Summary
1. Biodiversity is a central factor driving community invasibility. Diverse communities exploit
resources more efficiently, leaving less free niche space available to invaders. Niche partitioning,
however, is only possible in complex resource environments, and we hypothesized that
resource richness drives the biodiversity–invasibility relationship.
2. We tested the effect of two biodiversity indices, taxonomic richness and functional dissimilarity,
on the invasibility of Pseudomonas fluorescens communities in microcosms of varying
resource richness, herein used as a proxy for niche dimensionality, because different P. fluorescens
genotypes differed in their ability to use those resources.
3. Invader success was negatively correlated with the diversity of the resident community, with
functional dissimilarity being of greater significance than taxonomic richness. Varied niche
dimensionality revealed different mechanisms determining community invasibility: at low niche
dimensionality, invasibility was driven by the presence of particular genotypes (identity effect)
rather than by the biodiversity of the resident community. At high niche dimensionality,
functional dissimilarity increased community productivity and reduced invasion, most likely
through complementarity effects.
4. The results show that functionally dissimilar bacterial strains efficiently exploit their
environment, reducing the resources available for invasive species. These findings call for the
preservation of functionally dissimilar taxa to warrant resistance of communities against
invasive species, in particular, in environments of high niche dimensionality.