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Opuntia polyacantha Haw. (plains prickly-pear) is a common cactus in the Great Plains of North America. We used two data sets, from Montana and Colorado, to test the hypothesis that there is a range of precipitation event sizes upon which O. polyacantha specializes. Events smaller than this range (>2 to ≤6 mm) do not moisten sufficient soil to be utilized, and larger events have negative effects on the status of O. polyacantha because they favor the development of taller and denser grass canopies. Multiple regressions of either green cladode density (northern mixed prairie) or O. polyacantha frequency (shortgrass steppe) with precipitation event sizes indicated negative effects of large precipitation events on the yearly changes in the either density or frequency of O. polyacantha. We suggest that weather conditions in the Great Plains may cause O. polyacantha to be controlled almost entirely by light competition from grasses and other negative biotic effects.