Community phenological mismatch through invasion: no climate change required

Background/Question/Methods

Alterations of plant community composition during invasion, as exotic species increase in abundance, can lead to changes in the frequencies of functional traits in the plant community. This in turn is likely to affect interacting species that respond to plant traits, such as pollinators.  If an exotic plant species and a native plant species differ in traits relevant to foraging pollinators (such as nectar provision or flowering phenology), competitive displacement of the native by the exotic will cause a shift in community pollinator resources.  We asked whether plant communities along an invasion gradient differ in the type, amount, and seasonal timing of floral resources provided to pollinators. 

We determined the pollination functional type (bee-pollinated, other insect-pollinated, self-pollinated) of the most abundant species in native and exotic floras of Pacific Northwest prairies using observations, literature searches, and expert opinion.  We quantified floral abundance and flowering phenology of all forb species weekly at five sites along an invasion gradient within one prairie preserve in 2012, with additional 2010-2011 phenological data from a single site. Finally, we used two years of abundance data from 15 differentially invaded prairie preserves to generate community seasonal resource availability curves based on community frequencies of these traits.

Results/Conclusions

We found that the exotic and native floras differed significantly in their proportions of pollination functional plant types, with the native flora containing significantly more bee-pollinated and nectar-rich species. Flowering phenology also differed; exotic species were much more likely than native species to bloom late in the season, while the native flora contained a greater diversity of resource phenologies.  Invasion diminished the abundance and diversity of native species, propagating these trait differences community-wide; thus, the seasonal distribution of floral resources for pollinators differed across communities according to their degree of invasion. Resources for pollinators are likely to vary considerably between highly invaded and less invaded prairie communities.  Changes in community-level floral resource provision during invasion, especially resource phenology, could feed back to affect both generalist and specialist pollinator populations, ultimately affecting native plants indirectly.