Insight into invasion: Interactions between a critically endangered and invasive crayfish

Invasive species are known to competitively displace native species via direct behavioural interactions, or contests, which can result in limited access to valuable resources such as food or shelter for the native. Here, we aimed to determine the dynamics and outcome of agonistic contests between the critically endangered Fitzroy Falls Spiny Crayfish (Euastacus dharawalus) and the invasive yabby (Cherax destructor) in a controlled laboratory setting. Although C. destructor has been listed as a key threatening process for E. dharawalus, the magnitude and nature of its impact have not been formerly assessed. We recorded the frequency and intensity of aggressive interactions between the species, as well as the winner of the contest and subsequent dominance during staged encounters. In addition to this, we recorded the time individuals spent consuming food and actively moving on the substratum. Contrary to our predictions, the frequency of aggressive behaviours performed by the two species did not significantly differ, but E. dharawalus engaged in more intensely aggressive behaviours and was less submissive than C. destructor. As a result, E. dharawalus won significantly more contests than its invasive rival and subsequently achieved a higher dominance score overall. Despite this result, C. destructor spent significantly more time consuming food than did E. dharawalus. These results suggest that dominance in agonistic contests did not translate into dominance over food resources in our study. Ultimately, reduced access to food, combined with the costs of agonistic contests including injury and the relatively low reproductive rates of E. dharawalus indicate that C. destructor may have a negative effect on the survival and persistence of the critically endangered E. dharawalus