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Disentangling drivers of colonization success in laboratory and natural systems




Vahsen, Megan, author
Hufbauer, Ruth, advisor
Brown, Cynthia, committee member
Hobbs, N. Thompson, committee member

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Understanding why colonizing populations successfully establish is important for predicting dynamics of invasive species. Propagule pressure, or the number of individuals in a founding group, is considered the most consistent predictor of establishment success, however, there remains considerable variance around predictions that demography alone cannot explain. The identity of individuals within a founding group (e.g. level of pre-adaptation to the recipient environment, diversity) as well as how individuals are introduced (e.g. frequency and timing of discrete introduction events) can influence establishment. The relative importance of these factors is unclear, and could vary across species and environmental contexts. To address these inconsistencies, we conducted two experiments: one with Tribolium castaneum (red flour beetle) populations maintained in controlled laboratory conditions, and one with Bromus tectorum (cheatgrass) founding populations introduced to a natural environment. For the Tribolium experiment, we varied the level of prior adaptation, diversity, and introduction frequency and timing for groups of eggs colonizing in a novel environment across three levels of propagule pressure (n = 15, 30, 60). Founding groups that were larger and more adapted to the novel environment survived the founding event better than smaller and less adapted groups. Further, we found that a high frequency of smaller introductions reduced initial survival. After a generation of mating, establishment success was driven predominantly by adaptation to the novel environment and diversity of founders. In the second experiment, we introduced groups of B. tectorum seeds at a constant propagule pressure (n = 32) to a common garden in Colorado, varying in source diversity (1, 2, 4, 8, or 16 source populations) and source region (Colorado = pre-adapted or Nevada = unadapted). We evaluated establishment success by deriving the number of seeds produced by each founding group after one generation of growth and reproduction using a hierarchical Bayesian model. We found that increasing source diversity increased the number of seeds produced per founding group, but source region did not influence establishment success. Results from these experiments particularly speak to the context-dependency of the importance of pre-adaptation and diversity in predicting establishment success. This suggests that propagule pressure alone is not enough to explain why founding populations establish.


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invasive species
genetic diversity


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