Does community production drive species diversity or vice versa? Towards resolution of a diversity paradox

Ecologists, paleobiologists and evolutionary biologists have long known that species diversity is controlled by the productivity of ecosystems. But contemporary studies have turned this paradigm on its head by showing that biodiversity controls, rather than simply responds to, the production of biomass.  These contrasting perspectives have led to a seeming paradox … "Does community production drive species diversity or vice versa?"  Several recent conceptual and mathematical models have tried to resolve this paradox by showing how species diversity might simultaneously be a cause and a consequence of community production.  Nearly all of these argue that the supply rates of resources known to limit biological production place an upper bound on the diversity that a site can support, but the actual number of species available to colonize a site determines how efficiently resources are utilized and converted to biomass.  We tested this prediction experimentally by performing concurrent nutrient addition experiments in 25 stream ecosystems that ranged from fewer than 30 to greater than 140 species of primary producers (periphytic algae).  As nutrient supply rates were augmented by 8 orders of magnitude using diffusing agar substrates, the fraction of streamwide species found on the substrates either increased monotonically, or showed a unimodal response.  At the same time, net primary production measured as O₂ evolution from agar substrates was a monotonically increasing function of streamwide diversity.  These results lend support to the idea that as nutrient supply rates limit biodiversity, biodiversity dictates how efficiently nutrients are converted into biomass.