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The importance of the ambient community on early community establishment has
been demonstrated in other studies (e.g., Zajac and Whitlatch, 1982a) but the effect of
dense assemblages of biogenic polychaetes has not previously been investigated.
Although several studies have experimentally shown that such tube-builders affect
colonisation (e.g., Gallagher et al., 1983; Savidge and Taghon, 1988; Trueblood,
1991; Ragnarsson, 1996), in contrast to this study these studies have implanted tube-
building polychaetes or tube mimics into the defaunated sediments and, therefore,
their observations apply to the later stages of succession only.
In order to determine which process(es) were responsible for the faunal differences in
colonisation between patch and non-patch areas in this study, it is important to
indicate how these areas differed hydrodynamically. Flume experiments have shown
that isolated animal tubes, and those below a certain threshold density, may cause
sediment destabilisation through a sufficiently high transfer of turbulent kinetic energy
to the bed (Eckman et al., 1981; Eckman, 1983). At greater densities, the interactions
of flow perturbations created by individual tubes may produce a 'skimming flow'
(sensu Morris, 1955). In 'skimming flow', the region of maximum turbulent kinetic
energy and shear stress production occurs away from the bed (Nowell and Church,
1979). The observed effects of animal tubes creating destabilisation on the faunal
communities are very different from those where the tubes are dense enough to create
sediment stabilisation (Eckman, 1983; Lukenbach, 1986, 1987).
Experiments by Nowell and Church (1979) suggested that the transition from the
destabilising effect of low tube density to a stabilising one occurs when approximately
1/12th of the plan area of the sediment is occupied by tubes. For P. elegans, with a
mean tube diameter of lmm (Morgan, 1997), this would mean that a tube density of at
least 100,000/m2 is needed to evoke sediment stabilisation. This is far in excess of the
densities attained within the patches used for this study, i.e., 22,500/m 2 (April, 1997,
see Chapter 8). However, worm density may not be a suitable criteria for estimating
the direct role of tubes. Brey (1991) and Morgan (1997) found that many P. elegans
have tubes with more than one opening, each opening presumably acting as roughness
elements, and that many tubes lack an occupant due to emigration (Fauchald and
Jumars, 1979; Wilson, 1983) and/or predation (Woodin, 1984). Furthermore, it is
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