368
K. Reise
oystercatchers. This lack of correlation still does not imply that shorebirds
are unable to exert any selection pressure on the life-history traits of their
prey.
Field experiments consistently indicated that small epibenthic predators
preying on early bivalve recruits exert a very strong effect on population
dynamics. Due to the small size of predator and prey, the biomass consumed
is much less than that by the shorebirds that feed on the adult bivalves.
Nevertheless, in terms of community structure and dynamics, the former link
is more important than the latter. Often it requires a spatio-temporal mismatch between the bivalve recruits and the juvenile epibenthic predators to
give the former a chance to reach maturity and to grow up into the prey size
spectrum of the birds.
Sedimentary shores are strongly subsidized from the sea and in estuaries
also from the land (Raffaelli and Emmerson, Chap. 16). Pelagic and benthic
algae thrive where nutrients are plentiful, and then also the zoobenthic fauna
may abound. This bottom-up effect may also be a negative one when an excess
of nitrogen is supplied. Mats of green algae often quickly cover the entire
sediment surface, and then most of the benthic macrofauna either dies or
escapes the sulphidic conditions underneath the mats. On a larger scale, however, the green algae constitute an organic enrichment and, as detritus, they
benefit those deposit feeders that have not been covered by them. Thus,
bottom-up effects are basic for the abundance of benthic invertebrates at sedimentary shores while small predators affect their dynamics.
Increasing nutrient supply is a common corollary of high human population density and intensive agriculture next to sedimentary shores. The effects
differ between sites, depending on the hydrodynamic energy and the nutrient
supply level. When hydrodynamics are strong, the response potential of the
biota to increased resource supply is physically limited. When conditions are
sheltered and oligotrophic, an enrichment may increase biotic production
and diversity. However, when conditions are already eutrophic, a further
enrichment may have no or even negative effects. This has been confirmed
by field experiments following the same protocol on a coast in southern
Australia with a low and in a Scottish estuary with a high nutrient status.
Within artificial habitat units of identical size and structure, diversity of
colonizing organisms was higher in units with fertilizers than in unfertilized
controls at the oligotrophic site. At the eutrophic site, colonizer diversity was
lower in the fertilized habitat units.
K. Reise
oystercatchers. This lack of correlation still does not imply that shorebirds
are unable to exert any selection pressure on the life-history traits of their
prey.
Field experiments consistently indicated that small epibenthic predators
preying on early bivalve recruits exert a very strong effect on population
dynamics. Due to the small size of predator and prey, the biomass consumed
is much less than that by the shorebirds that feed on the adult bivalves.
Nevertheless, in terms of community structure and dynamics, the former link
is more important than the latter. Often it requires a spatio-temporal mismatch between the bivalve recruits and the juvenile epibenthic predators to
give the former a chance to reach maturity and to grow up into the prey size
spectrum of the birds.
Sedimentary shores are strongly subsidized from the sea and in estuaries
also from the land (Raffaelli and Emmerson, Chap. 16). Pelagic and benthic
algae thrive where nutrients are plentiful, and then also the zoobenthic fauna
may abound. This bottom-up effect may also be a negative one when an excess
of nitrogen is supplied. Mats of green algae often quickly cover the entire
sediment surface, and then most of the benthic macrofauna either dies or
escapes the sulphidic conditions underneath the mats. On a larger scale, however, the green algae constitute an organic enrichment and, as detritus, they
benefit those deposit feeders that have not been covered by them. Thus,
bottom-up effects are basic for the abundance of benthic invertebrates at sedimentary shores while small predators affect their dynamics.
Increasing nutrient supply is a common corollary of high human population density and intensive agriculture next to sedimentary shores. The effects
differ between sites, depending on the hydrodynamic energy and the nutrient
supply level. When hydrodynamics are strong, the response potential of the
biota to increased resource supply is physically limited. When conditions are
sheltered and oligotrophic, an enrichment may increase biotic production
and diversity. However, when conditions are already eutrophic, a further
enrichment may have no or even negative effects. This has been confirmed
by field experiments following the same protocol on a coast in southern
Australia with a low and in a Scottish estuary with a high nutrient status.
Within artificial habitat units of identical size and structure, diversity of
colonizing organisms was higher in units with fertilizers than in unfertilized
controls at the oligotrophic site. At the eutrophic site, colonizer diversity was
lower in the fertilized habitat units.