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probably exist around Drum Sands which may help retain P. elegans larvae.
Furthermore, since the largest P. elegans populations within the Forth are found
within the less saline Forth Estuary (SEPA, pers. comm.), planktonic larvae produced
by them are likely to pass Drum Sands since this water passes along the south shore of
the Forth (Dyke, 1987). This could be another potential input of larvae for the Drum
Sands population. However, the mode of reproduction of P. elegans within the less
saline Forth Estuary has not been studied.
The importance of pelagic larvae for shaping community structure has been discussed
by Lewin (1986). He suggested that previous models of marine benthic community
structuring, sensu Paine and Connell, have overlooked the importance of the larval
fluxes since these models were based on experiments carried out where larval influxes
were saturating. In areas where larval settlement is saturating, evidence suggests that
communities are shaped by post-settlement processes such as predation and
competition. In areas where larval recruitment is not saturated however, larval-supply
fluctuations become more important in shaping demographic processes (Lewin, 1986;
Bachelet, 1990). Therefore, it is possible that the P. elegans population at Drum
Sands is structured by fluxes in larval settlement and the processes which effect it,
rather than processes acting on the adult population. Consequently, the effect of larval
recruitment on population density changes is specifically addressed in this study in
response to a number processes including macroalgal mat establishment (Chapters 4
and 5), small-scale disturbances (Chapter 6) and the generation of micro-scale spatial
heterogeneity due to adult-juvenile interactions (Chapter 7). However, further studies
have to be carried out in order to assess whether P. elegans larval recruitment is
indeed saturated or not on this sandflat.
Since the life history of a species governs that species' dispersal dynamics and
recruitment in a particular environment, Levin (1984a) suggested that it determined
the scale of disturbance a species was potentially capable of exploiting. She noted
that polychaetes with small adult size, brood protection, small brood size and reduced
larval phases (e.g., Capitella spp., Streblospio benedicti) were adapted to small-scale
disturbances such as those caused by ray foraging and human pit-digging, while
polychaetes with larger brood sizes and longer-lived larvae, whose planktonic
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