Diversity indices

diversity incorporates richness, commonness and rarity

An important aspect of community structure is completely ignored, though, when the composition of the community is described simply in terms of the number of species present. It misses the information that some species are rare and others common. Consider a community of 10 species with equal numbers in each, and a second community, again consisting of 10 species, but with more than 50% of the individuals belonging to the most common species and less than 5% in each of the other nine. Each community has the same species richness, but the first, with a more 'equitable' distribution of abundances, is clearly more diverse than the second. Richness and equitablity combine to determine community diversity.

where S is the total number of species in the community (i.e. the richness). As required, for a given richness, D increases with equitability, and for a given equitability, D increases with richness.

Equitability can itself be quantified (between 0 and 1) by expressing 'equitability' or Simpson's index, D, as a proportion of 'evenness' the maximum possible value D would assume if individuals were completely evenly distributed amongst the species. In fact, Dmax = S. Thus:

equitability, E =

Another index that is frequently used and has essentially similar properties is the Shannon diversity index, H. This again depends on an array of values. Thus:

Shannon's diversity index diversity, H = -1 Pt ln Pt

and:

Hmax ln S

Control H

Control H

1900 Year

1950

Figure 16.4 Species diversity (H) and equitability (J) of a control plot and a fertilized plot in the Rothamsteard 'Parkgrass' experiment. (After Tilman, 1982.)

1850

1900 Year

1950

Figure 16.4 Species diversity (H) and equitability (J) of a control plot and a fertilized plot in the Rothamsteard 'Parkgrass' experiment. (After Tilman, 1982.)

An example of an analysis of diversity is provided by a uniquely long-term study that has been running since 1856 in an area of grassland at Rothamsted in England. Experimental plots have received a fertilizer treatment once every year, whilst control plots have not. Figure 16.4 shows how species diversity (H ) and equitability (J ) of the grass species changed between 1856 and 1949. Whilst the unfertilized area has remained essentially unchanged, the fertilized area has shown a progressive decline in diversity and equitability. One possible explanation may be that high nutrient availability leads to high rates of population growth and a greater chance of the most productive species coming to dominate and, perhaps, competitively exclude others.

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Responses

  • Ermanno
    What is the maximum possible value of Species Richness?
    1 year ago

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