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) Tissues of invasive B. nigra are less palatable to two generalist herbivores Theba pisana and Helix aspersa than tissues of native-range B. nigra. RESULTS Invasive B. nigra populations expressed 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 The biogeographical differences in glucosinolate profiles of invasive and native-range populations of B. nigra may be the result of differential herbivore selection pressures in the respective ranges.Funding provided by: DAAD Climate Research for Alumni and Postdocs in Africa*Crossref Funder Registry ID: Award Number: 91787649Funding provided by: Banco Bilbao Vizcaya Argentaria (BBVA)*Crossref Funder Registry ID: Award Number: BBVA-BIOCON06/06