Monday, April 30, 2007
From what we understand so far, five great extinction events have reshaped earth in cataclysmic ways in the past 439 million years, each one wiping out between 50 and 95 per cent of the life of the day, including the dominant life forms; the most recent event killing off the non-avian dinosaurs. Speciations followed, but an analysis published in Nature showed that it takes 10 million years before biological diversity even begins to approach what existed before a die-off.
Today we're living through the sixth great extinction, sometimes known as the Holocene extinction event. We carried its seeds with us 50,000 years ago as we migrated beyond Africa with Stone Age blades, darts, and harpoons, entering pristine Ice Age ecosystems and changing them forever by wiping out at least some of the unique megafauna of the times, including, perhaps, the sabre-toothed cats and woolly mammoths. When the ice retreated, we terminated the long and biologically rich epoch sometimes called the Edenic period with assaults from our newest weapons: hoes, scythes, cattle, goats, and pigs.
But, as harmful as our forebears may have been, nothing compares to what's under way today. Throughout the 20th century the causes of extinction - habitat degradation, overexploitation, agricultural monocultures, human-borne invasive species, human-induced climate-change - increased exponentially, until now in the 21st century the rate is nothing short of explosive. The World Conservation Union's Red List - a database measuring the global status of Earth's 1.5 million scientifically named species - tells a haunting tale of unchecked, unaddressed, and accelerating biocide.
When we hear of extinction, most of us think of the plight of the rhino, tiger, panda or blue whale. But these sad sagas are only small pieces of the extinction puzzle. The overall numbers are terrifying. Of the 40,168 species that the 10,000 scientists in the World Conservation Union have assessed, one in four mammals, one in eight birds, one in three amphibians, one in three conifers and other gymnosperms are at risk of extinction. The peril faced by other classes of organisms is less thoroughly analysed, but fully 40 per cent of the examined species of planet earth are in danger, including perhaps 51 per cent of reptiles, 52 per cent of insects, and 73 per cent of flowering plants.
By the most conservative measure - based on the last century's recorded extinctions - the current rate of extinction is 100 times the background rate. But the eminent Harvard biologist Edward O Wilson, and other scientists, estimate that the true rate is more like 1,000 to 10,000 times the background rate. The actual annual sum is only an educated guess, because no scientist believes that the tally of life ends at the 1.5 million species already discovered; estimates range as high as 100 million species on earth, with 10 million as the median guess. Bracketed between best- and worst-case scenarios, then, somewhere between 2.7 and 270 species are erased from existence every day. Including today.
We now understand that the majority of life on Earth has never been - and will never be - known to us. In a staggering forecast, Wilson predicts that our present course will lead to the extinction of half of all plant and animal species by 2100.