Assessment of ecological conditions and biodiversity

Biodiversity assessment

Biodiversity assessments can vary from a qualitative, more subjective survey to a quantitative survey, depending on the purposes, time constraints and budget of the assignment. Biodiversity is a very broad term which also includes individual variations within species and subspecies, i.e. genetic variation in a very broad sense. In general for the purposes of impact assessments and land use management species and subspecies are the units used to assess biodiversity.

Ecological diversity comprises the number of species, the proportional abundances of each species to the total abundance as well as the species composition. For a qualitative survey the word diversity often refer to the general biodiversity observed, of indigenous species in particular, in one habitat type or different habitats combined at a site.

In a quantitative ecological survey there is very specific meaning to the word diversity. Diversity in a quantitative study of species assemblages is the structural component of the ecological community or assemblage. This diversity consists of two main components: i) the number of species ii) the evenness, calculated by taken into account the proportional contribution of each species to the total abundance. Specific diversity indices such as the Shannon-Wiener index, Simpson index are used to estimate diversity in terms of species richness and species abundances. Species richness can be described in terms of number of species or as a species richness index such as Margalef’s species richness.

For quantitative assessments in particular at any particular site or demarcated area:

1) A higher diversity is the result when a high number of species with fairly even proportional abundances are present, whilst
2) Lowest diversity is a result from a low species richness combined with a high dominance, i.e. high proportional abundance, of one or few species.

The other component of an assemblage or ecological community is the species composition. Here, with the species composition, the kinds of species with their relative abundances are taken into account and compared between those of other sample plots/areas where assemblages are observed i.e. what species occur is important, compared to the assemblage structure where the numbers and abundances of species are important, no matter what species are present. Multivariate data analysis techniques such as DCA, PCA ordinations are used to analyse species composition. Environmental variables can then be superimposed on the species composition in canonical correspondence analyses. Obviously the methods used to obtain the assemblage data should allow for comparison.

From the assessment of the state of the biodiversity at the site various applications are possible, normally with the purpose of the assessment in mind.