Description of community

One way to characterize a community is simply to count or list the species that are present. This sounds a straightforward procedure that enables us to describe and compare communities by their species 'richness' (i.e. the number of species present). In practice, though, it is often surprisingly difficult, partly because communities can be recognized at a variety of levels - all equally legitimate composition species richness: the number of species present in a community


Community A

- /

Community B

V 1

1 1 1 1

0 400 800 1200 1600 Number of individuals in sample

0 400 800 1200 1600 Number of individuals in sample

Figure 16.3 The relationship between species richness and the number of individual organisms from two contrasting hypothetical communities. Community A has a total species richness considerably in excess of community B.

Knowing the numbers of individuals present in each species may not provide a full answer either. If the community is closely defined (e.g. the warbler community of a woodland), counts of the number of individuals in each species may suffice for many purposes. However, if we are interested in all the animals in the woodland, then their enormous disparity in size means that simple counts would be very misleading. There are also problems if we try to count plants (and other modular organisms). Do we count the number of shoots, leaves, stems, ramets or genets? One way round this problem is to describe the community in terms of the biomass per species per unit area.

The simplest measure of the Simpson's diversity character of a community that takes index into account both the abundance (or biomass) patterns and the species richness, is Simpson's diversity index. This is calculated by determining, for each species, the proportion of individuals or biomass that it contributes to the total in the sample, i.e. the proportion is P for the ith species:

of taxonomic problems, but also because only a subsample of the organisms in an area can usually be counted. The number of species recorded then depends on the number of samples that have been taken, or on the volume of the habitat that has been explored. The most common species are likely to be represented in the first few samples, and as more samples are taken, rarer species will be added to the list. At what point does one cease to take further samples? Ideally, the investigator should continue to sample until the number of species reaches a plateau (Figure 16.3). At the very least, the species richnesses of different communities should be compared on the basis of the same sample sizes (in terms of area of habitat explored, time devoted to sampling or, best of all, number of individuals or modules included in the samples). The analysis of species richness in contrasting situations figures prominently in Chapter 21.

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