What is Biodiversity and how do we measure it? Biodiversity refers to the variety of organisms on earth and is commonly expressed as the number of species in a given area or environment. The number of species that a biologist or paleontologist finds is related to the number of specimens they examine. A large sample of specimens will yield more species than a smaller sample of specimens. This graph shows the number of species recorded from Silurian reef collections in the Milwaukee Public Museum. Examination of 3000 specimens produced a record of 140 species. At 6000 specimens, 165 species were recorded. A total of 191 reef species were recorded at the final sample size of 10,201 specimens. Note that the curve does not reach a plateau. Examination of additional specimens would add to the number of recorded species.