Year

2022

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric, and Life Sciences

Abstract

Freshwater ecosystems are often extensively modified and impacted by anthropological activity. Therefore, many freshwater species encounter considerable risk of extinction. In Australian freshwaters, one taxon facing substantial endangerment from multiple stressors is the endemic freshwater crayfish genus Euastacus. However, in-situ investigations to examine the effect of these impacts on Euastacus populations are scarce. The critically endangered Fitzroy Falls spiny crayfish (Euastacus dharawalus) is facing considerable risk of extinction. This species is endemic to only one stream which has been subject to extensive disturbance through habitat destruction, modification, and non-native species introductions, including the highly invasive common yabby (Cherax destructor). However, the extent to which these threatening processes impact E. dharawalus is not known. Specifically, the in-situ competitive effect of C. destructor remains undetermined. Therefore, I aimed to address these knowledge gaps by using a variety of indirect and direct in-situ methods. In the first section of this thesis, I systematically review the literature to determine the key mechanisms through which invasive freshwater crayfish and other introduced species impact native freshwater crayfish as well as identify knowledge gaps in this area. I report that invasive crayfish affect native crayfish extensively through competition, predation, reproductive interference, and the introduction of disease, whereas other introduced species primarily impact native crayfish through predation. I also found the literature to be dominated by laboratory-based studies with little focus on field-based research. In the following chapter, I examine the history of the study system to provide context for this thesis and pinpoint key events of ecological disturbance. I also assess the efficacy of population control efforts on the abundance of C. destructor in the study system. Concerningly, I document numerous instances of historical disturbance to the system including the construction of a reservoir across the stream and continuous introductions of non-native freshwater species. Further, I report that invasive species eradication efforts had no significant effect on the abundance of C. destructor. In the following two chapters, I examine the degree of overlap in the activity and habitat-use patterns, diet, and trophic positions of E. dharawalus and C. destructor to assess the likelihood of competition between the species. Further, I perform a 12-month targeted population control of C. destructor to determine if the activity, habitat-use, diet, or trophic position of E. dharawalus shifts in the predominant absence of the invader. I found evidence of substantial overlap in the activity and habitat use patterns of E. dharawalus and C. destructor, further, both species occupied a similar trophic position and consumed similar proportions of the same dietary sources. Following the control of the C. destructor population, the activity of E. dharawalus was significantly reduced potentially indicating a diminished need to pursue resources in the invaders’ absence. Further, I report shifts in the diet of E. dharawalus following the removal of C. destructor; consistent with the two species competing over food resources in sympatry. In the final section, I observe in-situ interspecific and intraspecific behavioural interactions between E. dharawalus and C. destructor to evaluate patterns of dominance and differential contest dynamics between the species. I found E. dharawalus only exhibited dominance over C. destructor as a larger opponent, but when size-matched the dominance of E. dharawalus disappeared. I also report behavioural differences between the species that indicate greater relative aggression in C. destructor when contrasted with E. dharawalus. Together, the outcomes presented in this thesis indicate extensive competitive pressure by C. destructor on E. dharawalus in-situ, raising substantial concerns about the presence and further proliferation of C. destructor throughout the ranges of this and other Euastacus crayfish species. This work represents an important step towards the long-term management of this genus and the preservation of Australia’s endemic freshwater crayfish.

FoR codes (2008)

050103 Invasive Species Ecology, 050202 Conservation and Biodiversity, 050205 Environmental Management, 060204 Freshwater Ecology

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.