Most of us don’t think about extinctions when we think of Hawaii. But for conservation biologist Stuart Pimm that’s the first image that enters his mind. Since 1978 he’s been involved with studies of the last of these islands’ native birds—only 30 species. Scientists believe 135 bird species inhabited the isles before the first human settlers arrived by canoe 1,500 years ago. And of these 30, says Pimm, only ten have any chance of long-term survival. “You can find ten of them with a little searching,” he says. “Ten are really rare, and another ten are so rare that there are no hopes of our being able to save them. It’s like going to a restaurant to meet your friends, and suddenly finding that two-thirds of them are dead.”

And it’s not just native birds that are on the verge of becoming extinct; much of Hawaii’s plant and insect life is similarly threatened. Nor are such extinctions limited to these islands. As Pimm and other conservation biologists have discovered over the past two decades, species have either become extinct or are in the process of disappearing almost everywhere. Not since a meteorite slammed into Earth some 65 million years ago, eliminating much of life—including the dinosaurs—has our planet experienced such a high rate of extinctions among complex organisms.

Today’s mass extinction event, however, is different from those of the past. Earth’s plants and animals are not vanishing because of some external factor, such as a meteorite or climate change. Instead, the cause is us—humans. This is most easily documented, notes Pimm, in the Pacific Islands, since people arrived there relatively recently. But whatever lands humans have migrated to have experienced similar mass extinctions. He and others point to the loss of Australia’s megafauna—giant kangaroos and marsupial lions—and to the similar extinction of North America’s mammoths. “We are in the midst of a human-driven extinction,” says Pimm, “one that seems to have started as soon as modern humans moved out of Africa some 200,000 years ago and began entering new lands.”

Now with people spread across the globe—and our population rising—species are disappearing at an astonishing rate. “I think we are likely to lose 25 to 50 percent of God’s creation over the next century,” says Pimm. “And that raises many ethical questions. Don’t we have a moral responsibility to the other creatures around us?”

For Pimm that answer is yes. “I think we have to stop and think about the decisions we make in our individual lives and in our societies. We tend to be very shortsighted, driven by short-term gains. And somehow we’ve got to recognize that the environment is the center of our lives; it is not a marginal issue. We have to make that shift—or the environment will make it for us.” In driving Earth’s creatures to extinction, humans are diminishing their own world, and in an irreversible manner. Extinction, as Pimm says, is forever.