Do more diverse plant communities greater resistance to invasions?

Early theoretical analyses and observations of community structure have led to the hypothesis that a high diversity of communities increases their res istance to invasions. However, to this date, observation and experimental d ata have shown positive or negative relationships between plant community d iversity and invasibility. Studies of biological invasions proposed several others mechanisms explaining invasions: disturbances, presence of empty ni ches and interactions with others community trophic levels. The role of pla nt diversity can be decomposed in two factors: is specific richness per se and/or the diversity of functional groups that lead to invasion resistance of communities? A study carried out in old fields of Montpellier country sh owed a general picture of invasion and its complexity in this country. Our observations of these invasion patterns confirmed the difficulties to obser ve clear relationships with their diversity. We then resorted to semi-contr olled field experiments. We sowed annual old field communities with differe nt levels of diversity: variation in species richness, variation in functio nal richness and variation in functional identities for a fixed Functional richness. Demographic and vegetative parameters of two exotic probes transp lanted into these communities, Conyza bonariensis and C.canadensis, were me asured along their life cycle. Primary community functioning parameters of the model ecosystems were measured simultaneously. Species richness had lit tle effect on performance of the two Conyza species. Functional composition appeared more relevant than functional richness per se to explain communit y invasibility. Mechanisms of functional composition effects are proposed.