Species occupancy is inflated by sink populations in productive environments but not unproductive environments

For decades, community ecologists have examined how diversity varies with ecosystem productivity. Despite this long history, tests of hypothesized mechanisms, namely the interplay between environmental filtering, biotic interactions, and dispersal, are lacking, largely due to the intractability of using traditional approaches. Across a productivity gradient in a serpentine grassland (California, USA), for four annual plant species, we coupled local productivity estimates, occupancy surveys, and measures of persistence tested on transplants under natural conditions and when interactions with neighbors were experimentally reduced. We found a positive effect of productivity on diversity (i.e., the proportion of our focal species occupying a location) despite strong competition limiting species persistence in productive environments. Additionally, across species and for the community, we found a strong mismatch between species occupancy versus persistence, largely due to dispersal excess causing sink populations with negative growth rates. Our results suggest that diversity-productivity relationships can be largely driven by dispersal and its interactive effects with local biotic and abiotic conditions.