Abstract
The naturalisation hypothesis has been gaining attention recently as a possible mechanism to explain variations in invasion success. It predicts that exotic genera with native representatives should be less successful because of an overlap in resource use and of the existence of common specialised enemies.
In this study, we tested whether native congenerics have more negative impact on exotic species than heterogenerics by increasing the effects of soil pathogens. We sampled soil in populations of three exotic species (Epilobium ciliatum, Impatiens parviflora and Stenactis annua) at sites with and without respective congeneric species.
This soil was used as an inoculum for cultivating the first plant cohort, which included exotics, as well as native congenerics and heterogenerics. The conditioned soil was subsequently used for cultivating the second cohort of plants (exotics only).
We found no consistent impact of relatedness of conditioning species on exotic growth. Although soil conditioned by congeneric E. hirsutum had the largest reduction on the performance of E. ciliatum, the final biomass of S. annua was lowest when grown in soil conditioned by itself.
There was no effect of stimulating species on the biomass of I. parviflora. In both experimental phases, performance of exotics was improved when cultivated with sterilised inoculua, indicating the dominance of soil generalist pathogens.
However, the biomass of S. annua was increased most by congeneric-stimulated inoculum from congeneric sites, suggesting a possible role for specialised symbionts. Our results suggest that variations in invasion success of at least some exotics may be affected by species-specific interactions mediated by the soil biota