LAUSR

PREMISE Invasive plant species often escape from specialist herbivore and are likely to experience herbivory mostly from generalist herbivores in the exotic range. Consequently, the Shifting Defence Hypothesis predicts that invasive plants will produce 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 chemical 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) Tissues of invasive B. nigra are less palatable to the generalist herbivores Theba pisana and Helix aspersa than tissues of native-range B. nigra. RESULTS Invasive B. nigra populations produced a significantly lower diversity of glucosinolate compounds and a marginally higher concentration of total glucosinolate compounds. Leaf tissues of the invasive B. nigra were significantly less palatable to T. pisana and marginally less so to H. aspersa. Brassica nigra individuals that expressed high concentrations of total glucosinolate compounds were visited by a low diversity of generalist herbivore species in the field. CONCLUSIONS In line with the RCDH, the lower diversity of glucosinolate compounds produced by invasive B. nigra populations likely resulted from selection imposed by reduced herbivory pressure in the exotic range. This article is protected by copyright. All rights reserved.