Long-term shifts in intertidal predator and prey communities in the Wadden Sea and consequences for food requirements and supply

Abstract

Fluctuations in species composition can have major effects on the functionality of an ecosystem; however, studying such impacts is often complicated because fluctuations coincide with changes in other parts of the ecosystem. In this study, we explored long-term (1975 to 2014) changes in predator-prey interactions following concurrent changes in predatory epibenthic fish and crustaceans as well as their prey, macrozoobenthic bivalves and polychaetes, in the western Wadden Sea. Historical and recent invasions have resulted in an increase in relatively large and long-lived bivalves (Mya arenaria, Ensis directus, Crassostrea gigas) which have found a size refuge from epibenthic predators in the Wadden Sea. While bivalves dominated the macrozoobenthic biomass, polychaetes were the main food source of epibenthic predators, with the invasive polychaete Marenzelleria viridis probably becoming an important food source during the early 2000s. Food requirements of epibenthic crustaceans, mainly Crangon crangon, almost doubled from 5 to 10 g ash-free dry mass (AFDM) m-2 yr-1, and requirements of epibenthic fish decreased by more than 80%, mainly due to the local disappearance of Pleuronectes platessa. While the overall food requirements of the epibenthic predators stayed more or less constant, the edible fraction of the macrozoobenthic biomass increased from 5 to 20 g AFDM m-2 until the 2000s and decreased to 10 g AFDM m-2 thereafter. This was the result of changes in native (Nereis diversicolor and Heteromastus filiformis) and invasive (M. viridis) polychaetes. These findings illustrate that coinciding species-specific changes and interactions of both predators and prey should be taken into account to determine the impact of invasions on the food web structure and functioning of coastal systems.