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Simple Summary: Freshwater crayfish play critical roles in their ecosystems; however, non-native species are increasingly threatening native crayfish populations worldwide. Here, we review the literature examining or documenting the impacts of invasive freshwater crayfish and other non-native species on native freshwater crayfish. Four key mechanisms through which invasive crayfish impact native crayfish were identified: competition, predation, introduction of disease, and reproductive impacts, while other non-native species primarily impact native crayfish through predation. We highlight a need for more field-based investigations examining competitive interactions between invasive and native crayfish and identify a deficit of monitoring efforts for many non-native species that are likely to substantially impact native crayfish populations. Overall, the findings underscore the profound and diverse threats posed by non-native species to native crayfish, necessitating further research to safeguard native crayfish populations. Freshwater crayfish are vital species in ecosystems where they naturally occur, as they hold keystone and ecological engineering positions in these systems. Non-native species are common and widely spread throughout Earth's freshwater ecosystems and can have severe impacts on native crayfish populations. There has yet to be a comprehensive global review of the impacts of non-native species on native crayfish. Two literature searches were conducted using Web of Science and Google Scholar to find articles to address four key aims: (1) summarise trends in the literature; (2) examine the mechanisms by which invasive crayfish impact native crayfish species; (3) examine the mechanisms by which other non-native species, such as fish, impact native crayfish species; and (4) identify gaps in knowledge and research priorities. This review highlights that a far greater amount of research has addressed the effects of invasive crayfish than other non-native species. The research on invasive crayfish focuses on four types of interactions with native crayfish: competition, predation, introduction of disease, and reproductive impacts. Studies addressing the impacts of other non-native species on crayfish indicate that predation and habitat destruction by these species are the key processes impacting native crayfish. It is evident that field-based research, particularly concerning competition between invasive and native crayfish, is limited. Therefore, further in situ research is needed to assess the validity of laboratory results in a natural setting. Additionally, in many cases, the impact of certain non-native species on native crayfish populations has gone unmonitored. For this reason, it is recommended that additional research focus on assessing the impact of these non-native species. To conclude, the impacts of invasive crayfish on native crayfish are profound and wide-ranging, often leading to population decline or extirpation. Further, other non-native species are also likely to have a highly deleterious impact on native crayfish populations; however, more research is required to understand the scope of this impact. [ABSTRACT FROM AUTHOR]

Simple Summary: Aggressive invasive species can outcompete native species in contests over resources, which can lead to the exclusion of a native species by the invader. Invasive freshwater crayfish are often more aggressive than their native counterparts, however contests between invasive and native crayfish are typically investigated under laboratory conditions and are rarely examined in a natural setting. We used a baited underwater camera in a stream located in New South Wales, Australia to examine behavioural interactions between an invasive crayfish and a critically endangered native crayfish to determine which species was dominant. We found the native species dominant over the invader when larger, however when the species were size-matched the dominance of the native was lost and neither species exhibited a significant pattern of dominance. This outcome indicates the invasive crayfish represents a threat to the native since it may be able to outcompete the native over resources. Further, this outcome contrasts to previous laboratory findings, highlighting the importance of field observations in establishing the extent of impact an invader may be having on a native species. Competitive behavioural interactions between invasive and native freshwater crayfish are recognised as a key underlying mechanism behind the displacement of natives by invaders. However, in situ investigations into behavioural interactions between invasive and native crayfish are scarce. In Australian freshwater systems, the invasive Cherax destructor has spread into the ranges of many native Euastacus species, including the critically endangered Euastacus dharawalus. Staged contests between the two species in a laboratory setting found E. dharawalus to be the dominant competitor, however, this has yet to be corroborated in situ. Here, we used baited remote underwater video (BRUV) to examine in situ intra- and inter-specific behavioural interactions between E. dharawalus and C. destructor. We sought to evaluate patterns of dominance and differential contest dynamics between the species to provide indications of competition between the two species. We found E. dharawalus to be dominant over C. destructor based on pooled interspecific interaction data and size-grouped interactions where C. destructor was the smaller opponent. Alarmingly, however, when C. destructor was within a 10% size difference the dominance of E. dharawalus was lost, contrasting with the outcomes of the laboratory-staged study. In addition, we report that small C. destructor initiated significantly more contests than larger conspecifics and larger E. dharawalus, a pattern that was not observed in smaller E. dharawalus. Further, intraspecific interactions between C. destructor were significantly longer in duration than intraspecific interactions between E. dharawalus, indicating a willingness to continue fighting. Concerningly, these outcomes point towards inherent and greater aggressiveness in C. destructor relative to E. dharawalus and that only larger E. dharawalus hold a competitive advantage over C. destructor. Therefore, we conclude that C. destructor represents a substantial threat to E. dharawalus through competitive behavioural interactions. Further, due to the disparity between our findings and those produced from laboratory-staged contests, we recommend the use of in situ studies when determining the behavioural impacts of invasive crayfish on natives. [ABSTRACT FROM AUTHOR]

Aggressive encounters between invasive and native species are considered a key threat associated with the spread of invasive species. Extrinsic factors such as habitat complexity can profoundly influence the outcome of aggressive interactions between conspecifics, and this may also be the case in invasive‐native species interactions. This study used invasive and native freshwater crayfish species in Australia as a model system to investigate the influence of habitat complexity on aggression in both intraspecific and interspecific contests. Paired contests between the native Euastacus spinifer and the invasive Cherax destructor were conducted in the laboratory under high and low habitat complexity conditions. Furthermore, the influence of prior residency was examined, with one individual assigned the resident and the other as the intruder. In intraspecific contests, the number of interactions and aggression score were significantly reduced under high complexity conditions; however, complexity had no effect on the time spent interacting and contest outcome. Instead, the two crayfish species differed in terms of time spent interacting. In interspecific contests, complexity had no effect on any of the aggression metrics. Alarmingly, however, we demonstrate that when C. destructor was a resident the likelihood that E. spinifer won a contest fell to a mere 8% compared to 31% when E. spinifer was resident. This study suggests that while habitat complexity plays a role in shaping aggressive behaviour in intraspecific interactions, invasive species are competitively dominant if they are already established i.e. resident in a location and even as intruders. Therefore, preserving habitat complexity in freshwater systems may not be sufficient to sustain populations of native species, and other measures that mitigate the potential for range overlap between the species would also be needed to prevent the further spread of invasive species such as C. destructor within freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Population densities of invasive species fluctuate spatially and temporally, suggesting that the intensity of their aggressive interactions with native species is similarly variable. Although inter‐specific aggression is often thought to increase with population density, it is often theorized that it should be exceeded by intra‐specific aggression since conspecifics share a greater degree of resource overlap. Yet, the magnitude of intra‐specific aggression is seldom considered when examining aggressive interactions, particularly those between invasive and native species. Here, we manipulated the density of the invasive eastern mosquitofish, Gambusia holbrooki, and observed its aggressive interactions with juveniles of the native Australian bass, Macquaria novemaculeata in a laboratory setting. For both species, the magnitudes of intra‐ and inter‐specific aggression were recorded. Regardless of density, the native M. novemaculeata was more aggressive towards heterospecifics than G. holbrooki was. In addition to this, M. novemaculeata was more aggressive to G. holbrooki than towards conspecifics, at both low‐ and high‐density conditions. In contrast, G. holbrooki was similarly aggressive towards M. novemaculeata and G. holbrooki at a high density, yet at low density, displayed significantly more aggression towards conspecifics than M. novemaculeata. These findings demonstrate the importance of considering intra‐specific aggression when exploring behavioural interactions between native and invasive species. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]