Allysa C. Hallett, Randall J. Mitchell, Evan Chamberlain, and Jeffrey D. Karron
Department of Biological Sciences, UW-Milwaukee, ahallett@uwm.edu, karron@uwm.edu
Department of Biology, University of Akron, Akron, OH
Changes in the local pollinator community may shape the dynamics of plant-pollinator interactions. Since the quality of pollination services often varies markedly among pollinator species, the consequences of pollinator declines for plant reproductive success may be unpredictable. Therefore, changes in the composition of visiting pollinators, in terms of their pollination efficiency and visitation rates, may play a significant role in determining the effect of pollinator loss on plant reproductive success. If the loss of a major pollinator leads to increased visitation by another efficient pollinator, pollination success may remain unchanged or may even increase. We experimentally tested this hypothesis by removing bumble bees from plots of whorled milkweed (Asclepias verticillata). In three small and three large populations, we quantified pollinator visitation rates and pollination success for control plots and for plots where bumble bees were experimentally excluded. We found that exclusion of bumble bees led to a substantial increase in wasp visitation in all six populations. However, small populations experienced a less dramatic increase than did large populations. Since wasps and bumble bees are both efficient pollinators of whorled milkweed, increased wasp visitation offset the loss of bumble bees, and pollination success remained unchanged. This provides a vivid example of the challenges associated with forecasting how pollinator declines will influence plant reproductive success. This work was supported by a UWM Research Growth Initiative Grant. MS Thesis research, Dr. Jeffrey Karron, Major Advisor.