| Summary: | Biodiversity is of critical importance to the quality of life on Earth. In light of rapidly increasing extinction rates in recent years, understanding threats to biodiversity such as habitat destruction, over-exploitation, and the introduction of invasive species is of utmost importance to conservation efforts. Invasive species, in particular, are a threat of great concern in aquatic environments, due, in part, to their potential role in facilitating introgressive hybridization between closely related species. In recent decades, such a case of hybridization has led to genomic extinction in a benthic-limnetic species pair of threespine stickleback (Gasterosteus aculeatus) in Enos Lake, British Columbia, following the appearance of invasive American signal crayfish (Pacifastacus leniusculus) in this waterbody. In an effort to shed light on the potential role of these crayfish in this loss of diversity, research was conducted using an intact species pair from Paxton Lake, BC to determine if crayfish exert meaningful impacts on sticklebacks through the disruption of i) male stickleback reproductive behaviours and/or ii) juvenile stickleback growth rates. The results of reproductive behaviour trials demonstrated that limnetic male reproductive behaviour frequency was suppressed to a greater degree than that of benthic males in the presence of crayfish. This result suggests that crayfish disruptions may reduce conspecific mating opportunities for limnetic females, thus leading to increases in introgressive hybridization between benthic males and limnetic females. The results of the juvenile growth rate trial, on the other hand, demonstrated that crayfish do not disrupt growth rates of juveniles; however, significantly higher stickleback mortality levels were detected in the presence of crayfish. I conclude that invasive American signal crayfish have likely facilitated introgressive hybridization between the Enos Lake species pair, but other factors may have contributed to this loss of biodiversity. My study highlights the sensitivity of recently differentiated species to disruption by invasive species. By identifying important sources of ecological disruption and significant threats to biodiversity, such studies are critical for guiding conservation efforts in Canada and worldwide. === Science, Faculty of === Zoology, Department of === Graduate
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