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In Chapter 7 the micro-scale spatial distribution of P. elegans within small-scale high
density patches was investigated. Since the extent in this survey was relatively small,
a contiguous sampling design was used. This study indicated that P. elegans was non-
randomly distributed at this scale, forming patches mainly of less than 3cm 2, although
patches of 3-12cm 2 were detected. The detection of micro-scale clumping of P.
elegans within small-scale patches supported the proposition that most, if not all,
marine invertebrate species are aggregated at many spatial scales (Andrew and
Mapstone, 1987; Morrisey et al., 1992; Hewitt et al., 1996).
The multi-scale patchiness of marine benthic invertebrates, as has been shown to
occur for P. elegans in this study, has both practical and statistical implications for
most studies carried out within the soft-bottom environment. Most researchers have
previously ignored the presence of spatial patterns in marine benthic invertebrates
when carrying out surveys or experiments. Legendre and Trousellier (1988) and
Legendre (1993) pointed out that where a species has a distribution which is spatially
autocorrelated, the abundance of that species at any one location can be at least partly
predicted by the abundances at neighbouring points. Therefore, these abundance
values, or samples, are not statistically independent from one another. This affects the
subsequent power of statistical comparisons since each replicate does not bring one
whole degree of freedom (Legendre, 1993). Positive autocorrelation induces the same
bias with all standard statistical tests: computed tests are too often declared significant
under the null hypothesis. This should affect sampling design and numerical analyses
used in monitoring studies. While it is formally possible to estimate the statistical
bias of autocorrelation (Cliff and Ord, 1973; Legendre, 1993), a more sensible
approach would be to have an idea of the scale of patchiness of the most abundant
species or the species of particular interest before carrying out an experiment or
survey. Inter-replicate distances can then be selected accordingly. For P. elegans on
Drum Sands for example, inter-replicate distances should ideally be at least 1-1.5m
apart for each replicate to be totally independent from each other. For subsurface
species, this information can only be gained from spatial surveys, such as those
described in Chapter 2.
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