Plant species are generally subject to evolutionary tradeoffs between the production of secondary chemicals for defense, and allocation of resources to primary chemicals essential for growth and development. Whether and how these tradeoffs affect secondary chemistry as a species expands its range is relatively understudied, but important for understanding how “novel weapons”, or the introduction of new phytochemicals, will affect naïve species in pre-existing ecological communities. Thlaspi arvense is a biennial herbaceous mustard plant of European origin that produces the glucosinolate sinigrin, a type of secondary chemical that functions both as an appetite deterrent to generalist herbivores and oviposition cue for specialist herbivores. T. arvense is currently spreading through subalpine meadows of the Rocky Mountains of North America, where variation in snowmelt timing can pose allocation tradeoffs for plants. To test for population level variation in sinigrin production in invading populations of T. arvense, we surveyed 10 field populations for herbivory damage, fecundity and phenology, and conducted common garden experiments in the greenhouse and in the field. We measured plant size, herbivory damage, and reproduction, as well as foliar glucosinolate levels using high performance liquid chromatography in both field and common garden populations.
Results/Conclusions
We found significant genetic variation among T. arvense populations experiencing early vs. late snowmelt timing, suggesting the potential for local adaptation of these non-native plant populations in the new range. We observed higher proportions of plant populations with foliar herbivory on T. arvense at later snowmelt sites, as well as higher amounts of herbivory per individual plant, despite higher average foliar sinigrin levels in late snowmelt populations. Our field data suggest that higher herbivory pressures may maintain higher concentrations of secondary chemicals in locations with later snowmelt, despite the expected selection pressures of a shorter growing season on growth and reproduction. If edge populations continue to expand with higher glucosinolate expression, the potential impacts on native communities may be higher than anticipated. By documenting how biotic and abiotic environmental conditions interact with phenology to determine variation in glucosinolate production, we can improve our predictions of how current and future environments can shape the evolution of phytochemistry in a new biogeographic range, as well as better understand the causes and consequences of plant range expansion.