Cadmium pollution triggers a positive biodiversity-productivity relationship: evidence from a laboratory microcosm experiment.
Human activities are greatly changing the conditions of ecosystems. One of these major changes is that more and more ecosystems are facing increasing concentrations of highly toxic heavy metals like cadmium (Cd), lead (Pb) and mercury (Hg). Previous studies have shown this type of pollution may be associated with the loss of biodiversity, leaving some pollution-tolerant species to dominate the polluted ecosystems. To date, however, little is known about the extent to which the pollution caused by these toxins may affect the relationship between biodiversity and ecosystem function. In this study, we aimed to assess the role of Cd pollution in determining biodiversity-productivity relationships in aquatic ecosystems, by using data from a laboratory microcosm experiment that tested 110 algal communities with three levels of Cd pollution. Our results showed that the productivities of algal communities growing in non-Cd-polluted media did not significantly increase with increasing species richness, and this pattern did not change over time. In contrast, significantly positive biodiversity-productivity relationships were consistently found in algal communities growing in Cd-polluted media. We revealed that the putative facilitation among algal species in response to Cd pollution was the main driver of the observed significantly positive biodiversity-productivity relationships. Most importantly, we found that not only Cd-tolerant algal species but also Cd-sensitive algal species were able to dominate Cd-polluted polycultures and contribute to the increased productivities of these polycultures. Synthesis and applications. Collectively, our results provide the first explicit experimental evidence that Cd pollution can trigger a positive biodiversity-productivity relationship. We identify two profound implications: (i) conservation of biodiversity in all environments may reduce the future impacts of increasing environmental stresses (like Cd pollution) on ecosystem function (such as primary productivity); (ii) one possible approach to maintain or improve algal primary productivity in a polluted aquatic ecosystem may be to construct some suitable diverse algal assemblages that include not only pollution-tolerant species but also pollution-sensitive ones.