In this dissertation, we explored three issues central to the understanding of the dynamics of the invasions of non-native crayfish. First, we investigated how the spread and abundance of non-native crayfish are influenced by biotic and abiotic factors, in the context of a river restoration project. Second, we tested whether roundtail chub, an Arizona native fish, demonstrate as much predatory pressure on invasive crayfish compared to small mouth bass, a non-native sports fish. Third, we investigated the consequences of crayfish spread by testing if crayfish reduce emerging aquatic insects.
In the first chapter, we examined the response of a spreading and invading non-indigenous crayfish population to a stream restoration project in travertine forming Fossil Creek, Arizona. We tested three predictions: 1) Increased flow would increase crayfish by providing more shallow riffles and backwater areas for juveniles. 2) Travertine deposition would decrease crayfish by cementing the substrate making it difficult for crayfish to burrow. 3) Changes in fish assemblage from non-native to native would reduce predation pressure on crayfish causing an increase in crayfish abundance. In contrast to our predictions there were no large increases in crayfish abundance following restoration. More detailed observations of different reaches, however, indicates that fish species and density exert some regulation over crayfish. Crayfish abundance decreased following restoration where exotic fish remain but slightly increased where they were removed. Additionally, areas in the river with increased travertine deposition had the largest declines following restoration. These observations suggest that crayfish abundance in this system is limited by substrate modification by travertine, to a lesser extent by predator regulation from fish, and interactions with flow.
In the second chapter, we compared the predatory effects of a native fish, roundtail chub (Gila Robusta), to the non-native, predatory smallmouth bass (Micropterus dolomieu) on the exotic northern crayfish (Orconectes virilis). The investigation occurred in Fossil Creek and addressed three questions: 1) Are native roundtail chub as effective predators on invasive juvenile crayfish as non-native smallmouth bass? 2) Will the removal of the bass have unexpected management implications due to increases in crayfish? 3) Is the benthic invertebrate community affected by differences in the assemblage of fish and crayfish? Two in-stream enclosure experiments tested the survival of juvenile crayfish in the presence of each of the two fish species with cage controls without fish. Crayfish abundance and benthic macroinvertebrates were compared between a section of the stream above a fish barrier where non-native species were removed to a section of stream directly below the barrier where they remained. The results of the enclosure experiments showed that non-native bass more strongly reduced juvenile crayfish than native roundtail chub. Crayfish abundance was also reduced in the stream reach containing bass compared to the reach composed of the native fish, despite similar environmental conditions. These results support the hypothesis that the native roundtail chub have reduced predatory pressure on crayfish abundance. Contrary to our predictions there were no significant differences in the benthic macroinvertebrate community suggesting that abiotic factors are more important than fish and crayfish in structuring this assemblage.
In the third chapter, we conducted two experiments using in stream enclosures to determine if the invasive crayfish Orconectes virilis in Arizona reduced the density and biomass of different groups of emerging aquatic insects. The two experiments occurred in different stream reaches that varied in geomorphic features and benthic community composition. One reach consisted of a riffle-pool morphology typical of many southwestern streams and an insect community composition dominated by dipterans. The other reach was characterized by travertine dam formation and a step-pool morphology with a more diverse benthic insect community. Crayfish had the greatest negative impact on the emergence density of mobile non-dipteran insect taxa and larger dipterans leading to declines in overall emergence biomass. Sedentary sediment dwelling taxa were relatively not impacted. We were not able to determine the exact mechanism of reductions but a disruption of insect colonization could have been due to either predation or dislodgement from the foraging activity of the crayfish. The large decline in biomass and a shift in insect emergence composition towards smaller bodied taxa rather than larger bodied taxa could have implications for insect predators in the riparian environment. Crayfish effects were not sensitive to reach differences in geomorphology or other environmental factors and were strikingly similar across both experiments. Relatively minimized impacts by high densities of crayfish indicated some type of density dependence or interference.
|Advisor:||Marks, Jane C.|
|Commitee:||Gibb, Alice C., Theimer, Tad, Ward, David|
|School:||Northern Arizona University|
|School Location:||United States -- Arizona|
|Source:||DAI-B 75/09(E), Dissertation Abstracts International|
|Keywords:||Aquatic insect emergence, Aquatic insects, Crayfish, Fish predation, Fossil creek, Invasive|
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