Trophic complementarity and ecosystem functioning

Background/Question/Methods

How biodiversity determines ecosystem functioning is a central research question in ecology, and has been so for over ten years. While the patterns and mechanisms concerning the impact of "horizontal" diversity were extremely well characterized, the consequences of varying "vertical" diversity, i.e. an explicit integration of the trophic structure, are more complicated. Here, we adopt a new perspective on this problem, by adapting a simple tri-trophic model of population dynamics (akin to a resources-plants-herbivores system), and exploring how varying degrees of trophic complementarity have an impact on ecosystem functioning. We illustrate our findings with a scenario related to current global changes: we simulate the extinctions of species at different trophic levels, and measure ecosystem functioning once stable state is reached.

Results/Conclusions

Through mathematical analyses of a simplified case of the model, we show that trophic complementarity among consumers or predators increases functioning, and maximal productivity allows the occurrence of a strong transgressive over-yielding. In more complex systems, measures of trophic complementarity at both trophic levels correlates positively with functioning, both measured by biomass or primary productivity at equilibrium. These values are often easily measurable in BEF experiments, allowing to use our work as a baseline prediction for patterns of functioning in trophic systems. In addition to providing new insights on how the structure of food-webs is a major force driving ecosystem functioning, we highlight possible outcomes of species loss, which we believe to be an important information to guide conservation policies. Specifically, removing predators increased biomass in the system, but results in a strong decrease in productivity, regardless of the changes in trophic complementarity. We discuss how these results are complementary to previous theoretical works on BEF relationships in food web, and how they allow designing experimental studies to test the patterns we report.