Human encroachment on ecosystems is one of the key environmental stressors facing natural habitats. Understanding how individual species are affected by these disturbances is therefore fundamental to preserving the stability of vital ecosystem services, such as pollination. Roughly 87.5% of plants are animal pollinated, with pollination contributing over $175 billion to the global economy each year. Despite its ecological and economic importance, studies that have comprehensively evaluated how multiple components of pollination success are impacted by human disturbances are limited. Here we evaluate how different aspects of pollination success, including patterns of pollinator visitation, pollen removal (male fitness), and deposition, fruit and seed production (female fitness) vary across an urbanization gradient in populations of Asclepias syriaca (Common Milkweed). Preliminary data was collected on four naturally occurring Asclepias syriaca sites (Watauga Lake, Warrior’s Path, Lake Ridge, Jacob’s Nature Park) in Northeast Tennessee. Urbanization around each site was quantified using ArcGIS and the National Land Cover Database. Sites were visited every other day (5-12 visits per population) during the flowering season and data was collected on male reproductive success (proportion of pollinaria removed), female reproductive success (proportion of flowers receiving pollinaria), pollinator visitation rate and composition, fruit production, and seed set.
Results/Conclusions
Preliminary data on four Asclepias syriaca populations shows large among-site variation in male and female reproductive success, pollinator visitation rate, pollinator community composition, fruit abundance, and seed set. Specifically, the quantity and quality of pollen received varies between all four sites. Pollen quality was highest at Lake Ridge (mean 2.57 fruits per plant and 278 seeds per fruit), and lowest at Warrior’s Path (mean 1.54 fruits and 199 seeds), the most agricultural site. Male success was highest at Warrior’s Path (0.492) and lowest at Watauga Lake (0.308). Female success was approximately equal between three of the sites (between 0.208 and 0.179), but lower at Jacob’s Nature Park (0.072). There was significant among-site variation in pollinator community, not only in terms of composition, but also visitation rate. Visitation rate was highest at Warrior’s Path and Lake Ridge (0.214 and 0.283, respectively), and lowest at Watauga Lake and Jacob’s Nature Park (0.126 and 0.123, respectively). Interestingly, Jacob’s Nature Park and Watauga Lake represent the most and least developed sites, respectively, though the Watauga Lake population was adjacent to a major road. Among-site variation in pollination success is likely due to variation in pollinator community across the sites.
