Biodiversity depends upon ecosystem diversity.
The study of diversity involves questions of what species are present, what species are rare, and why species are present, rare, or abundant at specific sites. Of the up to 100 million species estimated to exist, only 1.4 millions species have been described. Over 4 billion species of plants and animals are estimated to have been present in the past and which are now extinct. Major extinction events have occurred regularly throughout the geological past. For the past 600 million yr biodiversity has shown a general increase.