The number of organisms on Earth is being reduced at a rate 1,000-10,000 times higher than the time before humans evolved. Present estimates say we are losing 27,000 species each year. Note that these are all species easily observed and recorded. Other losses are unknown. These losses are numerically comparable to those in the 5 mass extinctions that occurred on earth during the last 500 million years.
Losses of biodiversity are irreversible.
For more discussion on rates of extinctions, especially in the tropics, check out the Rainforest Action Network . Here there's lots of links to rates of rainforest destruction . More than 20,000 species have been used for medicinal purposes. These are the most effective drugs for the treatment of childhood leukemia and Hodgkin's disease. From Madagascar, this is the source of two of the most effective anticancer agents ever discovered.