340
D. Raffaelli and M. Emmerson
(but not always) functionally insignificant, whilst those links along which
virtually no energy flows may be all-important in the organisation of the
dynamics of the system. Specifically, sea urchins Strongylocentrotus ingested
little energy from kelps, because they grazed the settling spores, but removal
of urchins resulted in a large increase in the kelp Hedophyllum, a competitive
dominant (Paine 1980). Surprisingly, there have been few such comparisons
carried out on sediment shores, although Raffaelli and Hall (1996) reached a
similar conclusion to that of Paine (1980) for a mudflat food web.
There is clearly a pressing need to understand the links between, on the one
hand, energy flow and, on the other, the dynamics of community organisation; a need also highlighted by the Second International Food Web
Symposium held in Colorado in 1993 (Polis and Winemiller 1996). Here, we
describe several areas where the production and organisation of sediment
shore communities may well be linked. The investigations described are
focused heavily on a well-worked system, the Ythan estuary, Aberdeenshire,
UK (see Baird and Milne 1981; Gorman and Raffaelli 1993 for an overview of
this system), because few similar studies have been carried out elsewhere.
However, we hope that the approaches described here may be taken up by
workers in other regions so that between-region comparisons can become
possible. Specifically, we discuss the relationships between production subsidies and food chain dynamics, between production and body-size relationships, and between production and biodiversity.
16.2 Effects of Production Subsidies on Food Chain
Dynamics
Polis et al. (1997) have suggested that the consumer-resource dynamics in a
food web may be profoundly affected by subsidies of material originating
from outside the system. For example, terrestrial insects falling into a stream
may sustain a high abundance of predatory fish, leading to a reduction in
invertebrate grazers and a subsequent benthic algal bloom. The fish can have
no effect on the supply rate or production of the subsidy (terrestrial insects)
and the dramatic effect of the subsidy on the primary producer is indirect.
The most compelling evidence for such effects comes from studies on desert
food webs subsidised by carbon from the adjacent oceanic fringe (Polis and
Hurd 1996b), but such effects may be more common than realised (Polis et al.
1997). However, experimental tests of the hypothesised effects of subsidies on
food web dynamics are lacking (Riley et al. 2000)
By virtue of their open nature, intertidal flats receive large amounts of
allochthonous material (subsidies) from the sea, and, in the case of estuarine
D. Raffaelli and M. Emmerson
(but not always) functionally insignificant, whilst those links along which
virtually no energy flows may be all-important in the organisation of the
dynamics of the system. Specifically, sea urchins Strongylocentrotus ingested
little energy from kelps, because they grazed the settling spores, but removal
of urchins resulted in a large increase in the kelp Hedophyllum, a competitive
dominant (Paine 1980). Surprisingly, there have been few such comparisons
carried out on sediment shores, although Raffaelli and Hall (1996) reached a
similar conclusion to that of Paine (1980) for a mudflat food web.
There is clearly a pressing need to understand the links between, on the one
hand, energy flow and, on the other, the dynamics of community organisation; a need also highlighted by the Second International Food Web
Symposium held in Colorado in 1993 (Polis and Winemiller 1996). Here, we
describe several areas where the production and organisation of sediment
shore communities may well be linked. The investigations described are
focused heavily on a well-worked system, the Ythan estuary, Aberdeenshire,
UK (see Baird and Milne 1981; Gorman and Raffaelli 1993 for an overview of
this system), because few similar studies have been carried out elsewhere.
However, we hope that the approaches described here may be taken up by
workers in other regions so that between-region comparisons can become
possible. Specifically, we discuss the relationships between production subsidies and food chain dynamics, between production and body-size relationships, and between production and biodiversity.
16.2 Effects of Production Subsidies on Food Chain
Dynamics
Polis et al. (1997) have suggested that the consumer-resource dynamics in a
food web may be profoundly affected by subsidies of material originating
from outside the system. For example, terrestrial insects falling into a stream
may sustain a high abundance of predatory fish, leading to a reduction in
invertebrate grazers and a subsequent benthic algal bloom. The fish can have
no effect on the supply rate or production of the subsidy (terrestrial insects)
and the dramatic effect of the subsidy on the primary producer is indirect.
The most compelling evidence for such effects comes from studies on desert
food webs subsidised by carbon from the adjacent oceanic fringe (Polis and
Hurd 1996b), but such effects may be more common than realised (Polis et al.
1997). However, experimental tests of the hypothesised effects of subsidies on
food web dynamics are lacking (Riley et al. 2000)
By virtue of their open nature, intertidal flats receive large amounts of
allochthonous material (subsidies) from the sea, and, in the case of estuarine