Fish predators maintain estuarine biodiversity and benefit ecosystem engineers

Abstract

Loss of top predators may lead to the proliferation of mesopredators (i.e., intermediate consumers), with significant consequences for entire food webs. Termed mesopredator release, this process is typically attributed to a decline in the abundance of top predators. We investigated the potential for moderate environmental changes, that disrupt sensing abilities, to trigger mesopredator release by diminishing the foraging ability of top predators without affecting their abundance. In estuaries, fishes occupy the upper trophic levels and many species rely on visual cues to forage. We hypothesized that increased turbidity would attenuate fish foraging ability, increase the abundance of crabs and other mesopredators, and significantly alter coastal food webs. In oyster reef communities, turbidity triggered mesopredator release in 2016 and 2017, even though freshwater inflow and ambient salinity varied significantly between years, suggesting that turbidity’s effects on estuarine food webs are robust. Following experiments in 2017, our field site was struck by Hurricane Harvey, a category 4 storm. Oyster mortality was high following Harvey due to low salinity and trophic reliance on oysters as a basal resource. Hurricane Harvey removed human fishing pressure, causing fish populations to increase dramatically, resulting in stronger top-down control on crabs that are the primary predator of newly settled oysters. We also investigated the potential for barnacles as accessible and biologically relevant flow indicators since hydrodynamics are key regulators of speciesinteractions. In summary, fish predators are important components of oyster reef ecosystems, enhancing biodiversity and, at times, benefiting oysters through a trophic cascade depending on the environmental context. Turbidity and fishing reduce top down control by fishes, leading to increases in the abundance of intermediate consumers, less diverse reef communities, and often damaging juvenile oysters.