One of the most common questions addressed by ecologists over the past decade has been - how does species richness impact the production of community-level biomass? Recent summaries of experiments have shown that species richness does tend to enhance biomass production across a wide variety of trophic groups and ecosystems; however, the biomass of diverse polycultures only rarely exceeds that of the single most productive species in a community (a phenomenon called ‘transgressive overyielding’). Some have hypothesized that the lack of transgressive overyielding is because experiments have generally been performed in overly-simplified, homogeneous environments where species have little opportunity to express the niche differences that lead to ‘complementary’ use of resources and which enhance biomass production. We tested this hypothesis in a laboratory experiment where we manipulated the richness of freshwater algae in environments that were comprised of patches containing either homogeneous nutrient ratios (16:1 nitrogen to phosphorus (N:P) in all patches) or heterogeneous nutrient ratios (ranging from 4:1 to 64:1 N:P across patches).  

Results/Conclusions

After allowing 6-10 generations of algal growth, we found that algal species richness had similar impacts on biomass production in both homo- and heterogeneous environments. Although four of the five algal species showed a strong response to nutrient heterogeneity, a single species dominated algal communities in both types of environments. As a result, a ‘selection effect’ – where diversity maximizes the chance that a competitively superior species will be included in, and dominate the biomass of a community – was the primary mechanism by which richness influenced biomass in both homo- and heterogeneous environments.  Our study suggests that spatial heterogeneity, by itself, is insufficient to generate strong effects of biodiversity on productivity.