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Introduction The rapid range expansion of many invasive crayfish species resulted in the increasing co-occurrence of these invaders in freshwater ecosystems of Europe. The two most successful crayfish invaders of European freshwater ecosystems – the signal crayfish (Pacifastacus leniusculus) and the spiny cheek crayfish (Orconectes limosus) already co-occur in European water bodies or are destined to encounter each other in the near future. Despite their prevalence throughout Europe, the effects that these invaders could exert on each other and the effects of such interaction outcomes on their further invasion are poorly understood. Throughout this study we determined the distribution, dispersal rates and potential future co-occurrence of these invaders in Croatian water bodies through an intensive fieldwork covering over 220 sites in the Mura, Drava and the Danube. Also, we examined potential competition outcomes between the two invaders through pair-wise and group scale experiments. Results and discussion Distribution research showed that P. leniusculus is spreading downstream through the Mura and has entered the Drava river in 2011. Since O. limosus is spreading upstream through the Drava in addition to its downstream through the Danube, both invaders are expected to encounter each other in the Drava River in near future. The downstream dispersal rate of P. leniusculus recorded within this study was the highest in Europe and exceeded approximately four times the rates recorded in Austria, from where Croatian P. leniusculus populations originated. Experimental analyses showed that in all trials, P. leniusculus demonstrated the potential to outcompete O. limosus in both staged encounters and direct resource competition, being more likely to win heterospecific agonistic encounters and to acquire shelters at a higher rate. Observed dyadic dominance was translated to a broader social context of group-scale experiments, in which dominance of P. leniusculus was further strengthened by size differential between species. O. limosus was not able to compensate for competitive pressure by the dominant P. leniusculus and suffered wet weight loss and more frequent injuries in the presence of P. leniusculus. While both species are detrimental to native ecosystems, the ability of P. leniusculus to withstand competition pressure from another successful invasive species underscores its potential to establish dominant populations. Our results highlight the importance of understanding interspecies competition in prioritizing potential management activities or control efforts in future contact zones in Croatia as well as existing and future contact zones throughout Europe. |
