fast, thanks mostly to us.
One of the first great rules of terrestrial biology is that no species is forever. The Earth has gone through five major extinction events before—from the Ordovician-Silurian, about 350 million years ago, to the Cretaceous-Paleogene, 65 million years back. The likely causes included volcanism, gamma ray bursts, and, in the case of the Cretaceous-Paleogene wipeout, an asteroid strike—the one that killed the dinosaurs. But the result of all of the extinctions was the same: death, a lot of it, for 70% to 90% of all species, depending on the event.
As increasingly accepted theories have argued—and as the Science papers show—we are now in the midst of the sixth great extinction, the unsettlingly-named Anthropocene, or the age of the humans.
The numbers are sobering: Over all, there has been a human-driven decline in the populations of all species by 25% over the past 500 years, but not all groups have suffered equally. Up to a third of all species of vertebrates are now considered threatened, as are 45% of most species of invertebrates. Among the vertebrates, amphibians are getting clobbered, with 41% of species in trouble, compared to just 17% of birds—at least so far. The various orders of insects suffer differently too: 35% of Lepidopteran species are in decline (goodbye butterflies), which sounds bad enough, but it's nothing compared to the similar struggles of nearly 100% of Orthoptera species (crickets, grasshoppers and katydids, look your last).
As the authors of all this loss, we are doing our nasty work in a lot of ways. Overexploitation—which is to say killing animals for food, clothing or the sheer perverse pleasure of it—plays a big role, especially among the so-called charismatic megafauna. So we get elephants slaughtered for their tusks, rhinos poached for their horns and tigers shot and skinned for their pelts, until oops—no more elephants, rhinos or tigers.
Habitat destruction is another big driver, particularly in rainforests, where 25,000 miles (75,000 km) of tree cover are lost annually—the equivalent of denuding one Panama per year, year after year. And you don't even have to chop or burn an ecosystem completely away to threaten its species; sometimes all it takes is cutting a few roads across it or building a few farms or homes in the wrong spots. Environmental fragmentation like this can be more than sufficient to cut species off from food or water, to say nothing of mates, and start them in a downward spiral that becomes irreversible.
Then too there is global warming, which makes once-hospitable habitats too hot or dry or stormy for species adapted to different conditions. Finally, as TIME's Bryan Walsh wrote in last week's cover story, there are invasive species—pests like the giant African snail, the lionfish or the emerald ash borer—which hitch a ride into a new ecosystem on ships or packing material, or are brought in as pets, and then reproduce wildly, crowding out native species.
The result of all this species loss—what the Science researchers dub defaunation—goes far beyond simply leaving us with a less rich, less diverse world. After all, the Earth bounced back from far worse extinctions and did just fine. But it bounced back a different way each time, and the most recent version, the one in which we emerged, is the one we like—and it's easy to destroy.
Loss of species, the authors point out, means loss of pollinators—which is a real problem since 75% of food crops rely on insects if they're going to thrive. Nutrient cycles—the decomposition of organic matter that feeds the soil—collapse if mobile species can't get from place to place and do their living and dying in a fairly even distribution. The same is true for water quality, which relies on all manner of animals to prevent lakes and rivers and streams from becoming too algae-dense or oxygen poor. Pest control suffers as well — when animals like bats are no longer around the eat the insect pests that attack crops, it's bad news for autumn harvests. North America alone is projected to suffer $22 billion in agricultural losses as desirable bat populations continue to decline.
It oughtn't take appealing to our self-interest to get us to quit making such a mess of what we're increasingly coming to learn is an exceedingly destructible world. But it's that very self-interest that led us to make that mess in the first place. We can either start to change our ways, or we can keep going the way we are—at least until the Anthropocene extinction claims one final species: our own.