Importance Value Index

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 (i.e., the proportion of plots in which it occurs) relative to the sum of frequencies for all species.
• Relative dominance – typically based on basal area (for trees) or canopy cover, representing the area occupied by a species compared to the total.

IVI = Relative Density + Relative Frequency + Relative Dominance

Each of these components is expressed as a percentage, so the IVI ranges from 0 to 300.^[2]

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.

It offers insight into species' ecological roles beyond simple abundance by incorporating spatial and distributional data.^[3]

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.^[4]

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.^[3]

• Ecological dominance
• Biodiversity