Biodiversity and ecosystem functioning: recent theoretical advances

The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today. As experiments progress. there is a gro wing need for adequate theories and models to provide robust interpretation s and generalisations of experimental results. and to Formulate new hypothe ses. This paper provides an overview of recent theoretical advances that ha ve been made on the two major questions in this area: (1) How does biodiver sity affect the magnitude of ecosystem processes (short-term effects of bio diversity)? (2) How does biodiversity contribute to the stability and maint enance of ecosystem processes in the face of perturbations (lung-term effec ts of biodiversity)? Positive short-term effects of species diversity on ecosystem processes, su ch as primary productivity. and nutrient retention, have been explained by two major types of mechanisms: (1) Functional niche complementarity (the co mplementarity effect). and (2) selection of extreme trait values (the selec tion effect). In both cases, biodiversity provides a range of phenotypic tr ait variation. In the complementarity effect, trait variation then forms th e basis For a permanent association of species that enhances collective per formance. In the selection effect, trait variation comes into play only as an initial condition, and a selective process then promotes dominance by sp ecies with extreme trait values. Major differences between within-sire effe cts of biodiversity and across-sits productivity diversity patterns have al so been clarified. The local effects of diversity on ecosystem processes ar e 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 deca des seems necessary if we are to account for long-term effects of biodivers ity on ecosystem functioning. The classical deterministic, equilibrium appr oaches to stability do not explain the reduced temporal variability of aggr egate 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 environm ent: (1) a buffering effect, i.e., a reduction in the temporal variance, an d (2) a performance enhancing effect, i.e., an increase in the temporal mea n. The basic mechanisms involved in these long-term insurance effects are v ery similar to those that operate in short-term biodiversity effects: tempo ral niche complementarity, and selection of extreme trait values. The abili ty of species diversity to provide an insurance against environmental fluct uations and a reservoir of variation allowing adaptation to changing condit ions may be critical in a long-term perspective. These recent theoretical developments in the area of biodiversity and ecosy stem Functioning suggest that linking community and ecosystem ecology is a fruitful avenue, which paves the way for a new ecological synthesis.