The consequences of biodiversity loss for ecosystem functioning and ecosystem services have aroused considerable interest during the last decade. Experimental and theoretical work, however, has mainly focused on the impact of species diversity within single trophic levels, in particular in plant communities. An important current challenge is to understand how species interactions affect the relationship between biodiversity and ecosystem functioning in more complex systems with multiple trophic levels. Here I present a synthesis of new theoretical work on the effects of vertical diversity (number of trophic levels) and horizontal diversity (number of species within a trophic level) on the functioning of food webs and interaction webs.
Results/Conclusions

Trophic and non-trophic interactions make the relationships between biodiversity and ecosystem functioning potentially more complex in complex ecosystems than in the simple competitive communities that have been studied abundantly. Theory predicts that vertical diversity does not maximize ecosystem properties, but instead make them converge on intermediate values through damped oscillations as food-chain length increases. Overexploitation of biological resources is another factor that can deteriorate the functioning of diverse, strongly interacting ecosystems. Although horizontal species diversity is still expected to enhance ecosystem properties under a range of conditions, it can also reduce them when it promotes overexploitation. Furthermore, species additions or deletions often trigger abrupt changes in the structure and functioning of complex ecosystems. Thus, trophic and non-trophic species interactions are an important source of surprises and uncertainty in a rapidly changing world.