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A second example of deriving sets of coherent species curves, this time temporal rather than spatial, is for the benthic macrofauna sampled at one site in the Bay of Morlaix, on 21 occasions over 5 years, spanning the period of the Amoco-Cadiz oil tanker spill...

An alternative to line plots, and a technique that can often be even more useful, in terms of the range and quality of information it can present, is that of shade plots. These are visual displays in the form of the data matrix itself, with rows being species ...

This example, last seen in Chapter 3, consists of 24 (seasonally-averaged) zooplankton net samples at 57 sites in the Bristol Channel, UK. Fig. 7.8 shows the shade plot for fourth-root transformed abundances. All 24 species are used and this is again an exam...

An example where the biotic sample axis could have sensibly been ordered according to an a priori spatial layout, or in terms of environmental conditions (e.g. the first principal component of a suite of organics and heavy metal levels in sediments, PC1), is ...

The 39 sites sampled for benthic infauna at different distances from an oil-field in the N Sea were shown in the last chapter to demonstrate a clear gradient of community change with distance (nMDS, Fig. 16.3). The shade plot of Fig. 7.10a however, which order...

Dissimilarity breakdown between groups The fundamental information on the multivariate structure of an abundance matrix is summarised in the Bray-Curtis similarities between samples, and it is by disaggregating these that one most precisely identifies the spec...

Another clear generalisation is to a 2-way rather than 1-way layout, illustrated by the 16 meiofaunal cores from Eaglehawk Neck, Tasmania, Fig. 6.7. The MDS for the 59 nematode and copepod species from two crossed factors, treatments (disturbed or undisturbed ...

Bubble plots Abundance (or density, biomass, area cover etc) for a particular species can be shown on the corresponding ordination point by a circle (‘bubble’) of size proportional to that abundance, based either on its original scaling (e.g. counts), or on th...

A variety of different statistics (single numbers) can be used as measures of some attribute of community structure in a sample. These include the total number of individuals (N), total number of species (S), the total biomass (B), and also ratios such as B/N...

The purpose of graphical/distributional representations is to extract information on patterns of relative species abundances without reducing that information to a single summary statistic, such as a diversity index. This class of techniques can be thought of...

Plots of geometric abundance classes along a transect across the Garroch Head {G} sewage-sludge dump site (Fig. 8.3) are given in Fig. 8.4. Note that the curves are very steep at both ends of the transect (the relatively unpolluted stations) with many species...

ABC curves for the macrobenthos at site 34 in Loch Linnhe, Scotland {L} between 1963 and 1973 are given in Fig. 8.7. The time course of organic pollution from a pulp-mill, and changes in species diversity ($H ^ \prime$), are shown top left. Moderate pollutio...

Ekofisk macrofauna: testing dominance curves Fig. 8.5b compares the averaged community samples for the closest distances to the oil platform (< 250m) with the second closest group (250m – 1km), in terms of their k-dominance curves, and the closest samples appe...

Fig. 8.15 is from Coulter Counter data of particle-size distributions for estuarine water samples from 5 sites, over 92 logarithmically increasing size-classes, based on 4 replicate samples per site (A. Bale, pers. comm.), {P}. For clarity, the line plot¶ of F...

A large number of different diversity measures can be computed from a single data set and it is relevant to ask if anything is achieved by doing so. The classic ‘spot’ (alpha) diversity indices, many of which were listed earlier in equations (8.1) to (8.7), ar...

There are two distinct roles for transformations in community analyses: a) to validate statistical assumptions for parametric techniques – in the approach of this manual such methods are restricted to univariate tests; b) to weight the contributions of common ...

For purely illustrative purposes, Table 9.1 extracts the counts of a single Thyasira species from the Frierfjord macrofauna data {F}, consisting of four replicates at each of six sites.   Table 9.1. Frierfjord macrofauna {F}. Abundance of a single species (T...

There being no necessity to transform to attain distributional properties, transformations play an entirely separate (but equally important) role in the clustering and ordination methods of the previous chapters, that of defining the balance between contributi...

The transformation sequence in a multivariate analysis, corresponding to a progressive downweighting of the common species, is effectively: The choice of transformation from this sequence can affect the conclusions of an analysis, and in many respects it is m...

There is a clear dichotomy, in defining sample similarities, between methods which give each variable (species) equal weight, such as normalisation or species standardisation, and those which treat counts (of whatever species) as comparable and therefore give ...