## Margalefs Index

Margalef's index is a species diversity index developed by the Spanish ecologist Ramon Margalef Lopez during the 1950s. Diversity indices can be broadly divided into two types: those that assess species richness (how many types are there) and those that assess species evenness or dominance (how individual organisms are distributed among species). Margalef's diversity index is a species richness index. Many species richness measures suffer from the problem that they are strongly dependent on sampling effort. The greater the sampling effort, potentially the higher the index value. Thus comparing metrics from samples collected with differing levels of sampling effort can be difficult and possibly misleading.

Margalef's index was one of the first attempts to compensate for the effects of sample size by dividing the number of species in a sample by the natural log of the number of organisms collected.

The index is thus

where S is the number of species in a sample and N is the number of organisms in the sample. However, the use of

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Figure 1 ( Continued )

 Shannon • ! Î 1 1 1 . Number of individuals in subsample 101 102 103 104 Number of individuals in subsample Number of individuals in subsample Number of individuals in subsample Figure 1 Diversity indices calculated from random subsamples of increasing size drawn from a collection of 18 460 New Zealand stream invertebrates. the index rests on the assumption that there is a relationship between the number of species and the number of organisms in a sample such that S = constant x ln(A?). If this is not the case then the index will still be sensitive to the number of organisms collected, that is, it will increase as the number of organisms sampled increases. This is demonstrated in Figure 1, where increasing subsets of animals are randomly drawn from a collection of 18 460 New Zealand stream invertebrates and several standard diversity measures including Margalef's index are calculated. Margalef's index increases in much the same way as the number ofspecies increases. The Menhinick index is a similar index except that rather than dividing S by the natural log of the number of organisms you divide by the square root of the number of organisms. There are now several more efficient techniques for removing the effect of sampling effort from diversity estimates such as rarefaction. The approach involves randomly drawing a set number of individuals from the samples and calculating species richness, so that in effect samples have the same number of organisms and can then be compared directly. Although the index suffers from a sensitivity to sampling effort, it continues to be used as a measure ofspecies richness in ecological studies of freshwater, marine, and terrestrial systems as well as in other areas of science as diverse as genetics and sociology. It is an available option in several software packages that calculate diversity measures such as Primer. Certainly one of the key advantages of the index is that it is simple to use and easy to understand. If it is used with an awareness that it is sensitive to the number of organisms collected, then it will continue to be a useful tool in the ecologists' arsenal of diversity indices. See also: Average Taxonomic Diversity and Distinctness; Biodiversity.

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