An extinction event (also known as: mass extinction; extinction-level event, ELE) is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic groups present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They may be caused by one or both of.
Over 99% of species that ever lived are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because they are more plentiful and cover a longer time span than fossils of land organisms.
Since life began on earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, the Cretaceous--Tertiary extinction event, occurred 65 million years ago, and has attracted more attention than all others because it killed the dinosaurs. In the past 540 million years there have been five major events when over 50% of animal species died. There probably were mass extinctions in the Archean and Proterozoic Eons, but before the Phanerozoic there were no animals with hard body parts to leave a significant fossil record.
Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity. 6th Extinction
In the last 500 million years, the earth has experienced five mass extinctions.
Is humankind creating a sixth extinction?
In 1900, the earth held 1.6 billion people. Today, we have increased almost five-fold to 7 billion, and are projected to reach 15 billion by the end of this century.
Our rapid growth has led to habitat destruction, pollution and climate change. Through interlocking stories, this documentary will probe the extinction crisis and examine solutions to protect the earths gift of biodiversity.
樓主: 趙雲[子龍]
一定要戒 
人若無求品自高
在線雷達
發表於 2013-1-23 13:26:44
發表於 2013-1-23 14:12:00
