Invasive plant species that experience lower herbivory pressure may evolve lower diversities of chemical defence compounds in the exotic range

ABSTRACT
PREMISE
Invasive plant species often escape from specialist herbivore species and are likely to experience herbivory mostly from generalist herbivore species in the exotic range. Consequently, the Shifting Defence Hypothesis (SDH) predicts that invasive plants will express higher concentrations of qualitative defence compounds to deter dominant generalist herbivores in the exotic range. Here, I additionally propose a Reduced Chemical Diversity Hypothesis (RCDH), which predicts that reduced herbivory pressure will select for invasive plant genotypes that produce lower diversities of defence compounds in the exotic range.
METHODS
I tested whether: (1) Invasive Brassica nigra populations express a lower diversity and an overall higher concentration of glucosinolate compounds than native-range B. nigra; (2) Brassica nigra individuals that express high diversities and concentrations of glucosinolates are more attractive to specialist and deterrent to generalist herbivores; (3) Tissu...