Population Dynamics of Benthic Species and Shorebird Predation
319
a
LO
c 8
o
E.
E
:J
(/)
c
8 a
LO
a
a
a W
WI s W
50
100
production
W
150
Fig. IS.I. Annual consumption by
shorebirds C (g AFDM m- 2 ) versus
annual zoobenthic production P
(g AFDM m- 2 ) for various intertidal
areas (see Table 15.1). WWestern
Europe, S southern Europe and A Africa.
Line indicates the average elP ratio,
which equals 0.28
Another reason why the reliability of most of the published data should
be doubted is the short time span over which measurements of bird
numbers and benthic biomass were collected. The observation period
usually did not exceed a single year. However, even if the data were reliable
(direct methods applied over a long period), one might wonder whether
knowledge of the kind that 30 % of the overall production is consumed by
birds actually tells us much about the impact of bird predation on their prey
populations. It may say something about the amount of depletion of the prey
populations, but what does it mean for the dynamics and regulation of the
prey population, or for the structure of the benthic community as a whole?
Not much, we would say.
The question to what extent production of invertebrate biomass corresponds to consumption by birds can be stated in terms of population ecology. That is, what is the overall contribution of bird predation to prey
mortality? However, a much more interesting question in population dynamics is: what sort of relationship exists between prey mortality rate, on the
one hand, and densities of prey and predators, on the other? Not the mortality
level as such, but the density dependence of mortality rate is fundamental.
Studying this density dependence either requires long-term observations or
experiments at scales that are generally beyond reach.
15.3 The Balgzand Area: a Long-Term Study
Observational field studies on populations of both invertebrate prey species
and shorebird species have been carried out for about 30 years at the
Balgzand, a 45-km 2 tidal flat area in the westernmost Wadden Sea. We selected
319
a
LO
c 8
o
E.
E
:J
(/)
c
8 a
LO
a
a
a W
WI s W
50
100
production
W
150
Fig. IS.I. Annual consumption by
shorebirds C (g AFDM m- 2 ) versus
annual zoobenthic production P
(g AFDM m- 2 ) for various intertidal
areas (see Table 15.1). WWestern
Europe, S southern Europe and A Africa.
Line indicates the average elP ratio,
which equals 0.28
Another reason why the reliability of most of the published data should
be doubted is the short time span over which measurements of bird
numbers and benthic biomass were collected. The observation period
usually did not exceed a single year. However, even if the data were reliable
(direct methods applied over a long period), one might wonder whether
knowledge of the kind that 30 % of the overall production is consumed by
birds actually tells us much about the impact of bird predation on their prey
populations. It may say something about the amount of depletion of the prey
populations, but what does it mean for the dynamics and regulation of the
prey population, or for the structure of the benthic community as a whole?
Not much, we would say.
The question to what extent production of invertebrate biomass corresponds to consumption by birds can be stated in terms of population ecology. That is, what is the overall contribution of bird predation to prey
mortality? However, a much more interesting question in population dynamics is: what sort of relationship exists between prey mortality rate, on the
one hand, and densities of prey and predators, on the other? Not the mortality
level as such, but the density dependence of mortality rate is fundamental.
Studying this density dependence either requires long-term observations or
experiments at scales that are generally beyond reach.
15.3 The Balgzand Area: a Long-Term Study
Observational field studies on populations of both invertebrate prey species
and shorebird species have been carried out for about 30 years at the
Balgzand, a 45-km 2 tidal flat area in the westernmost Wadden Sea. We selected