Almost all past extinctions have occurred by natural processes, but today human activities are overwhelmingly the main cause of extinction.
Trends documented by the the World Bank and Worldwatch Institute, and reported to the Rio+5 conference in 1997, estimated 150 to 200 species of life become extinct every 24 hours.
Key areas of concern are: wild relatives of domesticated species which are essential to maintain the genetic variety of the domesticated breeds; harvested species and the threat to them through unsustainable rates of harvesting, leading to their biological extinction; totemic species, namely those held to be of special socio-cultural value to particular peoples; species of special importance to science; animal species that are sufficiently important and endangered to warrant captive breeding (especially in the light of the limited facilities for such breeding); wide ranging species that move between noncontiguous ecosystems, because of their ecological value and because an exclusively ecosystem-oriented approach will not conserve them; and indicator species because of their role in indicating the effectiveness of ecosystem maintenance.
Botanists at an International meeting in St Louis, USA in 1998 warned that two thirds of all species of mammals, birds, butterflies and plants will disappear by the end of the next century due to the destructive habits of man. They claim that the current rate is similar to the mass extinctions that wiped out the dinosaurs 65 million years ago. The destruction is piecemeal as humans encroach on the natural environment and fragment it, leaving little room for other species to survive.
Habitats are being destroyed and species presumed lost faster than at any other period of human existence and probably in the life of the planet. There is not enough time or resources to undertake any species-by-species approach to maintain species diversity.
The history of life on earth has been marked by extinction events on an unimaginable scale, where not just whole species have been wiped out but whole genera, families, orders, classes and phyla. There have been perhaps 5 to 50 billion species in 3.5 billion years of life's record on earth. At the very most, only 5-50 million species are living now. That means that life has had a 99.9% failure rate. As well as single, random causes events of species extinction, there have been major intervals of mass extinction brought about in a number of cases by asteroid or comet crashes on the planet. For example, the fossil record shows that 95% of all species perished in some unknown disaster at the end of the Permian era. Evidence for another disaster is a layer of iridium, associated with meteors, occurs everywhere in the world at the boundary between the Cretaceous and the Tertiary rocks. The scenario is of a 10 kilometre-wide object from outer space crashing into the earth 65 million years ago, causing shock, acid rain, forest fires, poison gas and water, and something like a nuclear winter or greenhouse effect which wiped out, once again a large proportion of life. Statistically, such events may happen every 26 million years or so.