EMPIRICAL-EVIDENCE THAT DECLINING SPECIES-DIVERSITY MAY ALTER THE PERFORMANCE OF TERRESTRIAL ECOSYSTEMS

We examined experimentally the association between species diversity a nd ecosystem processes in a series of terrestrial mesocosms. We develo ped and maintained 14 mesocosms whose biota were assembled from a sing le pool of plant and animal species and whose environmental conditions were identically controlled. Each community contained four trophic le vels: primary producers (annual herbs), consumers (herbivorous mollusc s and phloem sucking insects), secondary consumers (parasitoids) and d ecomposers (earthworms, Collembola and microbes). All mesocosms receiv ed the same diurnal pattern of light, temperature, relative humidity a nd water. The initial volume of soil, soil structure, composition, nut rient content and inocula of both soil microbes and nematodes were als o identical among replicates. The only experimentally manipulated fact or was the number of plant and animal species within each trophic leve l. High, medium and low diversity communities had nine, 15 or 31 plant and animal species, respectively. We measured five ecosystem processe s as response variables in these mesocosms over the course of 206 days : (i) community respiration; (ii) productivity; (iii) decomposition; ( iv) nutrient retention; and (v) water retention. The manipulation of d iversity produced communities that differed significantly in their eco system processes. Our results provide the first evidence (obtained by a direct manipulation of diversity under controlled environmental cond itions) that ecosystem processes may be affected by loss of diversity.