Recall the three domains representation of
life on Earth.
Very little is known about the distribution (or even
total number) of species on Earth. There are probably
between 5 and 100 million different species on Earth,
of which around 1.7 million have been identified.
It is likely -- though of course no one knows -- that
the vast majority of species are bacteria and archaea.
For example, the entire ocean may contain
2 million different bacterial species while
the average backyard might contain double
that amount in one ton of material
(http://news.bbc.co.uk/2/hi/science/nature/2079390.stm).