Top-down control of macrofauna: Are waterbirds passengers or drivers in wetlands?

Large animals including waterbirds are traditionally considered passengers in wetland ecosystems and used as indicator, flagship or umbrella species in wetland conservation, but whether they are drivers of wetland ecosystem patterns and dynamics is poorly understood. Combining a field experiment in a tidal flat in the Yangtze estuary and a global meta-analysis, we tested the hypothesis that waterbirds exert top-down control on wetland macrofauna. Waterbird exclusion more than doubled the abundance of grazing crabs in our field experiment but did not affect other macrofauna. Conversely, excluding grazing crabs to simulate above-current levels of waterbird predation increased macrofauna abundance and biomass and decreased their diversity, ostensibly by allowing the establishment of habitat-engineering plants otherwise eliminated by crab grazing. Meta-analysis of 135 tests revealed that waterbirds had negative, positive, or neutral effects on macrofauna, with an overall significantly negative effect. This finding was evident across different seasons, wetland types and macrofauna classes and was largely based on tests from coastal rather than inland wetlands in Europe and Americas and none in Asia and Australasia. Our results suggest that the roles of waterbirds in wetlands are more diverse and profound than currently recognized and that their driver roles in shaping wetland ecosystems warrant incorporation into conservation decisions.