These mass extinctions come typically every 20 - 100 million years.

There are lots of different possible reasons why mass extinctions could occur: climatological; geological (plate tectonics could wipe out a bunch of habitats); or even astronomical (next slide).

It has always been the case that the number of species increases again after a mass extinction. One way to think about this is that after many species die out, there are many empty ecological niches into which new families can adapt and succeed.

http://www.carleton.ca/~tpatters/teaching/intro/extinction/extinction1.html

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