Impact of predators and competitors on the depth selection by two invasive gammarids

Appropriate habitat selection driven by multiple environmental cues may help organisms avoid predation pressure and excessive competition. Moreover, identification of habitat selection mechanisms of invasive organisms is crucial for predicting their distribution and impact in novel areas. We evaluated experimentally the habitat selection by Dikerogammarus villosus and Pontogammarus robustoides (Gammaroidea, Amphipoda, Crustacea), invasive in European fresh and brackish waters, with regard to depth, the presence of predators, the racer goby Babka gymnotrachelus , and heterospecific gammarid competitors. We exposed gammarids in single‐ and mixed‐species treatments in a 100‐cm‐long depth gradient (depth: 10–100 cm, levels every 10 cm) or in a control shallow tank (constant depth: 10 cm) in the presence or absence of predators located behind a mesh barrier adjacent to the deeper side of the gradient. Dikerogammarus villosus always selected deeper locations than P. robustoides . Both species did not affect each other's depth selection, but P. robustoides limited the distribution of D. villosus and pushed it towards the mesh barrier in the shallow tank. Dikerogammarus villosus responded to predators only in the depth gradient, retreating to shallower sites and climbing towards the surface. Pontogammarus robustoides moved away from the predator area in the shallow tank, indicating that its response was independent of depth selection. Thus, different depth distributions of both gammarids result from their specific preferences rather than from competitive displacement. The observed behaviour is likely to result in their segregation in the field, limiting inter‐specific competition, allowing for optimum utilization of resources and decreasing predatory pressure.