Invasive plants are expected to evolve to become more competitive. They may do this through increased fecundity, more rapid growth rate and increased seed size among other means. However, the selective processes by which plants evolve to become more competitive may be reversed when biocontrols are introduced. In the invasive shrub Cytisus scoparius (Scotch broom), seed mass increased in plant populations introduced to New Zealand, but decreased again when a biocontrol species that eats seeds (Bruchidius villosus) was introduced (Paynter et al 2015). This plant is also invasive in the Pacific Northwest of the United States, where biocontrols were introduced in 1997, but have not been uniformly successful. Two biocontrol species (B. villosus and Exapion fuscirostre) have been introduced at different times and different rates, but both feed on seeds. I hypothesized that seed mass would vary between field sites, with the largest seeds found where biocontrol success was the lowest. I also hypothesized that greater seed destruction would be experienced by plants that have heavier seeds. To test this, I collected seeds from plants at six field sites. I counted the seeds per pod and proportion of seeds destroyed by biocontrols. I also weighed the seeds that had not been destroyed.
Results/Conclusions
Biocontrol species were easily distinguished from one another, and the species dominant at each site varied, as did the relative rates at which each species destroyed seeds. I found longer pods and heavier seeds at sites where the seed destruction rates by biocontrols were lower. Interestingly, there was no tradeoff, but rather a positive relationship, between seeds per pod and mean seed mass. Where biocontrol pressure was high, plants that produced heavier seeds had a higher seed destruction rate. Seed destruction ranged from 17% to 70% at sites, and between 0 and 89% for plants surveyed. I saw a positive correlation between seed mass and B. villosus attack rate at all but one site. This correlation implies natural selection by biocontrols on seed mass, with a potential future impact on seedling competitive abilities. Future common garden and heritability studies are planned, with a goal of measuring the rate of seed mass evolution in response to biocontrol pressure.