Understanding the key ecological factors that moderate the relationship between biodiversity and the productivity of ecosystems is a major challenge for ecological research. Here we show that the relationship between the species richness of primary producers and net rates of biomass production in streams depends on the history of discharge-related disturbances. Using common methods to study 83 streams in the mid-Atlantic United States, we demonstrate that a positive relationship between producer richness and the net production of biomass only occurs in streams characterized by highly variable, unpredictable discharge regimes that represent frequently disturbed environments to benthic organisms. This pattern is partly explained by predictable changes in the composition of species assemblages among disturbed streams. The pattern also results because, in disturbed systems, species have faster rates of biomass accumulation when they are a part of species-rich assemblages. We discuss several possible explanations for this result. Our study supports qualitative predictions from theoretical and small-scale experimental work that the strength of the diversity-productivity relationship will depend explicitly on the disturbance regime of an ecosystem. The results have important implications for the management and conservation of aquatic resources as they suggest that changes to the productivity and diversity of streams may have their greatest impacts on organisms and systems that are most sensitive to disturbance.

Univariate explanations of biodiversity have often failed to account for broad-scale patterns in species richness. As a result, increased attention has been paid to the development and testing of more synthetic multivariate hypotheses. One class of multivariate hypotheses, founded in successional diversity theory, predict that species richness is jointly influenced by periodic disturbances that create new niche opportunities in space or time, and the production of community biomass that speeds displacement of inferior by superior competitors. While the joint response of diversity to disturbance and productivity has gained support from theoretical and small-scale experimental studies, evidence that corresponding patterns of biodiversity occur broadly across natural systems is scarce. Using a data set that employed standardized methods to sample 85 streams throughout the mid-Atlantic United States of America, we show that biogeographical patterns of primary producer diversity in stream ecosystems are consistent with the predictions of a multivariate model that incorporates disturbance frequency and community biomass production as independent variables. Periphyton species richness is a concave-down function of disturbance frequency (mean no. floods year(-1)) and of biomass production (mu g of biomass accrual cm(-2) day(-1)), and an increasing function of their interaction. Changes in richness across the disturbance x productivity response surface can be related to several predicted life-history traits of the dominant species. Our findings complement prior studies by showing that multivariate models which consider interactive effects of community production and ecosystem disturbance are, in fact, candidate explanations of much broader patterns of richness in natural systems. Because multivariate models predict synergistic effects of ecological variables on species diversity, human activities - which are simultaneously altering both the disturbance regime and productivity of streams - could be influencing biodiversity more than previously anticipated.

One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '... is species diversity the cause or the consequence of resources that limit community biomass?' Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey.