Pervasive impact of intertidal migrant top predators on species diversity and size structure of its prey may lead to spatial knock-on effects between ecosystems

In the current biodiversity decline, predators are often the first to disappear. Predators are thought to play a positive role in biodiversity maintenance as they prevent certain prey species in achieving dominance, relaxing resource competition among prey, hence promoting prey growth rates, prey coexistence and diversity. Losses of species at the highest trophic levels in communities may therefore cause extinctions and shifts in size structure at lower trophic levels. However, this role of predators is still often underappreciated, largely because many ecosystems have already lost their top-predators and/or human-induced disturbances now blur the positive predation effects. We therefore need in-depth studies of predation in the most pristine ecosystems that still exist on Earth to set benchmarks against which more disturbed systems can be evaluated. I propose to study the role of molluscivore shorebirds for intertidal benthic communities of two pristine ecosystems: Banc dArguin (Mauritania) and Barr al Hikman (Oman). Both ecosystems have striking similarities but differ in one key aspect: Banc dArguin is dominated by molluscivore species (mostly red knot Calidris canutus), while Barr al Hikman is dominated by worm-eating shorebirds. By experimentally excluding (Banc d?Arguin) or introducing (Barr al Hikman) knots in small-scale plots, effects on prey competition and biodiversity will be quantified. In this respect, recent pilot experiments are encouraging: prey abundance was higher within exclosures than in controls, while mollusc biodiversity showed the opposite pattern. This mimics the comparison between Banc dArguin and Barr al Hikman (the absence of molluscivore shorebirds coinciding with higher prey abundance and lower biodiversity) and the changes that have taken place at Banc dArguin during the last 30 years (fewer knots coinciding with higher prey abundance and lower biodiversity). This decline in knots wintering at Banc dArguin is thought to be due to devastating shellfisheries at the knots major stopover, the Wadden Sea. If so, then benthic community changes observed at Banc dArguin may in fact represent spatial knock-on effects of human disturbance in the Wadden Sea.