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provide a rich food source for demersal fish. It is likely that all these apply to the P.
elegans patches on Drum Sands. In addition, the results presented here suggest that
tube-building species have a marked effect on the distributions of many invertebrate
species. C. volutator was found to be present almost exclusively within P. elegans
patches and, therefore, these tube-beds allow some species to occur on Drum Sands
which would otherwise not survive. A change in the population fitness of P. elegans
may result in the population fitness of other species. Consequently, elucidating the
processes which affect the distributions of tube-building polychaetes (the second aim
of this thesis) helps in the understanding of the processes spatially structuring the
benthic communities in such areas. This is particularly important in areas where tube-
beds occur on large scales, e.g., 150,000m 2 in Barnstable Harbour, Massachusetts
(Sanders et al., 1962). For this to be achieved, explicit studies addressing the reasons
why many species exhibit significantly higher densities in tube-beds should also be
conducted.
In conclusion, the approach adopted here of a combination of pattern documentation
and manipulative experiments to test hypotheses about the underlying mechanisms is
unusual in ecology. Only too often are post hoc explanations proposed to explain
patterns without serious attempts to experimentally falsify such hypotheses. In this
respect the present thesis makes, I believe, an important contribution to understanding
the structure and dynamics of intertidal assemblages.
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