Phenotypic plasticity has been suggested as one mechanism by which invaders proliferate in new environments. Plasticity has been studied mostly at single life history stages; however the traits that are adaptively plastic at earlier life history stages of a successful invader could be different from those that show plasticity at later stages.
Using a field experiment, I investigated plasticity at different life history stages, and its effect on the performance of invasive Centaurea stoebe (spotted knapweed) on Long Island, testing the hypothesis that earlier life history stages are more plastic for leaf traits in response to disturbance, and less plastic in response to competition than latter life history stages.
In May 2014, I subjected 768 spotted knapweed seedlings from 8 families to high and low density treatments, and to disturbance and no disturbance treatments, and monitored the plants as they progressed through juvenile to the adult stage. For juveniles, I measured the leaf length and leaf number, while for adults I measured the number of stems and the stem length. Using linear models, I explored the influence of disturbance, and density on plant traits. I also computed the plasticity index RDPI for of these traits under the treatment conditions.
The effect of family on all of the response variables was not significant at the 0.05 alpha level indicating that most of the differences in plant traits could be explained by phenotypic plasticity. Disturbance significantly influenced the length of leaves of juvenile plants (F 1,133 = 4.38, p<0.05), and the stem length of adult plants (F1, 265 = 5.41, p<0.05). For both life history stages, density did not significantly affect both stem length and leaf length. The stem length of adult bolted plants was more plastic than the leaf length of juveniles in response to disturbance (RDPI adults=0.13187, RDPI juveniles=0.008631), and density (RDPI adults=0.038329, RDPI juveniles=0.02389).
These results show that the amount of phenotypic plasticity in a population of invasive species can vary with life history stage. Contrary to our hypothesis, for both disturbance and density treatments, adults generally displayed more plasticity than juvenile plants. This suggests that plastic responses to environments could accumulate over time in a population of invaders, or that plasticity may be important at certain life history stages than at others. Understanding of the life stages at which phenotypic plasticity is important may have implications for management and control of invasive species.