Biodiversity-productivity relationships in a natural grassland community vary under diversity loss scenarios

Understanding the biodiversity-productivity relationship and underlying
mechanisms in natural ecosystems under realistic diversity loss scenarios
remains a major challenge for ecologists despite its importance for
predicting impacts of rapid loss of biodiversity worldwide. Here we report
the results of a plant functional group (PFG) removal experiment conducted
on the Mongolian Plateau, the largest remaining natural grassland in the
world. Our results demonstrated that the biodiversity-productivity
relationship varied among positive linear, neutral, and unimodal forms
under different PFG loss patterns. Moreover, the form of this relationship
with the same PFG loss pattern sometimes changed through time. The
abundance of the remaining PFG(s) before removal and their compensation
following the loss of other PFGs were two major mechanisms affecting the
biodiversity-productivity relationship under diversity loss scenarios. The
abundance effect promoted positive responses of productivity to
biodiversity, but the compensation effect caused several
biodiversity-productivity relationships, hinging on its direction
(positive or negative) and strength. As indicated by the values of the
compensation index, negative, zero and partial compensations contributed
to the positive relationships, while full compensation resulted in a
neutral relationship. Over-compensation at intermediate PFG richness
levels created a unimodal curve in our system, but it could also lead to a
negative linear relationship. Synthesis. Our experiment provides a vivid
picture of how the form of the biodiversity-productivity relationship
varies among different diversity loss patterns in a natural ecosystem. We
argue that compensation by the remaining species, which is not revealed by
synthesized biodiversity experiments, plays a critical role in shaping the
form of this relationship when diversity is lost from existing systems.
The direction and strength of compensation are highly dependent on
extirpation scenarios. Thus, impacts of biodiversity loss on natural
ecosystems are likely more complex than predicted by the canonical
positive saturating curve obtained from the synthesized biodiversity
experiments. We suggest that models forecasting the consequences of
biodiversity declines on natural ecosystems should take into account
diversity loss patterns and the ensuing compensation.