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Biological invasions are among the leading causes of species diversity loss; however, the impacts of invasion are context-dependent and can vary with the local environment. The mechanisms governing variation in impact and their relationship to specific abiotic and biotic factors remain largely unexplored. Recent local declines in native unionid mussels have been attributed to the invasion of North American lakes and rivers by the Eurasian zebra mussel ( Dreissena polymorpha), as a result of intense fouling of unionid shells by zebra mussels. My research investigated the role of abiotic and biotic factors in mediating the impact of zebra mussels on native mussels. I examined the impact of zebra mussels on unionids in a habitat thought to be suboptimal for zebra mussels and compared this to the impact observed in other invaded habitats. A predictive model relating fouling intensity to local environmental variables (calcium concentration, sediment particle size) was developed, and a predator-exclusion experiment was conducted to investigate the role of predation in mediating fouling intensity. Overall, I found that two abiotic factors of the local environment were significant predictors of fouling intensity and that relationships used to predict the impact of zebra mussels could be extended to a broader range of habitats.

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Gabriel Christie (1722-1799), a British military officer, acquired a vast estate in Quebec after the Seven Years war, including five timber-rich seigneuries in the Upper Richelieu Valley, our study area. These were inherited by two of his sons in succession: Napier Christie Burton (1758-1835) and William Plenderleath Christie (1780-1845). An examination of the available deeds of concession for our study area shows the legal framework of the tenure and the seigneurs' survey and land granting policies. Seigneurial rents increased between 1785 and 1820, but it was the accumulation of seigneurial arrears, followed by strict collection practices after 1835, which contributed most to social stratification and unrest. A seigneurial monopoly on mill construction and the use of water power was decentralized after 1815 so that manufactures were established by entrepreneurs with capital who acquired a share of the seigneur's rights through patronage. The seigneur's role in regional development--the rise of villages, settlement, and industrial growth--was significant particularly as a system of clientage which helped shape the social structure.

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