Importance Value Index
The Importance Value Index in Ecology is the quantitative measure of how dominant a species is in a given ecosystem. It combines multiple parameters to reflect a species' overall dominance, helping to describe the structure and composition of ecosystems.
Components
The IVI is calculated by summing three relative measures for each species in a given area:- Relative density – the number of individuals of a species divided by the total number of individuals of all species.
- Relative frequency – the frequency of a species relative to the sum of frequencies for all species.
- Relative dominance – typically based on basal area or canopy cover, representing the area occupied by a species compared to the total.
Each of these components is expressed as a percentage, so the IVI ranges from 0 to 300.
Applications
IVI is commonly used in vegetation analysis and forest ecology to:- Identify dominant and co-dominant species.
- Understand successional stages in ecological communities.
- Guide conservation efforts and habitat management.
Example
In a forest plot, three tree species are sampled. If Species A has high abundance, occurs frequently across plots, and occupies a large basal area, its IVI would be significantly higher than that of a rare, spatially restricted, or small-canopy species. Researchers often present IVI rankings to show the ecological dominance hierarchy within a study area.Limitations
Although useful, the IVI has some limitations:- It is scale-dependent and can vary with sampling methods and plot size.
- It gives equal weight to density, frequency, and dominance, which may not always reflect ecological significance.
- It may not be directly comparable across different ecosystem types.