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Environmental determinants of threespine stickleback species pair evolution and persistence Ormond, Chad Indar


Southwestern British Columbia is unique in that it is apparently the only place on Earth with sympatric species pairs of threespine stickleback (Gasterosteus aculeatus). As such, these species pairs are listed as endangered and are protected under Canada’s Species at Risk Act. Historically these species pairs inhabited seven known lakes but their distribution is currently limited to five lakes in three watersheds. Within these lakes stickleback are found in two forms: benthic and limnetic. These species pairs are thought to have originated through a double-invasion process followed by character displacement. While this hypothesis is supported by morphology and genetics it does not take into account the environmental factors needed to support threespine stickleback in two separate microhabitats, which may be essential for their continued persistence. It is conceivable that the divergence and persistence of the threespine stickleback species pairs may be due in part to unique environmental characteristics of the lakes they inhabit, including physical lake characteristics, water chemistry, or biological productivity. Alternatively, divergence and persistence could be due to specific attributes of fish community structure or random colonization events unrelated to lake attributes. To establish this, I measured a number of abiotic (water chemistry, physical lake parameters) and biotic (food resources, macrophyte abundance) variables from both species pair and non-species pair threespine stickleback lakes in order to identify any factors that may discriminate species pair lakes from non-species pair lakes, thereby leading to a clearer understanding of their critical habitat. While I did not find any clear environmental differences between species pair and non-species pair lakes, my study suggests that a major determinant of the existence of stickleback species pairs is the community of fish present in the lakes and that nesting habitat heterogeneity and its influence on pre-mating reproductive barriers may be important to the persistence of these species pairs. My study provides a baseline to monitor the environments for all known species pair lakes and suggests that the fish assemblage in stickleback lakes may affect resource availability as strongly as the limnological attributes of the lakes themselves although predation may also be an important factor in the evolution of these species pairs.

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