Population Dynamics of Benthic Species and Shorebird Predation
327
by shorebirds. The two bivalve species have a planktonic stage during their
first life phase. In spring eggs are spawned into the water and after a few weeks
post-larvae leave the water column and settle on the sediments (Beukema
1993). Growth of individual bivalves mainly takes place in spring and
summer. Shell growth of M. balthica occurs entirely in the period between
March and July (Beukema et al. 1985). Most birds arrive from their breeding
areas at Balgzand in late summer or autumn, at a time of the year when the
productive season of the bivalves is over, and leave again in late winter. Hence
for the birds full dinner is served right at the beginning of the annual period
that they spend at Balgzand. The impact of bird predation on bivalve populations can thus be stated in terms of the depletion of their prey stocks in the
course of winter. After the birds have left, only the surviving bivalves will contribute to the renewal of the population. The important question is thus
whether the reduction in prey stocks has any consequences for the renewal of
the prey population in the next spring. Stated otherwise: does the reduction in
adult stock due to bird predation affect the number of bivalve recruits at the
next spat fall?
Recruitment was more variable from year-to-year in C. edule compared
with M. balthica. For M. balthica the ratio between the highest and the lowest
yearly recruitment equalled 22.9. For C. edule the same ratio was almost three
times higher: 66.4. Yet, a linear relationship between log recruitment and adult
stock at the end of the preceding winter (in terms of log density m- 2 ) was
lacking for M. balthica (n=26, F=0.19, P=0.67). The relationship in C. edule
was even negative (n=26, F=6.92, P=0.015). The adult stock was expressed as
numerical density (m- 2 ), but the use of alternative expressions like biomass
density (g/m2) did not make much difference. For neither of the two species
was recruitment auto-correlated (M. balthica, n=26, r 1 =-0.13, P>0.05; C.
edule, n=26, r J =-O.17, P>O.05), which is a requirement for testing the stockrecruitment relationship.
Adult fecundity is almost unrelated to succeeding recruitment (Honkoop et
al. 1999), and the lack of (or a negative) stock-recruitment relationship
implies that the survival probability from egg to recruit decreases with increasing adult stock. The log-ratio density of recruits/density of the adult
stock at the end of the preceding winter, which expresses this survival probability, indeed significantly decreased with increasing adult stock (Fig. 15.8;
M. balthica, n=26, F=9.69, P=0.005; C. edule, n=26, F=70.5, P
place up to the time of recruitment, that is, up to the time of the sampling
survey in the first summer. The increased mortality rate with increasing
stocks during this first part of bivalve life may occur in the egg phase, during
the early larval stages in the pelagic phase, or during or after the settlement
processes. Beukema (1982) found a density-dependent mortality during the
first benthic period, between the first of July and late August. Yet, not much
327
by shorebirds. The two bivalve species have a planktonic stage during their
first life phase. In spring eggs are spawned into the water and after a few weeks
post-larvae leave the water column and settle on the sediments (Beukema
1993). Growth of individual bivalves mainly takes place in spring and
summer. Shell growth of M. balthica occurs entirely in the period between
March and July (Beukema et al. 1985). Most birds arrive from their breeding
areas at Balgzand in late summer or autumn, at a time of the year when the
productive season of the bivalves is over, and leave again in late winter. Hence
for the birds full dinner is served right at the beginning of the annual period
that they spend at Balgzand. The impact of bird predation on bivalve populations can thus be stated in terms of the depletion of their prey stocks in the
course of winter. After the birds have left, only the surviving bivalves will contribute to the renewal of the population. The important question is thus
whether the reduction in prey stocks has any consequences for the renewal of
the prey population in the next spring. Stated otherwise: does the reduction in
adult stock due to bird predation affect the number of bivalve recruits at the
next spat fall?
Recruitment was more variable from year-to-year in C. edule compared
with M. balthica. For M. balthica the ratio between the highest and the lowest
yearly recruitment equalled 22.9. For C. edule the same ratio was almost three
times higher: 66.4. Yet, a linear relationship between log recruitment and adult
stock at the end of the preceding winter (in terms of log density m- 2 ) was
lacking for M. balthica (n=26, F=0.19, P=0.67). The relationship in C. edule
was even negative (n=26, F=6.92, P=0.015). The adult stock was expressed as
numerical density (m- 2 ), but the use of alternative expressions like biomass
density (g/m2) did not make much difference. For neither of the two species
was recruitment auto-correlated (M. balthica, n=26, r 1 =-0.13, P>0.05; C.
edule, n=26, r J =-O.17, P>O.05), which is a requirement for testing the stockrecruitment relationship.
Adult fecundity is almost unrelated to succeeding recruitment (Honkoop et
al. 1999), and the lack of (or a negative) stock-recruitment relationship
implies that the survival probability from egg to recruit decreases with increasing adult stock. The log-ratio density of recruits/density of the adult
stock at the end of the preceding winter, which expresses this survival probability, indeed significantly decreased with increasing adult stock (Fig. 15.8;
M. balthica, n=26, F=9.69, P=0.005; C. edule, n=26, F=70.5, P
place up to the time of recruitment, that is, up to the time of the sampling
survey in the first summer. The increased mortality rate with increasing
stocks during this first part of bivalve life may occur in the egg phase, during
the early larval stages in the pelagic phase, or during or after the settlement
processes. Beukema (1982) found a density-dependent mortality during the
first benthic period, between the first of July and late August. Yet, not much