Biodiversity maintenance mechanisms differ between native and exotic communities

Background/Question/Methods The ongoing homogenization of Earth’s biota is affecting nearly every region of the globe.  Grasslands contain perhaps the most disrupted and homogenized communities.  We studied 9-species plant communities containing all exotic (i.e. introduced) or all native species under controlled field conditions to test two hypotheses: 1) species diversity declines more rapidly in assemblages of exotic than native species, and 2) exotic assemblages are characterized by overyielding by highly productive species, which increases dominance, whereas native species assemblages are characterized by having more complementary resource use among species, which leads to more even and complete resource use across space and time.  These differences should manifest themselves as differences in stabilizing and equalizing mechanisms of diversity maintenance.  Communities were established in April-May 2007 using exotic or native species that were paired to control for growth form, mode of photosynthesis and phylogeny.  Experimental plots were planted with equal-mass seedlings and species diversity, net primary productivity, and yielding behavior were measured at the end of the first growing season.  Rainfall during 2007 was above normal, and mixtures achieved complete canopy closure by this time.  Results/Conclusions There were dramatic differences between native and exotic communities by the end of the first growing season.  Species diversity declined more in exotic than native communities (Simpson’s diversity, P < 0.01), with exotics having 26% lower diversity (1.7) than natives (2.4).  Differences in diversity were associated with declines in evenness (0.6 unit drop in natives vs. 0.7 unit drop in exotics) and not richness.  Aboveground biomass differences between exotics and natives were larger in 9-species mixtures than in monocultures: with a 2-fold difference found in monocultures, and a 3-fold difference in mixtures (richness x origin, P < 0.05).   Diversity declines were influenced by different mechanisms in exotic and native communities.  In exotic communities, the selection effect was positive (i.e. overyielding occurred in species with high biomass in monoculture), and diversity declined linearly as the selection effect increased (P<0.01).  In native communities, however, the selection effect was negative (i.e. overyielding occurred in species with low biomass in monoculture) and there was no relationship between the selection effect and diversity decline.  These results suggest that native communities have stronger stabilizing mechanisms than comparable exotic communities, and that native-exotic status is important to understanding relationships between biodiversity and ecosystem functioning and other ecological questions.