Measuring Diversity

How many organisms live in the ecosystem? How many species are present? Are there equal number of each species present? Why are some species common and others rare? Such questions are routinely asked by ecologists who seek to understand the structure and functioning of ecosystems.

While the sampling of plant populations is relatively simple (plants, after all, do not run away or hide), it is virtually impossible to count all plants in a given area (unless plot size is relatively small). Various statistical sampling methods have been developed to determine the distribution and abundance of plant life.

Similarly, animal counting and identification poses unique problems. How can you possibly count all rabbits, sparrows, elephants, ants, etc. in a field? Animals move, hide, are born, die, etc. making it very difficult to determine which you have counted (or not) within a reasonable amount of time. Again, various statistical sampling techniques have been developed to determine animal population size and distribution.

A relatively straightforward concept is that of diversity: how many different species are present in a particular place? But, is this really enough? (If you re-read the preceding paragraphs you'll identify some potential pitfalls...). But, is just knowing the species richness (i.e. the total number of species present in the community) enough? Imagine two communities: A contains 10 species, each with 50 individuals; B contains 10 species, but one of these species makes up 99% of the total individuals. Is it enough then to say that both these communities have the same species richness? Ecologists use the Simpson Index to obtain a measure of diversity which combines both the idea of species richness and of evenness:

Simpson's Index: Diversity = SUM pi2, where pi is the proportion of species i in the community

Many other ways of calculating diversity exist, depending on what, where, when is being measured.

We will use a very simplified measure of diversity to compare various "communities" of candy organisms. Simply put, we will:
  1. Count how many organisms are present in our community
  2. Identify how many species are present in the community
  3. Calculate diversity by dividing the number of species by the number of organisms
  4. Predict which "real" biomes these might represent...
What assumptions have we made?