Origins of the Berger Parker Index for Measuring Dominance

The Berger-Parker index was originally presented by Wolfgang H. Berger and Frances L. Parker to study the diversity of planktonic foraminifera in deep-sea sediments. They developed the index to measure species dominance in an assemblage of species. In their equation, dominance was defined as

In the equation, dominance, D, is equivalent to pmax, the maximum proportion of any one species in a sample. Robert May expanded upon this index by writing it as d — Nmax/^T

In May's writing of the equation, species dominance, d, is given as the maximum number of individuals of any one species, Nmax, divided by the total number of individuals observed, NT. The lower the value of d, the more equitable or less dominant any one species is in the community and, thus, the more diverse the sample is thought to be. The reciprocal of the value can also be derived so that an increase in the value of the index represents an increase in diversity and a reduction in dominance. May suggested that the Berger-Parker index is one of the simplest yet best single measures for assessing diversity in a community. The measure of the Berger-Parker index is conceptually equivalent to the parameter ' k from the geometric series where k represents the proportion of niche space preempted by the dominant species in an assemblage. However, the geometric series is based on the assumption that all subsequent colonists occupy the same proportion, k, of the remaining niche space.

sites or at the same sites throughout time, it gives little insight into the species compositional differences between sites. Thus, it is possible that species abundances and, thus, the Berger-Parker index values may be similar across sites while species composition may be very different. Furthermore, the higher dominance found within species poor assemblages is not, in itself, evidence of an environmental impact, disturbance, or impairment. Unless there is a verifiable loss of richness, diversity measures such as the Berger-Parker index are probably of limited use in environmental impact assessments. For these reasons, it has been suggested that the Berger-Parker index is most useful for assessing differences in community structure across minimally complex environments where the composition of communities remains relatively constant but where abundance varies as a result of known environmental variables or disturbances of interest. Typically, this index has been most useful when used in combination with other indices and analytical techniques because of the limited information this index provides.

An analytical tool that may be a useful accompaniment to the Berger-Parker index is the k-dominance curve. This curve provides additional information on dominance because it graphically displays the distribution of individuals across species in a sample and, thus, reveals the nature of dominance rather than just a numerical measure of dominance. The k-dominance curve is a graph of the cumulative relative abundance, or cumulative percentage dominance, of species plotted against the log rank of species in the sample (from least to most common). For this type of plot, the steepness of the curve indicates the degree of dominance in a system: a very steep curve would indicate a less even assemblage and a flatter curve would represent a more even distribution.

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