Rank abundance curve

A rank abundance curve is a chart used by ecologists to display indicators of biodiversity -specifically species richness and species evenness. This is achieved using the relative abundances of different species in a sample. Rank abundance curves are useful as they provide a simple way to represent both species richness and evenness, which are important measures of biodiversity. It overcomes the shortcomings of the biodiversity index, that cannot display the relative role different variables played in its calculation.



The curve is a 2D chart with relative abundance on the Y-axis and the abundance rank on the X-axis.
 * X-axis: The abundance rank. The most abundant species is given rank 1, the second most abundant is 2 and so on.
 * Y-axis: The relative abundance. Usually measured on a log scale, this is a measure of a species abundance (e.g., the number of individuals) relative to the abundance of other species.

Interpreting a rank abundance curve
As mentioned, the rank abundance curve provide a means for visually representing species richness and species evenness. Species richness can be viewed as the number of different species on the chart i.e., how many species were ranked. Species evenness is derived from the slope of the line that fits the graph. A steep gradient indicates low evenness as the high ranking species have much higher abundances than the low ranking species. A shallow gradient indicates high evenness as the abundances of different species are similar.