Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis

Although the effect of biodiversity on ecosystem functioning has become a m ajor focus in ecology, its significance in a fluctuating environment is sti ll poorly understood. According to the insurance hypothesis, biodiversity i nsures ecosystems against declines in their functioning because many specie s provide greater guarantees that some will maintain functioning even if ot hers fail. Here we examine this hypothesis theoretically, We develop a gene ral stochastic dynamic model to assess the effects of species richness on t he expected temporal mean and variance of ecosystem processes such as produ ctivity, based on individual species' productivity responses to environment al fluctuations. Our model shows two major insurance effects of species ric hness on ecosystem productivity: (i) a buffering effect, i.e., a reduction in the temporal variance of productivity, and (ii) a performance-enhancing effect, i.e., an increase in the temporal mean of productivity. The strengt h of these insurance effects is determined by three factors: (i) the way ec osystem productivity is determined by individual species responses to envir onmental fluctuations, (ii) the degree of asynchronicity of these responses , and (iii) the detailed form of these responses, In particular, the greate r the variance of the species responses, the lower the species richness at which the temporal mean of the ecosystem process saturates and the ecosyste m becomes redundant. These results provide a strong theoretical foundation for the insurance hypothesis, which proves to be a fundamental principle fo r understanding the long-term effects of biodiversity on ecosystem processe s.