Soil microbes regulate ecosystem productivity and maintain species diversity

One of the major goals in ecology is to determine the mechanisms that drive the asymptotic increase in ecosystem productivity with plant species diversity. Niche complementarity, the current paradigm for the asymptotic diversity-productivity pattern, posits that the addition of species to a community increases productivity because each species specializes on different resources and thus can more thoroughly utilize the available resources. At higher diversity the increase in productivity decreases because resources become limiting, resulting in the classic asymptotic diversity-productivity pattern. An alternative but less tested explanation is that density-dependent disease from species-specific soil microbes drive the diversity-productivity relationship by increasing disease and thus decreasing productivity at low diversity. At higher diversity, productivity asymptotes because disease decreases with increasing diversity until it reaches a uniformly low level. Using a series of field experiments, we found that the classic asymptotic diversity-productivity pattern existed only when soil microbes were present. Soil microbes created the well-known pattern by depressing plant growth at low productivity though negative density dependent disease. In contrast, niche complementarity played only a weak role in explaining the diversity-productivity relationship because productivity remained high at low abundance in the absence of soil microbes. Based on our findings, the ongoing loss of species in natural ecosystems will likely increase per capita plant disease and lower ecosystem productivity. Furthermore, recent evidence suggests that negative density dependent disease maintains plant species diversity, and thus this single mechanism appears to link diversity maintenance to the diversity-productivity curve--two important ecological processes.