Integrity of predator assemblages controls the abundanceof the alien crab Percnon gibbesi

Ecosystem resistance to species invasion is considered to be related to the abundance anddiversity of native species (i.e. diversity-resistance hypothesis). In particular, the integrity ofpredator assemblages may enhance the control of prey populations through direct and indirectinteractions (e.g.: different predation strategies by different predators, facilitative interactionsamong predators). Depletion of predators due to overfishing is therefore expected to enhancethe abundance and ultimately the spread of alien prey species. Manipulative field experimentswere performed to evaluate the effects of different abundance and diversity of predator assemblageson the invasion success of the alien crab Percnon gibbesi. The experiments wereconducted in northwestern Sicily (Italy, Mediterranean Sea), in summer 2015, on bare rockyboulders, the habitat preferred by P. gibbesi. The study sites were characterized by significantlydifferent species richness and abundance of the potential predators of P. gibbesi. Predationrate of different predator assemblages was assessed by means of tethering experiments on thealien crab. As predicted, predation rate on P. gibbesi resulted significantly higher in sites characterizedby higher predator abundance and species richness than in sites with lower values.Our findings, although restricted to small spatial scales, support the diversity resistance hypothesis,revealing a negative relationship between the diversity of predator assemblages andthe invasiveness (average abundance) of the alien species P. gibbesi.