All species eventually become extinct, but drastic changes in environmental conditions can eliminate large groups of species. As local environmental conditions change, species disappear at a low rate, called background extinction. Based on the fossil record and analysis of ice cores, biologists estimate that the average annual background extinction rate is one to five species for each million species on the earth.
In contrast, mass extinction is a significant rise in extinction rates above the background level. In such a catastrophic, widespread (often global) event, large groups of existing species (perhaps 25-70%) are wiped out in a geological period lasting up to 5 million years. Fossil and geological evidence indicate that the earth’s species have experienced five mass extinctions (20-60 million year apart) during the past 500 million years.




所有種類最終變得絕種，但是在環境狀況上的猛烈變化可能消滅大小組種類。 當地方環境狀況改變，種類消失在低率，叫背景絕種。 基於對冰核心的化石紀錄和分析，生物學家估計平均每年背景絕種率是每百萬個種類的一個到五個種類在地球上。 相反，許多絕種是在絕種率的重大上升在背景強度之上。 在這樣一個災難，普遍(經常全球性)事件，大小組現有的種類(或許25-70%)在持續5百萬年的一個地質時期被消除。 化石和地質學的證據表明地球的種類體驗了五許多絕種(20-60百萬年單獨)在過去500百萬年期間。