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What is Biodiversity and how do we measure it?

Bar chart illustrating heights of different types of Silurian reef organisms.
Bar chart illustrating heights of different types of Silurian reef organisms.
   This method works great if you can count all the species in the community, but suppose you can only count some of the species in a community. Ecologists often have this problem because of difficulties in collecting all members of a community and paleontologist especially have problems because of the difficulties in collecting fossils and the possibility that not all organisms in a community were fossilized. An equation was developed to help compare diversity between different communities. This equation takes into account the size of the collection, the number of specimens, and the number of different species. Once all the numbers are plugged into the equation and the calculations are completed a single number is generated. This number is called the Shannon Index. The higher the Shannon Index the higher the diversity. 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.  However this is often difficult or impossible with paleocommunities. Some fossil locations have thousands of fossils while others may only contain a fewer specimens. This leads to the problem of how do you compare a fossil collection that has a 1000 specimens to one that has 100 specimens. Scientists have developed a method of comparing communites diversity that takes into account the number of species and the number of specimens. In this exercise, we will use species level data for discreet faunas from several locations in Wisconsin. We will also use the Shannon Index as a measure of the biodiversity of these faunas. The Shannon Index is one of the most common measurements used for the statistical study of biodiversity. The Shannon Index is based on the equitability of abundances in a fauna and requires quantitative data. Our exercise will test a portion of the Sepkoski Curve using more detailed data from Wisconsin. The Shannon Index is a single figure (i.e. univariate) that numerically measures diversity and which incorporate species richness and equitability (i.e. evenness). H’ = -? ( Pi * ln ( Pi ) H’= Shannon Index ? = the sum of all species P = abundance of each species ln = natural log