Resources, recruitment limitation and invader species identity determine pattern of spontaneous invasion in experimental grasslands

1. A number of experimental studies have supported the hypothesis that diversity increases invasion resistance, but several mechanisms were proposed to explain this relationship. We studied spontaneous invasion in experimental grasslands varying in species richness (1-16 (60)) and plant functional group richness and identity (1-4; grasses, legumes, tall herbs small herbs) during the first 3 years after establishment on large-area plots of 20 x 20 m size. 2. Invader species number, biomass and density decreased with increasing community species richness. The identity of successful invaders changed through time as the relative importance of external invaders (= species not belonging to the experimental species pool) decreased, while internal invaders (= species belonging to the experimental species pool) became more important. The presence of legumes increased, while the presence of grasses in the plant communities decreased density, biomass and species number of external invaders and biomass of internal invaders in the second and third year after establishment. 3. Analyses of viable seeds in the topsoil, a higher invasion in communities with more potential invaders in adjacent plots and an edge effect with a higher invader density and species number at the margin of the plots suggested recruitment limitation of internal invader species in contrast to external invader species that were more strongly limited by community characteristics. 4. Resource availability, in particular percentage cover of resident species, root length density and soil nitrate, and propagule availability had significant effects on invasibility. However, statistical analyses revealed that these variables did not completely control for effects of community species richness and the presence of grasses and legumes respectively on invasion resistance. 5. Separate analyses of frequent invader species showed that the invasion success of individual species varied in response to biodiversity and resource niches. 6. Synthesis. Our results confirm that a more complete use of available resources in plant communities of increasing species richness generally decreases invasibility. However, the overall positive effects of biodiversity on invasion resistance are dependent on species identity of the invader species. Thus, mechanisms at species level are important to understand community invasion resistance.