| Empirical evidence from the last two decades has suggested that seed production increases with increasing plant density. I tested whether seed production for three bee-pollinated perennials increases with increasing patch size, neighbor distance and whether changes in seed production are further influenced by the surrounding floral neighborhood. I determined how pollinators (long- and short-tongued bees) respond to plant density, spatial distribution and floral neighborhood, and the consequences of their responses for the quantity and quality of offspring. A simple, game-theoretic model evaluated the role of plant-pollinator interdependence in the evolution of floral advertising.; Results show that edge effects negatively influenced the abundance of insect pollinators with negative consequences for seed production in the spring ephemeral, Claytonia virginica. In the prairie perennial, Penstemon digitalis, seed set per flower increased by 376% from the smallest to the largest Penstemon patches. Contrary to long-standing views, bees that visited proportionately more plants but fewer flowers per plant as a function of patch size increased seed set. Positive density dependence influenced both the quantity and quality of offspring in Lobelia siphilitica . Over a three-year period, seed set per flower from plants in rich floral neighborhoods was an average of 46.2% greater in near neighbor than in far neighbor arrays. Positive density dependent effects in degraded habitats were not found. Resulting offspring showed 28.3% greater mortality when maternal plants were from oldfield rather than prairie habitats.; Model results indicated that selection for floral advertising was most relaxed when plant competition for pollinators was low and when effective pollinators were dependent upon plants. Investment into floral advertising was highest when many plants competed for a few, ineffective pollinators.; These findings revealed that positive density dependence in pollination ecology influences both the quantity and quality of offspring. I have shown animal-pollinated plants in sparsely spaced aggregations and in degraded floral backgrounds can be a serious threat to reproduction. Pollinator abundance was only part of the mechanism for positive density dependence. For seed set to increase with plant density, pollinators needed to move between plants and visit proportionately fewer flowers per plant. |
