Pollination is known to be sensitive to environmental change but we lack direct estimates of how quantity and quality of pollen transferred between plants is altered by disturbance. Thus, our understanding of how species compositional change impacts pollen receipt per species and overall structure of pollen transfer networks is severely limited. Moreover, over the last century tropical island ecosystems have become altered by human activities, yet we have not previously had a window into how anthropogenic disturbances have changed plant-plant interactions mediated by pollinators. We use the threatened dry forest ecosystems of the island of Hawai’i, HI, USA to explore how pollen transfer dynamics change along a contemporary invasion gradient and how they have been transformed by a century of environmental change. We constructed pollen transfer networks along a plant invasion gradient from pollen identified on stigmas from both native and introduced plants and characterized both plant floral traits and composition of pollen carried by the dominant pollinator (Apis mellifera). Finally, we characterized historical pollination interactions by identifying pollen on the stigmas of herbarium specimens (collected 1909-2002) from six remnant native species and compared results to contemporary data from the same species in remnant dry forest sites.
Results/Conclusions
In the contemporary timeframe, species flowering in native-dominated sites were more tightly connected by pollen transfer than those in heavily invaded sites. Compositional turnover in the pollen loads of A. mellifera (rather than turnover in floral abundances) was correlated with turnover in the composition of pollen transfer networks. Floral traits predicted species roles in the pollen transfer network, but differed qualitatively depending on whether plants were native or introduced. Over the century, there was no difference in the compositional variance of pollen on stigmas but there was substantial variance in the identity of species involved. Interestingly conspecific pollen loads were not affected by either contemporary invasion gradient or historical environmental change. Our work indicates that pollen transfer networks are reflective of disturbances on both time scales. In the contemporary data set floral morphology and foraging behavior of the introduced super-generalist pollinator were implicated as key determinants of the role introduced species play within native networks. Across the century, substantial changes in dry forest cover and composition translated into altered patterns of plant-plant interactions mediated by pollinators and observed on the stigma, even for species which have remained common in the midst of a rapidly changing ecosystem.