Induced chemical defenses in invasive plants: a case study with Cynoglossum officinale L.

Abstract

The ‘evolution of increased competitive ability’ (EICA) hypothesis is an extension of optimal defense theory and predicts that reduced pressure from insect herbivores in the introduced range results in evolution of reduced defenses in invading plant populations, allowing greater allocation of resources to competitive traits such as growth rate and reproduction. The EICA hypothesis considered levels of defensive chemistry to be fixed within a particular genotype. In this paper, we propose that if herbivory is reduced in the introduced range, but chemical defenses are inducible in response to herbivory, evolution of reduced defenses and any associated increase competitive ability should not occur. Rather, mean induced and constitutive levels of induced defenses should be similar in introduced and native ranges, but the variance about mean induced levels should be greater in the introduced range. This is predicted because induced levels will occur less frequently in the introduced range where herbivory is reduced, thereby insulating these levels from the stabilizing selection expected in the native range where induced levels occur more frequently. We conducted a preliminary study to examine this by comparing constitutive and induced concentrations of total pyrrolizidine alkaloids (PAs) from native (European) and introduced (western North America) populations of Cynoglossum officinale L. The mean constitutive and induced concentrations of PAs did not differ between continents, but the variability of the induced concentrations was significantly greater for plants from the introduced range. Although our study with C. officinale is provisional due to a small sample size, it supports our predictions for evolution of inducible defenses in introduced ranges where herbivore pressure is reduced. Most chemical defenses in plants have been found to be inducible, so similar patterns may occur widely. If so, this weakens the generality of EICA’s predictions concerning chemical defenses. The effects of inducible defenses should be considered in cross-continent comparisons of other invasive plant species.