Consequences of patch size and isolation for a rare plant: Pollen limitation and seed predation

In the relic prairies of the Pacific Northwest, sexual reproduction of a rare endemic plant, Aster curtus Cronq. (Asteraceae, white-topped aster), is generally poor and highly variable. This perennial is patchily distributed, and the movement of pollinators within and between A, curtus patches may constrain the production of viable seeds. For a range of patch sizes and degrees of isolation, I compared the number of viable seeds produced by hand-pollinated ramets to that of ramets exposed to ambient levels of pollination. This experiment was repeated at four different sites, separated by at least 0.5 km. On average, exposed and hand-pollinated A. curtus ramets produced an equal number of viable seeds, suggesting that ramets were generally not pollen limited. However, ramets were pollinator limited in one of the four study sites. Finally, although exposed A. curtus ramets in small isolated patches produced just as many viable seeds as hand-pollinated ramets, reproductive ramets in large crowded patches were more prone to attack by seed predators. In sum, A. curtus seed viability varied from 0% to 83% per ramet, with pollinator limitation explaining some of the variation among sites, and seed predation explaining some of the variation among plant patches within sites.