The Persistence of Rare Prairie Grasses in Missouri: Environmental Variation Buffered by Reproductive Output of Sparse Species

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Abstract
Sparse species have large ranges and are found in a variety of habitats but are never abundant. Over a 9-yr study, four sparse species of prairie grasses in Missouri showed lower variation in culm production and seed set and were less likely to show reproductive failure than were common grasses of the same habitat. The sparse species flower in the spring and early summer, when the rainfall is less variable from year to year. In general, rainfall is a good predictor of reproductive output for the common grasses but not for the sparse. When exposed to both fluctuating and constant moisture gradients in the greenhouse, the sparse and the common grasses do not differ in growth response. Thus, the pattern of environmental fluctuations in moisture buffers the reproductive output of the sparse species. The common species track environmental fluctuations. The mechanism of the sparse species'' buffering is not intrinsic physiological difference in response to moisture availability but reproduction when the environment is more equitable. These data provide no evidence that the local persistence of the chronically small populations is precarious. Rather, the buffered reproductive response is best viewed as a mechanism that offsets a hazard of small populations, demographic stochasticity.