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Transect survey - Micro-scale patterns of P. elegans - Table 7.1 presents the results
of the indices of dispersion of adult P. elegans densities from March 1997 to February
1998. An overall non-random distribution here is one in which at least 2 indices of
dispersion indicated a significant departure from random. The results suggest that
adult P. elegans exhibited an aggregated distribution all through the year, except
during October 1997. The greatest amount of clumping was generally observed
during the summer months of June and July (highest v:m ratios). There was some
variability between patches - for 4 months (April, May and December 1997 and
February 1998) adult P. elegans were randomly distributed in one patch replicate
while aggregated in the other 2 replicates. There was a good agreement between the 3
dispersion indices, only during May (replicates 2 and 3) and August (replicate 2) were
there discrepancies between the results obtained from the variance : mean ratio, Id and
Ip.
Figures 7.4(i-xxvii) present the density plots of P. elegans adults throughout the
sampling period. The plots of P. elegans with random distributions, i.e., April 1997
(replicate 1); October 1997 (replicates 1, 2 and 3); December 1997 (replicate 3); and
February 1998 (replicate 3), are included for comparison with plots of non-randomly
distributed P. elegans. Generally, the plots do not differentiate between random and
non-random distributions very clearly. However, it was pointed out in Chapter 2 that
a random distribution (according to indices of dispersion) could still have a significant
underlying spatial structure (e.g., N. hombergii, 1 m survey), especially for low-density
species. The plots show that for those transects in which P. elegans had statistically
significant aggregated distributions, patches of P. elegans were generally smaller than
3cm across, i.e., less than the size of the sampling unit, for most months. However,
since the pilot survey suggested that patches formed tended not to be symmetrical (see
Figure 7.2) it is possible that these micro-scale patches were larger than revealed by
the transects as some of them may have been perpendicular to the direction of the
tran sect.
The significant correlograms produced by spatial autocorrelation analysis are
presented in Figures 7.5(i-viii). The majority of transects with aggregated
distributions of adult P. elegans produced non-significant spatial autocorrelation
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