A North American native (Ragweed: Ambrosia artemisiifolia) escapes its enemies in France.
Kotanen, Peter*,1, Shykoff, Jacqui2, Genton, Benjamin2, 1 University of Toronto, Mississauga, Ontario, Canada2 Université Paris-Sud, Orsay, France
ABSTRACT- Ragweed (Ambrosia artemisiifolia) is a notorious North American weed which is now well-established in Europe, and has occurred in France for at least 140 years. We investigated whether its success in its new range is associated with a loss of natural enemies (as predicted by the Enemy Release hypothesis), and whether invading populations have reduced levels of defence (as predicted by the Evolution of Increased Competitive Ability hypothesis). Surveys of 29 natural populations in both the native (Ontario, Canada) and invaded (France) range of this species indicated that levels of herbivore damage are much lower in France. Reciprocal transplant experiments conducted on both continents with North American and French genotypes also demonstrated far lower levels of damage in France. This reduced damage occurs despite the fact that Ambrosia in its native range is host to a wide range of polyphagous herbivores, not just specialists. Native genotypes were as damaged as invading ones at all experimental sites, and laboratory feeding trials with an oligophagous beetle (Systena cf. elongata) indicated no preference for French genotypes. These results indicate that Ragweed has escaped its enemies in Europe, as predicted by the Enemy Release hypothesis, but that there is no evidence of loss of defence against either specialist or generalist herbivores.
Key words: invasions, enemy release hypothesis, evolution of increased competitive ability, Ambrosia artemisiifolia
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