Deer overbrowsing on autumn-flowering plants causes bumblebee decline and impairs pollination service

Abstract

Increased ungulate browsing has altered the composition of plant communities and food webs of forest ecosystems in many regions around the world. To evaluate the cascading impact of deer browsing on pollination and plant reproduction is critical to understand the roles of species interactions in maintaining plant populations and for conservation management. In this study, we investigated the relationships among floral resources of understory herbaceous plants, pollinator visitation, and fruit set of shrub species based on data accumulation over three years in six temperate deciduous forests with deer and without deer.We found that in deer browsed sites, the visitation rate of bumblebees had decreased due to severe reduction in the coverage of autumn-flowering herbaceous plants, while the effect varied between bumblebee species. On the other hand, other insect taxa showed no dependence on variation in autumn floral resources. The two genera of bumblebee-pollinated shrubs showed reduced fruit set due to severe decline in autumn-flowering herbaceous plants and bumblebee visitation (Weigela: 18.5% and Rhododendron: 21.9%). In contrast, the fruit set of shrubs pollinated by insects that did not show dependence on autumn floral resources were not negatively affected by deer browsing. Our results suggest that deer browsing have not only caused negative effects on herbaceous plants, but in addition have negative indirect effects on reproduction of woody plants through cascading effects of pollination linkages.