Ecological functioning and predator-prey relationships
Intertidal Mud and Sandflats
Subtidal Mobile Sandbanks
The previous section details
the components and structure of the communities found in these biotope complexes. However,
it is emphasised that these communities are relatively poor in diversity but that,
especially in the case of intertidal mud and sandflats, they have high abundances. Because
of this, it is necessary to detail the functioning of the ecosystems and the support by
prey for predator populations. Important predator populations in the intertidal and
subtidal sedimentary areas include bird and fish species. In addition to the effects of
other major biotic and environmental factors, the size of the any productive will area
affect its carrying capacity in supporting wading birds (Meire, 1993) and fish.
Intertidal Mud and Sandflats
Intertidal mud flats are important in the functioning of estuarine systems and may have
a disproportionately high productivity compared to subtidal areas (Elliott & Taylor,
1989b). Conversely, coastal sandflats have a very poor productivity (McLachlan, 1996).
Epifaunal organisms associated with these biotope complexes are predominantly mobile
predatory species such as crabs e.g. Carcinus maenus and shrimps e.g. Crangon
crangon, which take infaunal populations of small bivalves, polychaetes and crustacea.
Organisms associated with silty sands are predominantly mobile species, including the
crabs Liocarcinus depurator, Atelecyclus rotundatus and Macropodia
Carcinus maenus has been shown to significantly reduce the numbers of Manayunkia
aesturina on mudflats (McLusky, 1989) and Carcinus and Crangon may
reduce the population of Corophium volutator in estuaries by over 50% (Pihl, 1985).
The shrimp Crangon crangon is a significant predator of the smallest sizes of
plaice during and immediately after the fish settle on sandy beaches when predation rate
is strongly dependent on the size of both the predator and the prey (Gibson et al,
1995). This size dependency is caused principally by the superior escape capabilities of
larger fish once captured rather than differences in the ability of different sizes of
shrimps to capture prey.
Polychaete worms are dominant predators within the substratum and tend to be
opportunistic and actively pursue prey (although they may have size preferences); their
numbers may be closely related to those of their prey which includes other worms and
crustaceans (Meire et al, 1994). Many infaunal species also scavenge e.g. Nephtys
and the isopod Eurydice pulchra and quantity of food input determines the
density of scavengers (Hayward, 1994, Ansell et al, 1972). Scavengers may dominate
on coarse steep shores and are found in high numbers near kelp beds where there is a large
amount of macrodebris (McLachlan, 1983).
Intertidal areas are well-defined as juvenile fish feeding areas (Costa & Elliott,
1991). Mud and sandflats are important nursery areas for plaice (Lockwood, 1972; Marshall,
1995; Marshall & Elliott, 1997), as well as feeding areas for sea bass and flounder
(Elliott & Taylor, 1989). Fish such as dover sole, Solea solea and gadoids
frequent sandy areas, but many also occur on coarser and mixed grades of sediment. Smaller
fish (e.g. plaice) may settle on mudflats while larger fish are found on sandflats
(Gibson, 1973; Gibson & Robb, 1992). Migratory species such as salmon and shad can
also be found in these areas on passage to other wetlands, e.g. saltmarshes and freshwater
areas, although they appear to have no requirement for the mud and sandflats.
The most important marine predators on intertidal sand and mudflats are particularly
the flatfish Solea solea (sole), Limanda limanda (dab), Platichthys
flesus (flounder) and Pleuronectes platessa (plaice) which feed on polychaetes
and their tails (e.g. of Arenicola and Nereis), bivalve young and siphons
(e.g. of Macoma and Angulus) and tidally active crustaceans such as Bathyporeia
and Eurydice species (Croker & Hatfield, 1980; McDermott, 1983; McLachlan,
1983; Zwarts et al, 1985). In summer, large numbers of plaice and dab juveniles
move over flats at high tide to feed on mobile epifauna, sedentary infauna and protruding
siphons and tentacles (Elliott & Taylor 1989a). Within estuaries and on mud and
sandflats, however, many demersal fish are opportunistic predators and the prey choice
will reflect the infaunal species distribution of the area (Marshall & Elliott,
submitted; Costa & Elliott, 1991).
Flatfish use several feeding strategies in estuarine areas with plaice and flounder
using tidal migration feeding only at high tide on the intertidal flats. Dab and sole do
not migrate tidally and feed continuously in the subtidal areas (McLusky, 1989). Gobies
e.g. Pomatoschistus spp. are another important predator on mudflats and prey
heavily on Corophium volutator and they have a significant impact as both predator
and prey in estuarine ecosystems. Small juveniles (e.g. plaice) settling on fine sediments
are less likely to be predated than those that are settled but do not bury in coarser
sediments (Gibson & Robb, 1992).
