Intraspecific facilitation explains the spread of the invasive engineerSpartina anglicain Atlantic salt marshes

Abstract

AbstractQuestionsInvasiveness depends in part on the ability of exotic species to either exclude native dominants or to fill an empty niche. Comparisons of niches and effects of closely related native and invasive species enable the investigation of this topic. DoesSpartina anglicainvade European salt marshes through competitive exclusion of the nativeSpartina maritimaor due to the occurrence of an empty ecological niche in highly anoxic conditions?Study SiteThe Arcachon Bay (France).MethodsAt three intertidal levels, we quantified competitive response and effect abilities of the two species through a cross‐transplantation removal experiment. We also compared the biomass, root/shoot ratio, productivity and environmental conditions (elevation, salinity, redox potential and soil moisture) of salt marsh communities dominated by the exoticSpartina anglicaor the nativeSpartina maritimaat three intertidal levels.ResultsBoth established species showed similar biotic resistance to the invasion of the other species, but the exotic showed important intraspecific facilitation for growth. Species had similar niches and total biomass along a gradient of anoxic conditions, but the exotic had a much higher root/shoot ratio and productivity than the native. Owing to its rhizome density, the exotic showed high ability to increase sediment oxygenation, likely to explain its important intraspecific facilitation.ConclusionsOur results showed that the invasion success ofS. anglicacannot be explained by the competitive exclusion of the native or by its ability to fill an empty niche along a gradient of anoxia. Its behaviour as a self‐facilitator invasive engineer very likely explains its rapid spread in the Bay and biotic resistance to the colonization of other congeneric species when established in dense patches. Additionally, we suggest that physical disturbance in marsh communities dominated by the nativeS. maritimamay disrupt its biotic resistance against the invasion ofS. anglica.