Competitive abilities in experimental microcosms are accurately predicted by a demographic index for R*

Resource competition theory predicts that R*, the equilibrium resource amount yielding zero growth of a consumer population, should predict species' competitive abilities for that resource. This concept has been supported for unicellular organisms, but has not been well-tested for metazoans, probably due to the difficulty of raising experimental populations to equilibrium and measuring population growth rates for species with long or complex life cycles. We developed an index (R(index)) of R* based on demography of one insect cohort, growing from egg to adult in a non-equilibrium setting, and tested whether R(index) yielded accurate predictions of competitive abilities using mosquitoes as a model system. We estimated finite rate of increase (λ') from demographic data for cohorts of three mosquito species raised with different detritus amounts, and estimated each species' R(index) using nonlinear regressions of λ' vs. initial detritus amount. All three species' R(index) differed significantly, and accurately predicted competitive hierarchy of the species determined in simultaneous pairwise competition experiments. Our R(index) could provide estimates and rigorous statistical comparisons of competitive ability for organisms for which typical chemostat methods and equilibrium population conditions are impractical.