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The number of species present in a biological community can be measured
in a variety of ways. Each method requires different assumptions
to be made. An outline of each of the most common methods is presented
below.
Species richness
The simplest and oldest measurement is the total number of species
present in a natural community. Its main disadvantage is that it
assumes all species have been sampled, which as we know is almost
impossible without removing the entire community twig by twig. So
in many cases it becomes a relative
estimate.
Diversity indices
Species diversity is a dual concept that includes the
number of species in a community and the evenness in which the individuals
are divided among the species. These indices, based on statistical
sampling theory and information, investigate how communities are
structured. The most well-known and widespread index is the Shannon-Wiener
Index. This index is based on the probability of predicting the
presence of a species. Higher values suggest greater uncertainty
in being able to predict occurrence, therefore a greater number
of species is present (see Krebs 1989 for formulas and more indices).
Surrogates of diversity
With increasing pressure to identify and conserve areas
of biological diversity before they are lost, and decreasing resources
in which to carry out this task, much effort has gone into looking
for alternative ways of sampling diversity.
Recent research has focussed on surrogacy. Surrogancy is based
on the hypothesis that it may be possible to sample a variable that
directly correlates with the number of species present in that area,
instead of sampling the actual community of individuals. The main
types of surrogates being investigated are
- higher taxonomic orders (e.g. families)
- structural attributes of vegetation
- remotely sensed data.
The use of surrogate variables is controversial, and they will
never totally replace the need for fauna and flora surveys.
back to "types of statistical analysis"
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