24 years of red knot numbers and the relation with their prey in the Western Wadden Sea
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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.