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variance (TTLQV) techniques (see Ludwig and Reynolds, 1988) assess patch sizes
from a grid (or transect) of contiguous samples only.
Characterisation of populations in space requires the definition of both 'intensity' and
'form' (Pielou, 1969; Thrush et al., 1989; Thrush, 1991). The intensity of a spatial
distribution refers to whether it is clumped, regular or not significantly different from
random, while the form of a pattern describes the size of the patches or gradients
(Thrush, 1991). Methods which sort distributions into clumped, regular or random,
e.g., dispersion indices, rely on the distribution of density estimates about the mean
rather than the actual spatial arrangement of individuals and are therefore dependent
on the size of the sampling unit. Dispersion indices, although only giving information
about the intensity of the pattern, have been widely used by benthic ecologists for
assessing the degree of aggregation or uniformity of species distributions (e.g., Gage
and Geekie, 1973; Levin, 1981; Volckaert, 1987; Meire et al., 1989; Thrush et al.,
1989; Lamont et al., 1995; Lawrie, 1996). Indices such as the variance to mean ratio
or index of dispersion (0, Morisita index (Id), the standardised Morisita index (/),) and
Green's coefficient have been commonly applied to species data from a wide variety
of sampling designs. Such indices are simple to calculate and the majority of them
can be easily tested for significance using the Chi-square distribution. However, none
of these indices on their own are ideal since they tend to be affected by population
density and/or sample size (Elliot, 1977; Taylor, 1984; Andrew and Mapstone, 1987;
Meire et al., 1989; Hurlbert, 1990). Myers (1978) suggested that several indices
should be used in conjunction and when their results correspond, a stronger statement
can be made about the dispersion.
Dispersion indices give an indication of the intensity of pattern only, since they do not
utilise the information contained in the arrangement of individuals in space they
cannot give any idea as to the form of a pattern. Consequently, two distributions
which may have a similar intensity of pattern may have very different spatial
arrangements (Thrush, 1991; Hall et al., 1993). Furthermore, such indices give no
indication of the scales at which pattern might be found other than the scale at which
the data used to calculate them were gathered (Hill, 1973).
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