Propagule pools mediate community assembly and diversity-ecosystem regulation along a grassland productivity gradient

1 The relative importance of regional species pools and local ecological processes in governing landscape variation in plant species diversity and productivity was evaluated in a Kansas grassland. 2 We examined the impact of multispecies sowing treatments and experimental canopy disturbances on plant species diversity and ecosystem processes along a complex natural gradient of plant standing crop. 3 Data collected 4 years after sowing showed that plant invasion and diversity were seed limited in unproductive sites, but microsite limited in productive sites. Effects of sowing on plant diversity along the natural landscape gradient were paralleled by significant effects of sowing on measures of local plant production and community resilience to disturbance. 4 These results support the shifting limitations hypothesis (SLH) that landscape gradients in local plant diversity should reflect shifts in the major regulating factor, from species pools to local ecological processes, as one moves from sites of inherently low to inherently high productivity. 5 Our findings also indicate that diversity at the level of the available propagule pool acts to constrain ecosystem productivity and stability by mediating local community assembly, by determining the availability of key species, and by governing opportunities for functional compensation within the community. 6 In total, our results support an emerging view that community processes and ecosystem functions are dynamically linked and act reciprocally to constrain each other.