Population Dynamics of Benthic Species and Shorebird Predation
329
able bivalve density should not get below about half a gram per square meter.
The stock-recruitment relationships that we found show that when prey
densities are too low to be exploitable, they still are good enough for population renewal. Such prey refugia, as it were, may contribute to the stability of
the bivalve populations.
We cannot say much about the impact of bivalve population variability on
the regulation of the shorebird populations. Yet, we repeat (Piersma 1987) that
a simple comparison between secondary production of a prey species and
consumption of a predator (Faldborg et al. 1994) does not allow any conclusion about a possible food limitation of the predators, because higher consumption may simply enhance production.
15.7 The Scale of Population Studies
The natural system that we studied is not a simple closed predator-prey
system. Both the bird and bivalve populations are open. As the bivalves have a
planktonic phase during their early life stage, recruits at Balgzand may
originate from other neighbouring areas. Hence a totally different explanation of the apparent lack (at least in M. balthica) of a clear stock-recruitment
relationship is related to the open character of the bivalve population. Most of
the recruits may originate from neighbouring stocks outside the Balgzand,
which are out of phase with the adult stock that we measured. Yet, the high covariability among adult stocks over a much larger geographical area than the
present study area (Beukema et al. 1996) suggests that strong density dependence in the pre-recruitment period is a more likely explanation of the lack of
a stock-recruitment relationship than the occurrence of local sub-populations that are dynamically out of phase.
15.8 Exclosure Experiments
The large year-to-year variability in the fraction of the bivalve production that
is eaten by shorebirds also sheds some doubts on the generality of so-called
exclosure experiments aimed at estimating this fraction by excluding birds
from parts of the intertidal flats, if these experiments are only performed in a
single or a few years. Such estimates should not be based on short-term observations (Baird et al.1985). Due to logistic problems these experiments have to
be performed over rather short periods (Piersma 1987), so it may come as no
surprise that these studies have produced ambiguous results (Raffaelli and
Hawkins 1996).
329
able bivalve density should not get below about half a gram per square meter.
The stock-recruitment relationships that we found show that when prey
densities are too low to be exploitable, they still are good enough for population renewal. Such prey refugia, as it were, may contribute to the stability of
the bivalve populations.
We cannot say much about the impact of bivalve population variability on
the regulation of the shorebird populations. Yet, we repeat (Piersma 1987) that
a simple comparison between secondary production of a prey species and
consumption of a predator (Faldborg et al. 1994) does not allow any conclusion about a possible food limitation of the predators, because higher consumption may simply enhance production.
15.7 The Scale of Population Studies
The natural system that we studied is not a simple closed predator-prey
system. Both the bird and bivalve populations are open. As the bivalves have a
planktonic phase during their early life stage, recruits at Balgzand may
originate from other neighbouring areas. Hence a totally different explanation of the apparent lack (at least in M. balthica) of a clear stock-recruitment
relationship is related to the open character of the bivalve population. Most of
the recruits may originate from neighbouring stocks outside the Balgzand,
which are out of phase with the adult stock that we measured. Yet, the high covariability among adult stocks over a much larger geographical area than the
present study area (Beukema et al. 1996) suggests that strong density dependence in the pre-recruitment period is a more likely explanation of the lack of
a stock-recruitment relationship than the occurrence of local sub-populations that are dynamically out of phase.
15.8 Exclosure Experiments
The large year-to-year variability in the fraction of the bivalve production that
is eaten by shorebirds also sheds some doubts on the generality of so-called
exclosure experiments aimed at estimating this fraction by excluding birds
from parts of the intertidal flats, if these experiments are only performed in a
single or a few years. Such estimates should not be based on short-term observations (Baird et al.1985). Due to logistic problems these experiments have to
be performed over rather short periods (Piersma 1987), so it may come as no
surprise that these studies have produced ambiguous results (Raffaelli and
Hawkins 1996).