Good to the Last Drop? Nectar Depletion by Insect Pollinators Varies Among Plant Species and Through Time

Most insect pollinator species use floral resources exploited by many conspecifics and heterospecifics. In systems where floral resources limit pollinator populations and give rise to competition, understanding whether competition varies across the growing season is a fundamental question in explaining how species co-exist and how communities are structured. Recent work on nectar depletion rates in plant-pollinator communities has demonstrated the value of measuring nectar depletion rates as a proxy for competition among pollinators. One hypothesis on pollinator competition, hereafter, "equilibrium hypothesis," predicts that nectar depletion rates should be constant across the growing season within a given community. In contrast, an alternative hypothesis, hereafter, "dynamic hypothesis" suggests the opposite. To test these hypotheses, we sampled nectar depletion and floral abundance across an early successional plant community over the bulk of a growing season in central Kentucky, USA in spring/summer, 2023. We predicted that a dynamic ecosystem such as that studied here would most likely be characterized by dynamic depletion rates. Initially in the growing season, nectar depletion rates within this early successional vegetation community were moderate (e.g., 50%-70%) but steadily climbed to 85% as the growing season advanced. Plant species varied widely in depletion rate: invasive Lamium purpureum being the least depleted (48% depleted) and native Asclepias syriaca the most (95% depleted). After correcting for species-specific abundance, Vicia sativa and Lonicera japonica produced the greatest volume of nectar over the first 7 weeks, after which time Asclepias syriaca and Monarda fistulosa yielded a 2-3× increase in available nectar for pollinators. These data add to a growing body of literature suggesting that the dynamic hypothesis explains patterns of competition in early successional plant-pollinator communities. Importantly, future work should aim to examine nectar depletion dynamics within more stable ecological communities and those with few exotic species.