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exhibited by the tube-building polychaete has an effect upon the spatial pattern of
other species.
Species with similar structure functions (correlograms) do not necessarily occupy the
same spatial locations (Hewitt et al., 1996). For example, while the correlograms of
P. elegans, L. conchilega, B. sarsi and N. hombergii suggested that these species
formed patches of increased densities approximately 1-1.5m2, their density plots
revealed that their patches were not spatially correlated. For P. elegans, this was
examined more formally using correlation analyses which supported the observations
that P. elegans abundances were not correlated with the abundances of L. conchilega
or M. balthica (B. sarsi and N. hombergii were not considered sufficiently abundant
for correlation analyses). This suggests that the spatial patterns of P. elegans on Drum
Sands did not have a large positive effect on those of the other common species.
Pygospio elegans was only positively correlated with one species in this study, M.
balthica (8m and 40m surveys). However, positive association does not necessarily
imply a positive interaction, since negative interactions (e.g., predation) can take place
between species coincident in space, or coincidence may occur due to some common
ecological preference between two species. Controlled manipulative experiments,
together with spatial autocorrelation analysis, are needed to test explicitly for the
presence of an interaction between these two species.
Tube-building polychaetes are capable of affecting species distributions via many
direct and/or indirect interactions, e.g., hydrodynamic effects (Eckman et al., 1981;
Eckman, 1983), provision of refuges (Woodin, 1978, 1981; Lukenbach, 1986) and
increased food availability (Eckman, 1983; Noji, 1994) which suggest that the positive
association between P. elegans and M. balthica in this study may have been non-
coincidental. A change in spatial pattern of M. balthica or reduced survival, growth or
recruitment may, therefore, be expected in the absence of P. elegans. The processes
by which P. elegans affect other fauna are discussed further in Chapter 8 in which the
faunal communities and sediment variables of P. elegans patches are compared with
non-patch sediments using not a non-parametric correlation approach as in this study
but a more suitable, parametric method of statistical analysis, i.e., t-tests.
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