Stability of ecosystem properties in response to above-ground functional group richness and composition

While there has been a rapidly increasing research effort focused on unders tanding whether and how composition and richness of species and functional groups may determine ecosystem properties, much remains unknown about how t hese community attributes affect the dynamic properties of ecosystems. We c onducted an experiment in 540 mini-ecosystems in glasshouse conditions, usi ng an experimental design previously shown to be appropriate for testing fo r functional group richness and composition effects in ecosystems. Artifici al communities representing 12 different above-ground community structures were assembled. These included treatments consisting of monoculture and two - and four-species mixtures from a pool of four plant species; each plant s pecies represented a different functional group. Additional treatments incl uded two herbivore species, either singly or in mixture, and with or withou t top predators. These experimental units were then either subjected to an experimentally imposed disturbance (drought) for 40 d or left undisturbed. Community composition and drought both had important effects on plant produ ctivity and biomass, and on several below-ground chemical and biological pr operties, including those linked to the functioning of the decomposer subsy stem. Many of these compositional effects were due to effects both of plant and of herbivore species. Plant functional group richness also exerted pos itive effects on plant biomass and productivity, but not on any of the belo w-ground properties. Above-ground composition also had important effects on the response of below-ground properties to drought and thus influenced eco system stability (resistance); effects of composition on drought resistance of above-ground plant response variables and soil chemical properties were weaker and less consistent. Despite the positive effects of plant function al group richness on some ecosystem properties, there was no effect of rich ness on the resistance of any of the ecosystem properties we considered. Al though herbivores had detectable effects on the resistance of some ecosyste m properties, there were no effects of the mixed herbivore species treatmen t on resistance relative to the single species herbivore treatments. Increa sing above-ground food chain length from zero to three trophic levels did n ot have any consistent effect on the stability of ecosystem properties. The re was no evidence of either above-ground composition or functional group r ichness affecting the recovery rate of ecosystem properties from drought an d hence ecosystem resilience. Our data collectively point to the role of co mposition (identity of functional group), but not functional group richness , in determining the stability (resistance to disturbance) of ecosystem pro perties, and indicates that the nature of the above-ground community can be an important determinant of the consistency of delivery of ecosystem servi ces.