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Factors affecting pollination and seed set of a rare plant (Sidalcea malachroides)
Numerous studies have analyzed the effect of habitat fragmentation on plant pollinators and herbivores. These studies describe the decrease in pollinator services and seed set in small and isolated patches due to pollinator preference for large, contiguous patches. However, other studies have shown that small and isolated patches may escape herbivory due to dispersal limitations of many insect herbivores. Sidalcea malachroides (Maple-leaved checkerbloom) is a rare plant with a patchy distribution, which requires insect pollen vectors and experiences severe fruit loss to weevil larvae. To test the effect of patch size and isolation distance on pollen reception and pollen limitation, I measured pollen loads on stigmas and the difference in seed set for open and hand pollinated flowers in plants of various combinations of patch size and isolation. To examine the effect of patch reduction and isolation distance on herbivore damage, I scored the total number of damaged seeds per plant. I used multiple regression to analyze these three dependent variables against patch size, isolation distance and other potentially important variables, and separate GLM analysis to examine the effects of abiotic environmental factors. S. malachroides experiences a reduction in pollen reception and an increase in pollen limitation when it occurs in isolated patches. This effect is especially pronounced in small patches. The most striking result was the lack of pollination in small patches isolated by more than 17 meters. Plants in large patches received more pollen grains, but surprisingly, degree of pollen limitation was not related to patch size. Small patches and isolated patches experience reduced damage to weevil larvae; however the effect was weak in comparison to the effect of pollinator services. Since the distribution of S. malachroides is likely limited by seed production and dispersal, and small, isolated patches do not receive adequate pollinator service, care should be taken to consider patch size and isolation distances when analyzing the status of this plant and the effects of land management activities.