Feedbacks between community assembly and ecosystem function govern the diversity-productivity relationship

The interdependence of diversity and productivity has been evaluated from the perspective of diversity as dependent variable and productivity as independent variable and vice versa. These different perspectives have largely been investigated by separate research programs, each operating under the implicit assumption of unidirectional causation. Here we demonstrate reciprocity and feedback between community assembly and productivity in Kansas grassland, illuminated through experimental alterations of diversity and composition in the field achieved via propagule pool manipulations (multi-species seed additions).  By the sixth year of the experiment, seed addition not only led to increased plant diversity and altered species composition but also led to a three-fold increase in mean ANPP over control quadrats.  Seed addition also increased the variance and range in ANPP (range: 84.2-637.0 versus 439.9-2144.6 g m^-2 yr^-1 in control and sown plots, respectively) despite relatively similar starting conditions.  These substantial changes in production driven by seed addition shifted a weakly positive productivity-diversity relationship to a strongly negative relationship.  Hence, not only do shifts in community structure resulting from an expanded species pool feed back to alter productivity, but that productivity so altered, feeds back further to constrain diversity and alter the shape and fidelity of the diversity-productivity relationship. Our study illustrates that community assembly dynamics and ecosystem functioning are inextricably linked through reciprocal feedbacks that integrate bottom-up control of production and community structure with larger-scale and stochastic processes that generate alternative community assembly pathways.  Furthermore, these feedbacks suggest that attempts to restore ecosystem structure and function through species additions may have complex outcomes.