Ziptrek Ecotours

Custodians of a fragile planet

July 2009

More than ninety-nine percent of all species that have lived on earth have disappeared. Over the past half-billion years, there have been at least twenty mass extinctions, when the diversity of life on earth has suddenly and dramatically contracted. Five of these were so devastating that they are usually put in their own category.

The first took place nearly four hundred and fifty million years ago, when life was still confined mainly to water. Geological records indicate that more than eighty percent of marine species died out. The fifth occurred sixty-five million years ago. It brought to an end seventy-five per cent of all species on earth, including the dinosaurs.

This is not the slow extinction brought about by Darwinian “unfitness”. In fact Darwin did not recognize this phenomenon. The theory that the Cretaceous (dinosaur) era came to an end with a catastrophic meteor hit was posited in 1980, and only proven in 1991. Though missing from the original theory of evolution, mass extinctions have played a determining role in evolution’s course.

It is now generally agreed among biologists that another mass extinction is under way. Though it’s difficult to put a precise figure on the losses, it is estimated that if current trends continue, by the end of this century as many as half the earth’s species will be gone.

Fifty thousand years ago, when humans reached Australia, all the continents largest animals disappeared. For example, all species of marsupials weighing more than two hundred pounds – there were nineteen of them – vanished. Coincidence is discounted when the extinction of North America’s largest animals occurred when humans arrived eleven thousand years ago. The same patterns emerge in South America, Madagascar, New Zealand and Hawaii.

Why is contact with humans so catastrophic? Hunting is an insufficient explanation. Our impact can be more subtle and comprehensive. But there were maybe half a million people on earth fifty thousand years ago, and we expect to hit seven billion within the next three years. We have transformed between a third and a half of the world’s land surface, and these figures probably understate the effect, since land not being actively exploited may still be fragmented.

The most deadly aspect of human activity may be the pace of it. We can move organisms (intentionally or not) around the world in hours, where it would have previously taken millennia (if at all). Just in the last century, CO2 levels have changed by as much as they normally do in a hundred-thousand year glacial cycle. Meanwhile, the drop in ocean pH levels that has occurred over the past fifty years may well exceed anything that has happened in the seas during the previous fifty million.

Unlike the end-Cretaceous period, where catastrophe struck in an instant and the planet’s recovery began soon after, we are continuing to affect the planet. While a mass extinction is not imminent, the stress is ongoing, because it is us. Sustainable living requires an awareness of our role as caretakers and custodians of all life on earth.