The balance of niche and neutral processes dictate the effects of biodiversity on ecosystem production

How species diversity affects the productivity of ecosystems has been a prominent research question over the past 15 years.  Ecologists have increasingly recognized that the consequences of biodiversity loss for the functioning of ecosystems depend on the mechanisms that maintain diversity in the first place – that is, the mechanisms of species coexistence.  To date, diversity-function research has focused on niche based mechanisms of coexistence such as resource partitioning.  This is a short-coming given that (i) Hubbell has shown that neutral interactions can delay competitive exclusion, and (ii) Chesson has shown that stable coexistence depends on the balance of stabilizing forces, generated by species differences, and equalizing forces, which make species so similar that their interactions do not matter for long-term dynamics.  Here we incorporate the interaction between equalizing and stabilizing forces into theoretical expectations for the relationship between species richness and productivity using two models: a general continuous-time Lotka-Volterra competition model, and a discrete-time model of annual plant growth.  For the LV model, we find that (i) weak equalizing forces decrease the amount of overyielding, or the productivity gains from polycultures, generated by stabilizing forces, and (ii) strong equalizing forces increase the probability that the productivity of polycultures will exceed that of the greatest monoculture – a phenomenon called transgressive overyielding. Simulations from the annual plant model show that moderate equalizing forces enhance the probability of transgressive overyielding.  Thus, the effect of species diversity on ecosystem production depends on the balance of stabilizing and equalizing forces, which together determine competitive coexistence.