Functionally novel invasive predator eradicates herbivores of a littoral community

Abstract

<p>In the Archipelago Sea as in most other parts of the Baltic Sea, the bladder wrack (<em>Fucus vesiculosus</em>) is a foundation species of the littoral communities of the rocky shores. It sustains a community of epiphytic algae, herbivorous crustaceans and molluscs and various fish. Recently we have noticed a steep decline in the occurrence of the herbivorous crustaceans and molluscs in many sites in the Archipelago Sea. We hypothesise that a key factor contributing to this decline is the recent introduction of the Harris mud crab (<em>Rhithropanopeus harrisii</em>), which was first sighted in 2009 in this region. Importantly, because there are no native crabs in the northern parts of the Baltic Sea, the mud crab is a completely novel kind of predator in the ecosystem and the herbivorous crustaceans and molluscs may be particularly susceptible to it. Here, we document a dramatic decline of the typical herbivores occurring on the bladder wrack, possibly indicating an ongoing regime shift, by comparing our recent samples from across the Archipelago Sea with data collected a decade before the sighting of the mud crab. Moreover, we demonstrate a spatio-temporal association between the decline, particularly of the key herbivore species, the isopod <em>Idotea balthica</em>, and the establishment of the mud crab. We also present experimental evidence for a strong predator-prey -link between the mud crab and the isopod <em>I. balthica</em>. Finally, we discuss the possible consequences of the community change and scrutinise alternative explanations for our observations.<br></p>