If the expected life of a species is one million years, then that means there is one extinction for a given species every million years:

Eq. 1

Equation 1 is read as “one extinction per species per million years.” Note that equation 1 can also be rearranged to express a yearly probability of extinction:

Eq. 2

This is equivalent to saying that one species out of a million will go extinct every year.

To determine the expected current background rate of extinction, we multiply the extinction probability calculated above (either one will work, since mathematically they are the same) times the current number of species:

Eq. 3

If we also do this assuming a species life equal to 10 million years, we get:

Eq. 4

This gives us a high and low estimate for the expected current background rate of extinction – somewhere between 1 and 10 species each year. If you’re curious why we used 10 million species for the current number of species, don’t worry – we’ll cover that in class soon!