Population Dynamics of Benthic Species and Shorebird Predation
331
we assume that the bivalve sub-populations in the study area are synchronized with neighbouring sub-populations in terms of adult stock size, the lack
of a stock-recruitment relationship implies that strong density-dependent
processes take place up to the time of recruitment. Yet, not much is known
about the underlying causes of the relationships between early mortality and
the size of the adult stocks. Hence, our analyses also point out some avenues
for future research. Particularly interesting for understanding the population
dynamics of the bivalves are the causes of mortality of bivalves other than
predation by shorebirds, the density-dependent factors that operate during
the early life phase of the bivalves, and the spatial coupling between adult
stock and recruitment.
Still, there are important considerations that we have not touched upon
here. In fact, these considerations may be quite central to the evolutionary
histories of dynamic interactions between bivalve prey and their shorebird
predators. The bivalves living on intertidal flats typically show a wide variety
of behavioural and morphological traits that can be interpreted as anti predator adaptations, e. g. deep burying, fast growth, heavy shell armour (Boulding 1984; Vermeij 1987; Zwarts and Blomert 1992; De Goeij and Luttikhuizen
1998). Recent work by our group indicates that the deep-burying response, for
example, is relaxed in favour of behaviours enhancing food intake, growth
and reproduction when cues indicating the presence of predators are, even
temporarily, absent. Although the present analysis has failed to demonstrate
any evidence for a tight coupling between population sizes of bivalve prey and
shorebird predators, we nevertheless believe that the selection pressure
exerted by shorebirds will underlie aspects of the bivalves' population dynamics. Predators like birds and crabs continuously mould the life-history
traits of the prey species, as individual prey not expressing traits that prevent
predation are constantly weeded out of the population. It is likely that this
predation effectively includes selection pressures on life-history traits that
bear directly on growth, reproductive investments, and perhaps even on
settlement decisions (Beukema 1993). We regard it as our challenge to find out
whether in intertidal bivalves like Macoma and Cerastoderma antipredation
traits reflect adaptations to local sets of predators (the predatory environment, as it were) and to explore how such adaptations in turn affect the
characteristics of population dynamic processes.
Acknowledgements. We are very grateful to M. Otter for allowing us to use his bird count
data. We thank Karsten Reise for giving us a forum to present these ideas. In recent years
our work on bird-benthos interactions has been supported by a PIONIER grant to T.P.
from the Netherlands Organization for Scientific Research (NWO).
331
we assume that the bivalve sub-populations in the study area are synchronized with neighbouring sub-populations in terms of adult stock size, the lack
of a stock-recruitment relationship implies that strong density-dependent
processes take place up to the time of recruitment. Yet, not much is known
about the underlying causes of the relationships between early mortality and
the size of the adult stocks. Hence, our analyses also point out some avenues
for future research. Particularly interesting for understanding the population
dynamics of the bivalves are the causes of mortality of bivalves other than
predation by shorebirds, the density-dependent factors that operate during
the early life phase of the bivalves, and the spatial coupling between adult
stock and recruitment.
Still, there are important considerations that we have not touched upon
here. In fact, these considerations may be quite central to the evolutionary
histories of dynamic interactions between bivalve prey and their shorebird
predators. The bivalves living on intertidal flats typically show a wide variety
of behavioural and morphological traits that can be interpreted as anti predator adaptations, e. g. deep burying, fast growth, heavy shell armour (Boulding 1984; Vermeij 1987; Zwarts and Blomert 1992; De Goeij and Luttikhuizen
1998). Recent work by our group indicates that the deep-burying response, for
example, is relaxed in favour of behaviours enhancing food intake, growth
and reproduction when cues indicating the presence of predators are, even
temporarily, absent. Although the present analysis has failed to demonstrate
any evidence for a tight coupling between population sizes of bivalve prey and
shorebird predators, we nevertheless believe that the selection pressure
exerted by shorebirds will underlie aspects of the bivalves' population dynamics. Predators like birds and crabs continuously mould the life-history
traits of the prey species, as individual prey not expressing traits that prevent
predation are constantly weeded out of the population. It is likely that this
predation effectively includes selection pressures on life-history traits that
bear directly on growth, reproductive investments, and perhaps even on
settlement decisions (Beukema 1993). We regard it as our challenge to find out
whether in intertidal bivalves like Macoma and Cerastoderma antipredation
traits reflect adaptations to local sets of predators (the predatory environment, as it were) and to explore how such adaptations in turn affect the
characteristics of population dynamic processes.
Acknowledgements. We are very grateful to M. Otter for allowing us to use his bird count
data. We thank Karsten Reise for giving us a forum to present these ideas. In recent years
our work on bird-benthos interactions has been supported by a PIONIER grant to T.P.
from the Netherlands Organization for Scientific Research (NWO).