The impacts of pollinator abundance on benefits from pollination mutualisms

Background/Question/Methods Native pollinators play a crucial role in flowering plant reproduction.  Several North American pollinator species are federally listed as endangered, and many pollinator species are experiencing population declines.  Restoring lost pollinators to ecosystems may positively impact plant communities. This premise assumes that changes in pollinator abundance will impact pollination services to plants, with effects that cascade to fecundity and recruitment into plant populations.  Plants that can utilize alternative pollinating species may be less affected by varying abundance of one pollinator than plants that specialize on that pollinator.  We tested these ideas using a small network of two native alpine plants (Trifolium dasyphyllum and parryi) which vary in their level of associations with a shared bumblebee pollinator (Bombus kirbyellus).  We added varying densities of queen B. kirbyellus foragers to naturally occurring mixed patches of the two clovers and measured visitation rates, pollen removal/deposition rates and plant seed set.  Throughout the plants’ full flowering season, we also excluded bumblebees from clover plants near experimental plots to assess the contribution of other pollinator taxa to seed production and hand pollinated clovers to assess the extent to which seed set was limited by other resources.
Results/Conclusions Addition experiments showed that as B. kirbyellus density increased, flower visitation increased more rapidly for its specialist (T. parryi) compared to the generalist (T. dasyphyllum), which has an alternative pollinator.  There was also a positive relationship between flower visitation and seed production T. parryi but not for T. dasyphyllum.  Hand pollinations showed that resource limitation accounted for considerable variation in seed set.   Bumblebee exclusion experiments showed that clovers’ degree of pollen limitation was greater early in the season, and varied with seasonal changes in the pollinator community.  Results from these experiments provide some of the first data tracing the impacts of partner abundance on benefits from mutualisms that vary along the facultative-obligate continuum and are a first step in quantifying the potential impact of pollinator restoration on plant population growth.