Effects of species evenness can be derived from species richness – ecosystem functioning relationships

Although species richness effects on ecosystem functioning have been
studied thoroughly in countless experiments, the effects of the other side
of diversity – species evenness – remain less identified, especially at high
species richness. Due to the large number of different model ecosystems
that need to be created, the explanatory power of the experimental
approach for evenness is indeed limited. We show here that experimental
studies on the influence of species richness on ecosystem functions contain
hidden information on the influence of species evenness. Both the effects
of maximum and minimum evenness, and of a key set of intermediate
evenness levels, can be derived from species richness – ecosystem
function curves, and that for every richness level, by using communities
with low species richness as the equivalent of highly uneven communities
with higher richness. We show that evenness effects on ecosystem
functioning have the same direction as richness effects, however with
increasing effect sizes at higher richness levels. We validated our technique
for a wide range of ecosystem functions and applied it to the species
richness - community biomass data from an existing biodiversity
experiment. Our approach could provide a fast and easy alternative for
resource-intensive experiments in which evenness is experimentally varied,
as we can build on the elaborate existing literature on species richness to
assess its effects.