Synthesis: Comparative Ecology of Sedimentary Shores
367
Dynamic Structures and Trophic Supplies
Structural patterns in the benthos of sedimentary shores are highly dynamic.
Comparing patterns helps to reveal underlying dynamics. This often requires
long-term observations to discover significant correlations between variables, and manipulative field experiments to understand the mechanisms.
Patterns of spatial heterogeneity and biodiversity in marine sediments have
been explained with the disturbance mosaic model proposed by Johnson
(1973), assuming that local disturbances produce patches, and among these
the recovery of benthic assemblages may be out of phase. Disturbances are
biogenic on a small scale and physical over much larger scales. Anthropogenic
sediment disturbances are frequently caused by coastal engineering, changes
in land-use or bottom trawl or dredge fisheries.
All these disturbances are very common on sedimentary shores, and an
understanding of their role is important. Similar disturbance experiments
have been performed on coasts with very different biota, for example in New
Zealand and New England (USA) (Thrush and Whitlatch, Chap. 14). Biotic
interactions during the course of recovery are highly variable but the provision of habitat structure by larger organisms produces consistent patterns.
The differential mobility of benthic organisms is a key variable in the process
of recolonization. On a small scale crawling and bed-load transport prevail,
on a wider scale drifting of benthic stages with the tidal currents is of importance, and finally planktonic larvae allow for colonization from very
distant sources. Because of these different modes of dispersal, the scaling-up
from small and short field experiments to the recovery of an entire bay cannot
be a linear exercise and is not trivial. Studies on dispersal are an important
challenge for ecologists of sedimentary shores because of the complexity of
hydrodynamics in these environments and the multiple life stages over which
dispersal occurs.
High mobility in prey and predator populations also complicates attempts
to answer questions such as whether shorebirds differ in their impact on
their bivalve prey between regions (van der Meer et aI., Chap. 15). Respective
field experiments with cage exclosures have not been sufficiently large or
long. Difficulties also arise when consumption/production ratios are calculated by assuming species-specific production rates or assuming data from
a single year to be representative. Calculated ratios of shorebird consumption versus zoobenthic production varied along the latitudinal flyway
from Europe to Africa without any trend. On average some 30 % of the overall production is consumed by birds. However, what does this mean for the
dynamics and the regulation of the prey populations? A data set spanning
almost three decades revealed no relation between the death rate of adult
bivalves and the numbers of their main shorebird predators, knots and
367
Dynamic Structures and Trophic Supplies
Structural patterns in the benthos of sedimentary shores are highly dynamic.
Comparing patterns helps to reveal underlying dynamics. This often requires
long-term observations to discover significant correlations between variables, and manipulative field experiments to understand the mechanisms.
Patterns of spatial heterogeneity and biodiversity in marine sediments have
been explained with the disturbance mosaic model proposed by Johnson
(1973), assuming that local disturbances produce patches, and among these
the recovery of benthic assemblages may be out of phase. Disturbances are
biogenic on a small scale and physical over much larger scales. Anthropogenic
sediment disturbances are frequently caused by coastal engineering, changes
in land-use or bottom trawl or dredge fisheries.
All these disturbances are very common on sedimentary shores, and an
understanding of their role is important. Similar disturbance experiments
have been performed on coasts with very different biota, for example in New
Zealand and New England (USA) (Thrush and Whitlatch, Chap. 14). Biotic
interactions during the course of recovery are highly variable but the provision of habitat structure by larger organisms produces consistent patterns.
The differential mobility of benthic organisms is a key variable in the process
of recolonization. On a small scale crawling and bed-load transport prevail,
on a wider scale drifting of benthic stages with the tidal currents is of importance, and finally planktonic larvae allow for colonization from very
distant sources. Because of these different modes of dispersal, the scaling-up
from small and short field experiments to the recovery of an entire bay cannot
be a linear exercise and is not trivial. Studies on dispersal are an important
challenge for ecologists of sedimentary shores because of the complexity of
hydrodynamics in these environments and the multiple life stages over which
dispersal occurs.
High mobility in prey and predator populations also complicates attempts
to answer questions such as whether shorebirds differ in their impact on
their bivalve prey between regions (van der Meer et aI., Chap. 15). Respective
field experiments with cage exclosures have not been sufficiently large or
long. Difficulties also arise when consumption/production ratios are calculated by assuming species-specific production rates or assuming data from
a single year to be representative. Calculated ratios of shorebird consumption versus zoobenthic production varied along the latitudinal flyway
from Europe to Africa without any trend. On average some 30 % of the overall production is consumed by birds. However, what does this mean for the
dynamics and the regulation of the prey populations? A data set spanning
almost three decades revealed no relation between the death rate of adult
bivalves and the numbers of their main shorebird predators, knots and