Probably many of you have heard that 65 million years ago an asteroid struck the Earth and wiped out the dinosaurs and many other species, causing a mass extinction (50% of all species died out). However, this was by no means the only such catastrophic event in Earth's history - it was only the last one. There were about 20 such mass extinctions, the most famous ones, the "Big Five", were identified in 1982 by Raup and Sepkoski:

- 444 million years ago: probably caused by a cosmic gamma ray burst that happened too close to Earth and which generated a glacial age
- 360 million years ago: this was a prolonged "event" lasting for about 20 million years during which 70% of all species disappeared; it isn't clear what caused this "event", but some scientists believe an asteroid was involved
- 251 million years ago: the "Great Dying" - in less than a million year 95% of all species disappeared and the Earth was temporarily dominated by fungi; it isn't known what has caused it, many believe there probably has been a coincidence of many factors
- 200 million years ago: in less than 10 000 years 50% of all life (20% of marine life) including most large amphibians disappeared; this allowed dinosaurs to take over the Earth; cause - not known.
- 65 million years ago: 50% of all species have died including most dinosaurs, permitting mammals to take over the Earth; the duration of this extinction is not known, raising doubts about the cause of the extinction, but many scientists believe an asteroid is to blame.

The idea that the "sixth extinction" is happening right now on was pioneered by the famous biologist E.O. Wilson. In 1998 the American Museum of Natural History surveyed the biologists about the issue. The result was that "the majority (70%) polled think that during the next thirty years as many as one-fifth of all species alive today will become extinct, and one third think that as many as half of all species on the Earth will die out in that time." As Wilson notes, if true, this is by far the fastest ever mass extinction in Earth's history.

International Union for Conservation of Nature and Natural Resources (IUCN), an international organization dedicated to natural resource conservation, has estimated that one in four mammals, one in three amphibians, and one in eight birds are now threatened with extinction. The total number of endangered species is more than 15 500.

What does this mean?

IUCN has only studied a very small fraction of all the animals and plants on Earth. It is estimated that there are between 10 to 30 million species. Among these, only 1.9 million have been described and classified by biologists (i.e. around 10% of all species). Out of these 1.9 million, only 3% have been evaluated by the IUCN (i.e. around 0.3% of all species). Out of the evaluated species, more than 15 500 have been found to be threatened by extinction. The point is, there is no reason to consider these particular species in any way exceptional species - it's just that we happened to study them instead of others.

So, what does that mean for all life on Earth? If the 15 500 known endangered species represent 0.3 percent of all endangered species (both known and unknown), it means that in fact more than 5 million species are under threat. In other words, around 25% of all species may die out in the next few decades! This is indeed a mass extinction happening at an unprecedented rate.

The UN has also acknowledged this. The UN report released yesterday says: "In effect, we are currently responsible for the sixth major extinction event in the history of earth." According to their study, the global demand for biological resources now exceeds the planet's capacity to renew them by 20%.

"The direct causes of biodiversity loss - habitat change, over-exploitation, the introduction of invasive alien species, nutrient loading and climate change - show no sign of abating." The report concludes: "Biodiversity is in decline at all levels and geographical scales."

The reasons why biologists are concerned by this loss of biodiversity go beyond just conservationism. It's not only that life is valuable in itself so we should save it. To give you an example, if wild bees would disappear, around 30% of our crops would die out too because they are pollinated by bees. The point is the species are highly interconnected and there is no way one can predict what will happen if some species dies out. But if so many species die out, it's virtually certain that human life will be impacted in a very negative way - although one cannot predict from what exact direction disaster will strike.

Photo: 1870 - hunters on a top of a pile of bison skulls