Importance Value Index (IVI) - Synthetic Characters
In any study of community, the quantitative value of each of the frequency density, and cover has its own importance. But the total picture of ecological importance cannot be obtained by any one of these alone. For instance, frequency gives an idea as to how a species is dispersed in an area but we do not get any idea about its number or the area covered. Density gives an idea about the numerical strength and 40 on. To have an overall picture of ecological importance of a species with respect to the community structure, the percentage values of the relative frequency, relative density and dominance are added together and this value out of 300 is called the Importance Value Index or IVI of the species. IVI can also be ascertained by using diagrammatic representation in the following manner. Draw a circle and divide it into four equal segments by drawing two lines at right angles to each other passing through the centre.
Figure: Polygraphic method of showing sociological characters of individual species