The Role Of Species Diversity For Ecosystem Productivity And Stability In Aquatic Microcosms

Concerns over the unprecedented loss of biodiversity have motivated ecologists to conduct considerableresearches, to describe the relationship between biodiversity and ecosystem function, and detect the underlyingmechanisms. I used experimental aquatic microcosms to explore the long-term relationships between speciesrichness and biomass production,or stability (resistance, resilience and temporal variability). The influences ofresource availability, nutrient fluctuation and colonization sequence of species on biodiversity effects wereexamined. Additionally, I proposed a new subset-based analyzing method for biodiversity experiments.No consistent diversity-biomass production relationship was found, it was positive in some of myexperiments but neutral in other ones. Both selection and complementarity effects contributed to thebiodiversity on biomass. The biodiversity effects could become either stronger or weaker through time,depending on the environmental context.A manipulated cold perturbation led to greater biomass reduction in diverse communities than inspecies-poor systems. Resistance of community decreased with species richness. All the individual speciesshowed significant reduction in biomass when stressed, and the ratio of biomass change in each species wasindependent of diversity. The similar responses of different species to environmental perturbations may be themajor cause of the inverse diversity-resistance relationship.I independently manipulated nutrient availability and species richness in aquatic algal microcosms. Thecomplementarity effect was greater in nutrient-poor than in nutrient-rich environment. The effect of speciesrichness on the temporal variability of community was neutral in rich environment, while a stabilizing effect ofdiversity was found in poor environment. During a cold perturbation compensation growth among species waspresent only in the poor environment, while community biomass was reduced only in the rich environment.The responses of community and constituent species to cold perturbation were diversity-independent in eachenvironment.I independently manipulated species richness and nutrient fluctuation regimes in experimental aquaticmicrocosms. Either in the nutrient-constant or fluctuating environment, the biodiversity effect on productivityand the complementarity effect both were positive at the early stage but became neutral during the later period,while the selection effect was zero. Species richness showed no stabilizing or destabilizing effect for temporalvariability in community biomass, but the underlying mechanisms were different across environments. In thenutrient-constant environment stabilizing effect by the statistical averaging effect was neutralized by thediversity effect on population level variability;while in the fluctuating environment the more positivecovariance with species richness and population level variability eliminated the stabilizing effect by thecomplementarity effect and statistical averaging.For testing the influence of colonization sequence on biodiversity effects, I grew six green algal species inmonocultures and all possible 2-species mixtures, with two colonization sequences established for eachmixture, The complementarity effect was positive in assemblies with low-yielding species as the initialdominants and negative in ones with the opposite colonization order;the selection effect was negative in theformer and positive in the latter. The complementarity and selection effects in combination allowed theassemblies with productive-unproductive species colonization order produced more biomass thanmonocultures, but the unproductive-productive assemblies only yielded the same level of biomass withmonocultures.I proposed a new subset approach for analyzing biodiversity experiments, which determines whether eachmulti-species combination performs better than its subset combinations (the next less diverse mixturescomprised by its component species), and test whether the increase in species richness at any given diversitylevel lead to further greater complementarity effect. I applied the different analyzing method, overyiedingindex, relative yield total (RYT), additive partitioning and subset approach, to the same experimental data. Theadditive partitioning and RYT methods could accurately test the existence of niche complementarity in amixture, and the subset approach could directly evaluate whether this mixture is of more complementarity thanits next less diverse mixtures.