The resource balance hypothesis of plant species diversity in grassland

We hypothesize that plant species diversity is favoured when actual resourc e supply ratios are balanced according to the optimum resource supply ratio s for the vegetation as a whole. This 'resource balance hypothesis of plant species diversity' (RBH) follows from two different mechanisms of plant sp ecies coexistence, namely: 'differential resource limitation', which allows species to coexist in a competitive equilibrium in a homogeneous environme nt and 'micro-habitat differentiation', which builds on spatial heterogenei ty. Both mechanisms require that resource supply ratios are intermediate be tween the optimum supply ratios of the species present in the species pool. Additional conditions, concerning the resource acquisition and requirement ratios of the species, are easier to meet for the second mechanism than fo r the first. To test the RBH we measured species diversity parameters in 74 grassland plots, as well as the N, P and K concentrations in the above-gro und biomass. We used a new ceiling detection algorithm to examine the relat ionship between maximum observed diversity and the N/P-, P/K- and K/N-ratio s in the biomass. Most of these ceiling relationships could be described by parabolic curves with significant quadratic terms. This indicates that hig h diversity does not occur at the extremes of the observed ranges of nutrie nt ratios. This supports the RBH.