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surveys; 7 out of 9; 8 out of 10 and 11 out of 11 species exhibited significantly non-
random distributions respectively with very good agreements between the 3 dispersion
indices (i.e., I, Id and /p) used. All of these were aggregated distributions, i.e., their
variances were greater than their means. These dispersion indices gave a simple but
useful indication as to the degree of spatial heterogeneity exhibited by many of the
species within each survey. This is in agreement with many other studies of the
marine benthos (e.g., Gage and Geekie, 1973; Volkaert, 1987; Meire et al., 1989;
Thrush et al., 1989; Lamont et al., 1995; Lawrie, 1996). These studies have revealed
that the majority of macrobenthic populations in both intertidal and sublittoral areas
are aggregated and that apparent randomness or uniformity at the scales defined in the
present study are rare.
The lack of significance from a random distribution did not preclude a significant
spatial pattern in all cases. For example, according to its dispersion indices, the
distribution of N. hombergii (mean abundance of 5 individuals per core) in the lm
survey was random, yet spatial autocorrelation analysis indicated that it exhibited a
significant spatial structure. Gage and Geekie (1973) proposed that spatial analysis
based on variance estimates is influenced by the number of individuals and
consequently low density populations are less likely to show significant differences
from random. This may explain why P. elegans which was by far the most abundant
species had relatively high variance to mean ratios. Consequently, dispersion indices
do not give reliable indications of spatial intensity for rarer species. Therefore,
previous studies relying solely upon dispersion indices may have under-estimated the
presence of spatial heterogeneity for low-density species.
Mapping, together with spatial autocorrelation analysis, provided detailed and less
equivocal assessments of the form of spatial patterns. These techniques suggested that
the majority of the species within the study area indicated in Figure 1.2 formed
patches of increased density at one or more scales. In the 1 m survey, 4 of the 7
species with significantly non-random distributions displayed significant spatial
patterns in addition to N. hombergii. These species, P. elegans, A. marina, L.
conchilega, B. sarsi and N. hombergii, formed patches between 1-2m2. The other
species with non-random distributions, i.e., M. balthica, G. duebeni and E. cf flava,
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