Species richness destabilizes ecosystem functioning in experimental aquatic microcosms

The relationship between species diversity and ecosystem stability has long interested ecologists, yet no consensus has been reached and the underlying mechanisms remain unclear. We used five unicellular algal species, cultured in all possible combinations, to assemble microcosms containing 1 to 5 algal species, on which a cold perturbation was imposed. Our aim was to find whether and how species richness begets ecosystem resistance and resilience. In the experiment, the species-rich communities produced more biomass than the species-poor ones, either in pre-, under- or post-perturbation conditions. The positive diversity-biomass relationship was weakened by the perturbation, and fully restored one week after the perturbation. The diverse communities showed greater absolute biomass reduction during the perturbation than did species-poor systems. Resistance of community, measured by the relative change in biomass from pre- to under-perturbation, decreased with species richness. All the species showed significant reduction in biomass when stressed, without any density compensation among species in diverse communities; and the ratio of biomass change in each species was independent of diversity. The overyielding effect, measured as relative yield total, remained constant from pre- to under-perturbation; and the selection and complementarity effects played equal roles for the biodiversity effect on biomass production, and their relative importance was not altered by the perturbation. These results suggest that similar responses of different species to environmental perturbations may limit the insurance effect of biodiversity, and lead to an inverse diversity-resistance relationship.