High rates of nectar depletion in summer grasslands indicate competitive conditions for pollinators

Competition among pollinators for floral resources is a phenomenon of both
basic and applied importance. While competition is difficult to measure
directly under field conditions, it can be inferred indirectly through the
measurement of floral resource depletion. In this study, we conducted a
pollinator exclusion experiment to calculate nectar depletion rates in
summer across 16 grassland sites in the German regions of Franconia and
Saxony-Anhalt. Overall depletion rates were estimated at 95% in Franconia
and 79% in Saxony-Anhalt, indicating strong nectar limitation and likely
competition among pollinators for nectar. Despite being ubiquitous in our
study regions, honey bees were scarce at our sites at the time of nectar
sampling. This demonstrates that wild pollinators alone are capable of
massive nectar depletion, and the addition of managed honey bees to wild
pollinator communities may intensify already competitive conditions.
Nevertheless, the manifest diversity of the pollinator communities at our
sites indicates that other factors, such as non-trophic constraints or
temporal variation in nectar limitation, can mitigate competitive
exclusion despite immediate conditions of acute nectar scarcity.