Enemy release but no evolutionary loss of defence in a plant invasion: an inter-continental reciprocal transplant experiment

This is the accepted manuscript of an article published by Springer Verlag.When invading new regions exotic species may escape from some of their natural enemies. Reduced top-down control ("enemy release") following this escape is often invoked to explain demographic expansion of invasive species and also may alter the selective regime for invasive species: reduced damage can allow resources previously allocated to defence to be reallocated to other functions like growth and reproduction. This reallocation may provide invaders with an “evolution of increased competitive ability” over natives that defend themselves against specialist enemies. We tested for enemy release and the evolution of increased competitive ability in the North American native ragweed (Ambrosia artemisiifolia: Asteraceae), which currently is invading France. We found evidence of enemy release in 29 natural field populations from the invaded and native ranges. Further we carried out a reciprocal transplant experiment, comparing several life history traits of plants from two North American (Ontario and South Carolina) and one French population in four common gardens on both continents. French and Canadian plants had similar flowering phenologies, flowering earlier than plants from further south in the native range. This may suggest that invasive French plants originated from similar latitudes to the Canadian population sampled. As with natural populations, experimental plants suffered far less herbivore damage in France than in Ontario. This difference in herbivory translated into increased growth but not into increased size or vigour. Moreover we found that native genotypes were as damaged as invading ones in all experimental sites, suggesting no evolutionary loss of defence against herbivores.This is a publication of the Koffler Scientific Reserve at Jokers Hill. The first author acknowledges an FCPR grant from the French Ministry of Agriculture and Alimentation (MAAPAR). PMK was supported by an NSERC Discovery Grant, the Embassy of France Séjour Scientifique de Haut Niveau program and a Visiting Professorship at the University of Paris-Sud, Orsay