number of species found in a sample. If the numbers of individuals are equally spread amongst the species then the community is said to be even. The closer Pielou’s eveness is to 1, the more even the distribution of abundance is amongst the species. The nearer the value is to 0, the less even the community is with some species having much higher abundances than others. Simpson’s dominance index is a measure of the probability that two individuals randomly selected from a sample will belong to the same species. Simpson's dominance index ranges from 0 (all taxa are equally present) to 1.0 (one taxon dominates the community completely).
78.
Faunal data for multivariate analysis were imported into PRIMER and initially subjected to either a square root (grab samples) or fourth root (trawl samples) transformation to reduce the influence of any highly abundant taxa allowing less abundant species a greater role in driving the emergent multivariate patterns. The transformed data were then subjected to hierarchical clustering to identify sample groupings based on the Bray Curtis index of similarity. This process combines samples into groups starting with the highest mutual similarities and then gradually lowers the similarity level at which groups are formed. The process ends with a single cluster containing all stations and is best expressed as a dendrogram showing the sequential clustering of stations against relative similarity. To best describe the ecological differences between sites, the groups were identified on the basis of a slice at 21% similarity.
79.
The MDS (Multi‐dimensional Scaling) procedure uses the same similarity matrix as that used by the cluster analysis to produce an ordination of stations which is multi‐ dimensional. This is carried out to satisfy all of the between samples relationships indicated by the similarity matrix. This multi‐dimensional ordination is then reduced to a 2 or 3 dimensional representation that is a more accessible and useable representation. The representativeness of these low dimensional versions, in comparison to the multi‐dimensional array, is indicated by a stress level. The closer this stress level is to zero, the better the representation.
80.
SIMPER analysis was then applied to the data to rank species in terms of their contribution to both the internal group similarity and “between” group dissimilarity and thereby assist the assessment of the distinctiveness of each community identified and the identification of the characterising taxa.
Benthic Characterisation Report November 2013
East Anglia THREE and FOUR Offshore Cable Corridor
