Predators shape the behavior and distribution of prey organisms, driving their evolution and environmental impact. We studied relationships between interspecific competition and predation risk in Ponto-Caspian invasive gammarid crustaceans: Pontogammarus robustoides and Dikerogammarus villosus. We hypothesized that a stronger competitor D. villosus would displace P. robustoides from a preferred habitat, but the frequency of this displacement would be reduced in the presence of a higher fish predator, the racer goby. We studied gammarid preferences for stone or sand substrata in 24-h pairwise-choice tests in single-species or mixed-species treatments, at 2 densities (12 or 24 individuals per tank), in the presence or absence of a predator. D. villosus displaced P. robustoides from the stone habitat, even at the low density. As P. robustoides tested separately preferred stones at both densities, its displacement resulted from active avoidance of a stronger competitor even when the substratum was not actually a limited resource. The presence of a predator reduced the number of instances of displacement of P. robustoides by D. villosus, allowing the weaker species to stay on its preferred substratum. Nevertheless, the presence of D. villosus still increased fish predation on P. robustoides. The presence of a predator modifies relationships between prey organisms, allowing a weaker competitor to stay in its preferred habitat. Thus, antipredator responses of prey organisms may have cascading effects on the functioning of the community, affecting habitat choice and competition among species.

Pontogammarus robustoides has spread across Europe since the second half of the twentieth century. It is often regarded as a psammophilous and/ or phytophilous species. We studied preferences of P. robustoides for 5 mineral substrata of different grain sizes, 5 plant species and 3 artificial plants differing in leaf shape. We tested 10 gammarid individuals in 24-h pairwise choice tests in darkened tanks. Adults selected stones of 27 ± 4.8-mm grain diameter, rather than smaller or larger objects, whereas juveniles preferred particles of 15 ± 3.2-mm grain diameter. Structurally complex plants (natural and artificial) were most preferred for both age groups. Juveniles, unlike adults, preferred natural plants over artificial ones and mineral substrata. Adults preferred mineral materials over macrophytes and did not differentiate between natural and artificial plants. Juveniles did not avoid areas occupied by adults, but in their presence exhibited significantly stronger preferences for substrata providing them with suitable shelters. Habitats selected by gammarids in our study somewhat differed from those commonly used in the field, indicating their high plasticity. Different preferences of adults and juveniles may help them avoid competition and/or cannibalism in the field.

The racer goby is an invasive Ponto-Caspian fish spreading throughout Europe. They threaten a native species of similar biology, the European bullhead, by displacing them from shelters. These shelters are necessary for reproduction as well as for protection against predators and hydrodynamic forces. However, abiotic conditions may strongly modify the outcome of an interspecific competition in the wild. Nevertheless, little is known of the effect of flow velocity on the competition between these rheophilic species, although this factor is crucial for their distribution in the field. We video-recorded fish behaviour for 2 h in single-species and mixed-species pairs in the presence of single shelters at three flow velocities: 0, 10 (a velocity preferred by the racer goby) and 30 cmÁs À1 (a velocity greater than preferred by the racer goby) to determine whether the invader can deprive the native species of its shelter. At the flow of 0 and 10 cmÁs À1 , the racer goby exhibited aggressive behaviour towards bullhead, and this restricted the time spent by the bullhead in the shelter. Moreover, although the flow of 30 cmÁs À1 inhibited racer goby aggression, the time spent by the bullhead in the shelter in interspecific competition was still reduced when compared to intraspecific controls. Our results suggest that under natural conditions, the racer goby displace bullheads from their shelters even at flow velocities greater than optimal for the racer goby.