Humanity Leaves Its Mark

If these predictions hold true, the next few centuries will see another bout of mass extinctions, with well over half of all species disappearing. Given that we may have inherited a planet at the height of its diversity, it could be the biggest of all in terms of the total number lost.

In a few important ways, this extinction pulse will be different from past ones. An asteroid cannot change its own course, but humans can. How big the extinctions become depends on what humans do in the next hundred years. With habitats disappearing and fragmenting so quickly, conservation biologists have focused their attention on ways to save the most diversity with the least amount of effort. Diversity is not smoothly distributed over the globe, or even within the tropics. A few places, such as Madagascar, the Philippines, and Brazil’s Atlantic forests, represent “biodiversity hot spots.” The top 25 hot spots contain 44 percent of the diversity of plants and 35 percent of vertebrates. They also comprise only 1.4 percent of Earth’s land surface. They are going to disappear quickly if they aren’t conserved. On average, 88 percent of the original area of the hot spots has already been destroyed, and their human population is growing fast. These cradles of diversity demand our immediate attention and care.

If extinctions continue to accelerate, the world will become, in a matter of centuries, a homogenized place. While the majority of species with limited ranges continue to go extinct, a few rugged species will thrive. Over 90 percent of the world’s agriculture is based on only 20 species of plants and 6 species of animals. As the human population continues to rise, the fortune of these species will go on rising with it. Invaders will continue to spread–zebra mussels, for example, are expected to colonize much of the United States in years to come as they move from one waterway to another. The destruction of forests and other habitats will harm most native species, but a few will prosper. In South American forests, hylid frogs can lay eggs in potholes and other temporary ponds, while wolf spiders can spin their webs on weeds. Today quillworts and other lycopsids are growing where tropical rain forests once stood, just as they did 250 million years ago.

“As long as humans are here and don’t go extinct,” says Ward, “that evolutionary faucet that you turn on after a mass extinction that creates new species–that’ll never be turned on. As humans exist far into the future, I foresee a world in which biodiversity stays very low. And that to me is the tragedy.”

It is possible that we ourselves will not escape these mass extinctions. We depend on wetlands to filter our water, on bees to pollinate our crops, on plants to build soils. And these plants and animals depend, in turn, on healthy ecosystems to survive. Biologists have run experiments in which they’ve altered the diversity in simple ecosystems such as grassland plots. With few species, ecosystems become more susceptible to droughts and other catastrophes. Humans may not be able to survive if the impoverished ecosystems we depend on collapse. Of course, humans are the most resourceful species on the planet, so we may find ways to survive even beyond such a disaster.

After past mass extinctions, life has recuperated and even rebounded. How it recovers from the current one depends partly on the destiny of the human race. Man‑made global warming may end up being one of the most profound causes of extinction, but it cannot last forever. There is only so much oil and coal left–about 11 trillion tons. James Kasting, a climatologist at Penn State University, estimates that burning this much fossil fuel would increase atmospheric levels of carbon dioxide to about three times today’s levels, raising temperatures between 3 and 10 degrees Celsius. It would take only a few centuries to use up these reserves. It will take hundreds of thousands of years for Earth to draw down the carbon dioxide to the levels it was at before the Industrial Revolution.

But long after the atmosphere recovers from its binge of carbon dioxide, and even after Homo sapiens is gone, the biological invaders we have sown around the world will keep regenerating, keep controlling the ecosystems that surround them. They will continue to frustrate the evolution of other plants and animals.

“Evolution has now entered a new mode,” says Burney. “Something altogether new is happening, and it has to do with what humans do to the evolutionary process. And it’s a very scary thing, because it’s like we are taking evolution around a blind corner, something that nature hasn’t dealt with before: species that can just hop a plane and wind up on the other side of the world; combinations of species that have never been combined before. It’s a whole new ball game, and we don’t know, really, where it will end.”

PART THREE

Evolution’s Dance

Eight

Coevolution