Floral plasticity in response to herbivory differs between native and introduced plant populations

Background/Question/Methods

Understanding how plant populations evolve after introduction to a new range, as well as the consequences of these changes for the surrounding community, is key to understanding invader success. I therefore used Verbascum thapsus as a model system to test the following alternate hypotheses: (1) flower size decreases after introduction to a new range as reliance on pollinators decreases, (2) flower size increases after introduction to a new range as resources are reallocated from defense to reproduction, or (3) flower size is a plastic trait and differences between native and introduced populations depends on the environment.  To understand the consequence of changes in floral morphology, I then investigated whether floral visitors respond to variation in flower size.  To answer these questions, I measured floral characteristics of plants from 16 native European and 22 introduced North American populations of V. thapsus grown in a common garden, and recorded the rates at which pollinators visited each plant. Half of the plants were treated with insecticide to reduce insect feeding, while the other half of plants were sprayed with water, allowing for ambient levels of generalist herbivory. Comparing between herbivory treatments allowed me to explore whether leaf damage by insects influences floral traits. 

Results/Conclusions

European and North American populations of V. thapsus react differently to changes in herbivory.  In the absence of herbivory, North American populations produce larger flowers than European populations. However, in the presence of generalist herbivory flower size does not differ between ranges.  In other words, flower size is a plastic trait linked to herbivory, and the direction of plasticity differs between the native and introduced range. Given what we know about between-range differences in allocation of defenses, patterns of floral plasticity may be explained if a flower’s resources are finite; thus as investment in defenses increases, flower size decreases. Variation in flower size did influence floral visitation rates, but the preference of honeybees for plants of European as compared to North American origin was largely driven by a variable that has not yet been measured.  However, that the direction of floral plasticity differs between ranges is an important novel finding. Although plasticity is known to contribute to the success of invader establishment, my results suggest we need to adopt a more long-term perspective that investigates how plasticity evolves after introduced populations that have had the time to evolve and adapt to their new environment.