Beyond complementarity: Multi-factor effects of native richness underlie the invasion paradox

Background/Question/Methods: Diversity is predicted to regulate invasion by niche-based complementarity effects, but supporting evidence from 100s of studies ranging fine-scale experimental work to coarse-resolution observation studies and meta-analyses has been equivocal.

Results/Conclusions: Here, using a standardized multi-scale global data set, we confirm previous findings of wide context-dependent relationships between native and exotic plant richness. We detect evidence for fine-scale complementarity in some systems, where invasion resistance increases with native plant diversity. In most, however, native diversity predicted invasion through indirect pathways relating to anthropogenic impacts, coarse-scale environmental heterogeneity, and biogeographical factors relating to the relative sizes of the regional species pools and the length of invader residency. Each of these indirect factors sometimes created significant negative correlations between native and exotic diversity, but did not involve niche-based mechanisms associated with invasion resistance. These findings explain the long-standing difficulty in solving the ‘invasion paradox’ using single-factor mechanisms, demonstrating instead that the native and exotic diversity relationship derives from multivariate factors that unfold differently in different regions of the planet.