The stability of ecosystems has not only intrigued biologists for generations, but is essential for human well-being, as we depend on the stable provisioning of ecosystem services like food and fuel. A large body of research has explored how ecosystem stability depends on biodiversity; however, most of this research only manipulated producer diversity, despite evidence that predators face greater extinction threats than herbivores or producers. The influence of predator diversity on community stability has rarely been tested experimentally, either to relate predator diversity to the stability of natural ecosystems, or in an applied setting investigating the stability of crop yield via pest control.
Phytoplankton are cultured for a variety of products, and show much promise as a biofuel feedstock. However, culturing ponds are inevitably invaded by zooplankton grazers, causing culture crashes and subsequent large oscillations, and reducing the predictability and profitability of the crop.
We grew communities of local freshwater plankton in outdoor pond mesocosms with one, two, or three predator species varying in size (hemipterans and a copepod), as well as a phytoplankton-only treatment. We then sampled weekly for six weeks to test whether more functionally diverse predator communities lead to higher and less variable (lower CV) phytoplankton biomass.
Results/Conclusions
Phytoplankton biomass was as high in mesocosms with predators as it was in phytoplankton-only mesocosms, indicating that the invertebrate predators all prevented zooplankton grazers from significantly reducing phytoplankton yields. Functional predator diversity had no effect on mean phytoplankton biomass, because even a single predator species controlled grazers enough to allow the phytoplankton to flourish. However, temporal variability (CV) of phytoplankton biomass in mesocosms with all three predators was reduced by 50% compared to mesocosms with one or two predators. Thus, while predator diversity did not cascade down to affect average producer biomass, functional predator diversity had a stabilizing effect on producer biomass in this aquatic system. These results suggest that invertebrate predators can provide effective biological control of grazers in algae ponds, and further, that a functionally diverse predator community can improve the reliability of algal yields. Additionally, the results imply that predator diversity may play an important role in maintaining the stability of ecosystems.