Calculating Species Importance Values


1- Sum all point-to-plant distances taken for all species and compute the mean: d-bar = Σdi/Σni (d-bar is the mean point-to-plant distance, di is the point-to-plant distance for an individual plant, and Σni is the total number of individuals measured)

The mean area in which a single plant occurs is equal to the mean distance squared. This relation can be visualized as one individual in a square area in which the side of the square is equal to the mean point-to-plant distance A-bar = (d-bar)2 (A-bar is the mean area per plant)

2- Calculate Relative density (RD) for each species, which is RDi = ni/Σni (ni is the number of individuals of a given species counted and RDi is the relative density of the given species).

The absolute density (D) for a species is Di = (ni/Σni)(u/A-bar) (u represents the number of area units to be used in expressing density. When hectare is used as the standard unit, u equals 10000 m2. [note that Di is also equal to RDi*TD, where TD is total density for all species, TD = u/A-bar])

3- Frequency is the chance of finding a given species within a sample, and is given by fi = ji/k (fi is the frequency of a species, ji is the number of sampling points at which the species was counted, and k is the total number of points sampled.)

Calculate Relative frequency. (Rf) is the frequency of a given species (fi) as a proportion of the sum of the frequencies for all species (Σf) Rfi = fi/Σf

4- Coverage (C) is the proportion of the ground occupied by a vertical projection to the ground from the aerial parts of the plant, and is given by Ci = (ai)(Di)/ni (ai is the sum of the basal areas (computed from DBH) for a species, Di is the density of the species, and ni is the total number of individuals sampled for that species.)

Calculate Relative coverage for a species, which is RCi = Ci/ΣC (ΣC is the total coverage or basal area for all species.)

5- The sum of the above three relative measures for a species is an index called the Importance Value (IVi) IVi = RDi + Rfi + RCi The value of IV may range from 0 to 3.00 (or 300%). Dividing IV by 3 will result in a figure that ranges from 0 to 1.00 (or 100%). This value is referred to as the importance percentage. The importance value, or the importance percentage, gives an overall estimate of the influence of importance of a plant species in the community.

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