Experimental Approaches to Sediment Communities
343
bers of which have increased in the same areas as those of deposit feeders
(Raffaelli 2000a).
16.2.2 Effects of Subsidised Predators
Given the highly mobile nature of many of the predators in the Ythan
(shorebirds, fish and epibenthic crustaceans), it could be argued that the
abundance of these species is regularly subsidised (tidally or seasonally) by
resources located outside of the system. In other words, their abundance
within the estuary is maintained at a level higher than can be supported by the
invertebrate prey production within the estuary itself. We therefore tested the
hypothesis that subsidised predator biomass can depress prey densities
through several field experiments where enhanced densities of gobiid fish
(Pomatoschistus minutus and P. microps) and epibenthic crustaceans (Carcinus maenas and Crangon crangon) were maintained on the mud flat (Jacquet and Raffaelli 1989; Raffaelli et al.1989). In none of these experiments was
there evidence of significant top-down effects of predators on the abundance
of their invertebrate prey (Raffaelli and Hall 1992, 1996; Fig. 16.I).
In summary, our research on the role of subsidies in the dynamics of the
Ythan estuary food web indicates that there is a significant positive effect of
nutrients on macro algae, which, in turn, has a negative effect on the
abundance of invertebrates (and hence their predators) locally, through the
production of an organically enriched and hostile physicochemical environment. In contrast, subsidies of organic matter (fuelled ultimately by the nutrient subsidy) may enhance densities of deposit feeders and increase shorebird
abundance at the whole estuary scale (Raffaelli 2000a). However, manipulative
field experiments provide no evidence to support the idea that top-down
effects by these subsidised consumers reduce the abundance of their resources, as postulated by Polis et al. (1997).
16.3 Production and Body-Size Distributions
The association of levels of production with organism body size is well
documented for marine benthic systems, as are the reciprocal effects of body
size on in situ secondary production (e.g. Gerlach et al. 1985).A feature of the
classic Pearson and Rosenberg (1978) model of the response of benthic
communities to organic enrichment is the relative predominance of small
body-sized species under the most enriched conditions, with larger bodysized species only accommodated under normal inputs or organic matter. The
cause-effect relationships in this model are complex, involving size-related
343
bers of which have increased in the same areas as those of deposit feeders
(Raffaelli 2000a).
16.2.2 Effects of Subsidised Predators
Given the highly mobile nature of many of the predators in the Ythan
(shorebirds, fish and epibenthic crustaceans), it could be argued that the
abundance of these species is regularly subsidised (tidally or seasonally) by
resources located outside of the system. In other words, their abundance
within the estuary is maintained at a level higher than can be supported by the
invertebrate prey production within the estuary itself. We therefore tested the
hypothesis that subsidised predator biomass can depress prey densities
through several field experiments where enhanced densities of gobiid fish
(Pomatoschistus minutus and P. microps) and epibenthic crustaceans (Carcinus maenas and Crangon crangon) were maintained on the mud flat (Jacquet and Raffaelli 1989; Raffaelli et al.1989). In none of these experiments was
there evidence of significant top-down effects of predators on the abundance
of their invertebrate prey (Raffaelli and Hall 1992, 1996; Fig. 16.I).
In summary, our research on the role of subsidies in the dynamics of the
Ythan estuary food web indicates that there is a significant positive effect of
nutrients on macro algae, which, in turn, has a negative effect on the
abundance of invertebrates (and hence their predators) locally, through the
production of an organically enriched and hostile physicochemical environment. In contrast, subsidies of organic matter (fuelled ultimately by the nutrient subsidy) may enhance densities of deposit feeders and increase shorebird
abundance at the whole estuary scale (Raffaelli 2000a). However, manipulative
field experiments provide no evidence to support the idea that top-down
effects by these subsidised consumers reduce the abundance of their resources, as postulated by Polis et al. (1997).
16.3 Production and Body-Size Distributions
The association of levels of production with organism body size is well
documented for marine benthic systems, as are the reciprocal effects of body
size on in situ secondary production (e.g. Gerlach et al. 1985).A feature of the
classic Pearson and Rosenberg (1978) model of the response of benthic
communities to organic enrichment is the relative predominance of small
body-sized species under the most enriched conditions, with larger bodysized species only accommodated under normal inputs or organic matter. The
cause-effect relationships in this model are complex, involving size-related