Tidal elevation influences population size in fish, for example, plaice populations are
largest at the waters edge at a depth of 1-2m suggesting that they migrate with the
tide up and down shore (Gibson, 1973). The young of many species, such as plaice, enter
the intertidal zone to feed as the tide floods. There is also a relationship between the
size of the fish and depth (for plaice specifically), as the body length increases, the
depth of the water that the fish inhabits increases (Gibson et al, 1995). Depth and
salinity may also influence flounder distribution (Armstrong, 1997).
Wading birds and wildfowl
These biotope complexes are used by important wintering and passage birds for feeding
Figure - a simplified
food web of a depositing shore
Shorebirds form important predators on NW European intertidal mud and sandflats during
long migrations over long distances from breeding to wintering grounds. Particularly
dependent species are brent geese, shelduck, pintail, oystercatcher, ringed plover, grey
plover, bar-tailed and black-tailed godwits, curlew, redshank, knot, dunlin and
sanderling, whilst grey geese and whooper swan may use this habitat for roosting (Jones
& Key, 1989; Davidson et al, 1991). Bird communities are highly mobile and
usually exhibit patterns of activity related to tidal water movements. However, the
carrying capacity, in terms of space or food, of these biotope complexes has not been
determined. The carrying capacity is reached when every new individual entering the
habitat causes emigration or death of another bird (Goss Custard, 1984).
Where mudflats occur within estuaries, any change in salinity will affect their prey
structure but not necessarily their functioning. For example, on mud flats Nereis
may be replaced by Nephtys following an increase in salinity with reduced river
flows (McLusky & McCrory, 1989). Although the species composition is seen to have
changed along the environmental gradient, the community still functions as prey for the
In analysing the feeding preferences of different species, it was initially suggested
(Green, 1968) that both physical and behavioural adaptations have been shown for shore
birds including bill lengths which correspond to the depth of specific prey items. Species
such as the bar-tailed godwit takes cues from new Arenicola casts and the shelduck
which feeds extensively on Hydrobia ulvae has five distinct feeding methods
relating to the tidal state and the behavioural patterns displayed by Hydrobia (Bryant
& Leng, 1976). However, more recently, waders are regarded as opportunistic feeders
with only a general relationship between depth of prey and size of bill (McLusky, 1989).
Different bird species exploit different areas of an intertidal area, for example the
redshank and the shelduck feed on the intertidal area at low water or the waters
edge, usually on Macoma, Hydrobia and other small invertebrates such as Corophium.
Waders such as sanderling are efficient croppers of macrofauna at the low tide
waters edge both in summer and winter (McLachlan, 1983). Eider ducks, however, feed
in the shallow water at low tide on species such as Mytilus edulis. The diet of the
black-tailed godwit consisted mainly of the bivalve mollusc Scrobicularia plana with
a small proportion of ingested biomass being Nereis and Hydrobia (Moreira,
1994). The sizes taken reflect those available although optimal foraging occurs.
Feeding behaviour differs within intertidal sand and mudflats with variations in the
percentage of shorebirds engaged in feeding, roosting and other behaviours dependent on
location, date, time, tide and species. Burger et. al (1997), studying spring
migrations in Delaware Bay, found that a higher percentage of shorebirds fed during the
middle of migration, in early to mid-morning, and during low and rising tides than at
other times. Some shorebirds fed on the marshes and mudflats during all tidal states, but
none fed on beaches at high tide (beaches were too narrow). Within each habitat, the
highest percentage of shorebirds foraged during low tide (marshes) or rising tides
(mudflats and beaches). The percentage of shorebirds engaged in foraging as an indication
of foraging value for each habitat type, showed that a mosaic of habitat types ranging
from mudflats to high marshes is essential to sustain the high populations of shorebirds.
Subtidal Mobile Sandbanks
The subtidal mobile sandbanks provide prey for demersal fishes, especially the mobile
small crustaceans which migrate from the sediment and thus become available for predation
(Costa & Elliott 1991; Marshall & Elliott, submitted). The areas are often
important as fish nursery areas, e.g. plaice (Gibson, 1973), and may be characterised by
low organic enrichment though there may be localised pockets of organic matter or areas
which receive anthropogenic waste, e.g. the Dogger bank.
The sandbanks are also important areas for crab populations, for example the Race Bank
and Docking Shoal off the Norfolk coast support a large population as well as numerous
other epifauna, particularly echinoderms. The epifaunal component may represent a large
proportion of the biomass of the sand bank fauna with large numbers of echinoderms such as
Asterias rubens and brittle stars such as Ophiura albida. Predatory fauna
such as hermit crabs e.g. Eupagurus bernhardus, Liocarcinus depurator and
the edible crab Cancer paguras may also be present.
Birds such as the guillemot, razorbill, puffin and the terns will feed on the fish such
as sandeels (Ammodytes spp.) which are found in mobile subtidal sands (Batten et
al, 1990). Both the arctic tern and the puffin rely on populations of sandeel as their
predominant food source. The sandeel is also an important food source for wintering birds
such as scoters, little terns and the red-throated diver (Gibbons et al, 1993).
Guillemots and razorbills although not as selective as puffins and terns will also eat