The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today. As experiments progress. there is a growing need for adequate theories and models to provide robust interpretations and generalisations of experimental results. and to Formulate new hypotheses. This paper provides an overview of recent theoretical advances that have been made on the two major questions in this area: (1) How does biodiversity affect the magnitude of ecosystem processes (short-term effects of biodiversity)? (2) How does biodiversity contribute to the stability and maintenance of ecosystem processes in the face of perturbations (lung-term effects of biodiversity)? Positive short-term effects of species diversity on ecosystem processes, such as primary productivity. and nutrient retention, have been explained by two major types of mechanisms: (1) Functional niche complementarity (the complementarity effect). and (2) selection of extreme trait values (the selection effect). In both cases, biodiversity provides a range of phenotypic trait variation. In the complementarity effect, trait variation then forms the basis For a permanent association of species that enhances collective performance. In the selection effect, trait variation comes into play only as an initial condition, and a selective process then promotes dominance by species with extreme trait values. Major differences between within-sire effects of biodiversity and across-sits productivity diversity patterns have also been clarified. The local effects of diversity on ecosystem processes are expected to be masked by the effects of varying environmental parameters in across-site comparisons. A major reappraisal of the paradigm that has dominated during the last decades seems necessary if we are to account for long-term effects of biodiversity on ecosystem functioning. The classical deterministic, equilibrium approaches to stability do not explain the reduced temporal variability of aggregate ecosystem properties thar has been observed in more diverse systems. On the other hand, stochastic, nonequilibrium approaches do show two types of biodiversity effects on ecosystem productivity in a fluctuating environment: (1) a buffering effect, i.e., a reduction in the temporal variance, and (2) a performance enhancing effect, i.e., an increase in the temporal mean. The basic mechanisms involved in these long-term insurance effects are very similar to those that operate in short-term biodiversity effects: temporal niche complementarity, and selection of extreme trait values. The ability of species diversity to provide an insurance against environmental fluctuations and a reservoir of variation allowing adaptation to changing conditions may be critical in a long-term perspective. These recent theoretical developments in the area of biodiversity and ecosystem Functioning suggest that linking community and ecosystem ecology is a fruitful avenue, which paves the way for a new ecological synthesis.
