How important are species richness, species evenness and interspecific differences to productivity? A mathematical model

We present a theoretical model to quantify the influence of diversity on productivity and nutrient acquisition in plant communities during exponential growth. The model fractionates diversity into three components, namely species richness ( S ), species evenness ( E ) and the degree of difference between species ( D ). The influence of each of these components is assessed individually: S is varied by changing the number of species, E by changing their population size, and D by changing the range of species traits critical to productivity (specific nutrient uptake rate, Σ r , or nutrient use efficiency, NUE ). D was quantified as the coefficient of variation of Σ r or NUE . All three components of diversity enhance the biomass and nutrient stocks in the community, but the response patterns are different. Species richness has a saturating influence, whereas effects of E and D are linear and exponential, respectively. In all cases the non‐linear dependence of productivity and nutrient acquisition on Σ r and NUE during exponential growth was the single mechanism underlying these effects. This causes the presence of plants with extreme traits to promote productivity, and S , E and D all affect the abundance and/or intensity of these extremes. The model offers a framework to explain differences between experimental observations, and suggests a concept of diversity where S and E are structural components and D a qualitative or functional component, which modulates the influence of the two others. We propose to explicitly recognise D as an integral constituent of plant diversity in future studies.