Linking biodiversity to ecosystem function: implications for conservation ecology

We evaluate the empirical and theoretical support for the hypothesis that a large proportion of native species richness is required to maximize ecosys tem stability and sustain function. This assessment is important for conser vation strategies because sustenance of ecosystem functions has been used a s an argument for the conservation of species. If ecosystem functions are s ustained at relatively low species richness, then arguing for the conservat ion of ecosystem function, no matter how important in its own right, does n ot strongly argue for the conservation of species. Additionally, for this t o be a strong conservation argument the link between species diversity and ecosystem functions of value to the human community must be clear. We revie w the empirical literature to quantify the support for two hypotheses: (1) species richness is positively correlated with ecosystem function, and (2) ecosystem functions do not saturate at low species richness relative to the observed or experimental diversity. Few empirical studies demonstrate impr oved function at high levels of species richness, Second, we analyze recent theoretical models in order to estimate the level of species richness requ ired to maintain ecosystem function. Again we find that, within a single tr ophic level, most mathematical models predict saturation of ecosystem funct ion at a low proportion of local species richness. We also analyze a theore tical model linking species number to ecosystem stability. This model predi cts that species richness beyond the first few species does not typically i ncrease ecosystem stability. One reason that high species richness may not contribute significantly to function or stability is that most communities are characterized by strong dominance such that a few species provide the v ast majority of the community biomass. Rapid turnover of species may rescue the concept that diversity leads to maximum function and stability. The ro le of turnover in ecosystem function and stability has not been investigate d. Despite the recent rush to embrace the linkage between biodiversity and ecosystem function, we find little support for the hypothesis that there is a strong dependence of ecosystem function on the full complement of divers ity within sites. Given this observation, the conservation community should take a cautious view of endorsing this linkage as a model to promote conse rvation goals.