| dc.description.abstract | Food availability and food stock depletion by foraging populations can have substantial
effects on the population size. How these aspects combine is rarely studied on long time
scales, considering the complexity of ecological systems and the challenges to collect all
relevant data. Migratory red knots (Calidris canutus) have been studied in the Western Dutch
Wadden Sea over 24 years (from 1993 until 2017), and so has their and food availability.
Here, we examine the relationships between numbers of red knots and their food in the
Western Wadden Sea. First, the relation between red knot population size in the Vlie tidal
basin and the availability of edible sizes of the preferred prey species (Limicola balthica) and
an alternative prey (Cerastoderma edule) in is considered. The effect of prey quality, defined
as the flesh to shell ratio of the prey, on the red knot population size is also taken into account.
Furthermore, the relation between red knot numbers and elimination of prey in edible size
classes in the Vlie tidal basin is analysed. To validate the relations between red knots and
their food, the total available prey biomass and total calculated prey elimination are
compared with the theoretical energy requirements of all red knots counted in the Vlie tidal
basin each year. The numbers of red knots correlated positively with the amount of preferred
prey (L. balthica) available, whereas there was no such relationship for the alternative prey
(C. edule). Moreover, the number of red knots counted yearly correlated positively with mean
quality of the preferred prey. This implies that any change in the availability of high-quality
L. balthica will affect the red knot population. With increasing numbers of red knots, the
calculated elimination of the edible size classes of L. balthica also increased, whereas there
was no relationship with the estimated elimination of the alternative prey. The fraction of L.
balthica eliminated was constant with the yearly number of red knots in the Vlie tidal basin.
The fraction of eliminated prey was higher in edible prey size classes than overall, indicating
that the elimination could indeed be caused by red knots. In general, the total prey biomass
available and the total eliminated prey biomass in the Vlie tidal basin were of the same
magnitude as the estimated energy requirements of all red knots counted in the study area.
This work suggests strong causal links between the number of red knots in the Wadden Sea
and the population of L. balthica both for availability of prey and prey depletion by red
knots. This means that when there is insufficient high-quality prey available, a part of the red
knot population cannot reside in the Western Wadden Sea. | |
