Spatiotemporal variation in the pollination systems of a supergeneralist plant: is Angelica sylvestris (Apiaceae) locally adapted to its most effective pollinators?

Background and Aims: In terms of pollination systems, umbellifers (plants of the carrot family, Apiaceae) are regarded as generalists, since their (usually dichogamous) flowers are visited by a wide range of insects representing several taxonomic orders. However, recent analyses of insect effectiveness revealed that these plants may be pollinated effectively by a narrow assemblage of insect visitors. Of particular interest was whether populations of an umbellifer species varied in pollinator assemblages and whether this could lead to local specialization of the pollination system. We also explored whether variation in pollinator assemblages was associated with variation in floral traits, and whether this variation influences reproductive output.

Methods: The focus was on Angelica sylvestris, a common European species visited by a taxonomically diverse insect assemblage. In three populations, located along an ~700-km transect, over three growth seasons insect visitors were identified, their effectiveness was assessed by surveying pollen loads present on the insect body, insect activity on umbels, nectar and scent composition was studied, and transplantation experiments were performed.

Key Results: The populations investigated in this study differed in their nectar and scent profiles and, despite the similar taxonomic composition of insect visitor assemblages, were effectively pollinated by disparate pollinator morphogroups, i.e. flies and beetles. Although this suggested local adaptations to the most effective pollinators, analyses of body pollen loads and behaviour on umbels demonstrated functional equivalency of the visitor morphogroups, which is probably related to the fact that A. sylvestris bears few ovules per flower. The transplantation experiments confirmed that reproductive success was not related to the source of experimental plants and that the insects do not exhibit preferences towards local genotypes.

Conclusions: Angelica sylvestris is morphologically well adapted to ecological generalization, and there is little evidence that the surveyed populations represent distinct pollination ecotypes. Most likely, the observed variation in floral characters can be interpreted as 'adaptive wandering'. Specialization in this family seems possible only under very special circumstances, for example when the pollinator community comprises insect visitor groups that clearly differ in their pollination capacity (e.g. due to differences in their functional morphology) and/or have different perceptional biases (e.g. for colour or scent). However, the barrier to the evolution of morphological adaptations resulting in the fine-tuning of the flower towards particular pollinator types may arise from the architectural constraints on the floral bauplan that make umbellifers so uniform in their floral displays and so successful in attracting large numbers of pollinators.