Annemarie Wurz, Jörg Albrecht, Katrin Böhning-Gaese, Roland Brandl, Eike Lena Neuschulz, Jörg Bendix, Markus Fischer, Andreas Hemp, Jürgen Homeier, Ralf Kiese, Yakov Kuzyakov, Christoph Leuschner, Marcell K. Peters, Stefan Scheu, Ingolf Steffan-Dewenter, Andre Velescu, Wolfgang Wilcke, Matthias Schleuning, Nina Farwig

Aim
Significant progress has been made in understanding the links between biodiversity and ecosystem functioning in both experimental and real-world ecosystems. Yet, we have limited understanding to which extent biodiversity affects ecosystem functioning in natural heterogeneous environments and whether changes in ecosystem functions are related to changes in species richness and/or turnover. Here we (1) quantify the contribution of diversity to variation in ecosystem functioning (i.e., the diversity effect) in heterogeneous environments and (2) test whether spatial variation in species richness and/or turnover between communities mediate effects of environmental heterogeneity on the diversity effect.

Location
Two tropical elevational gradients: Andes (Ecuador) and Mt. Kilimanjaro (Tanzania).

Time period
Current.

Taxa studied
Multiple: plants, oribatid mites, springtails, ants and birds.

Methods
We expand an analytical framework based on the Price equation to quantify the relative contribution of species richness or turnover to variation in ecosystem functioning within and across ecosystem types. We applied this framework using data on seven ecosystem functions collected in six ecosystem types on two tropical elevational gradients.
Results
We found a consistent increase in the diversity effect on ecosystem functioning with increasing environmental heterogeneity. Both species richness and turnover contributed similarly to the diversity effect. The increase in the diversity effect with environmental heterogeneity was solely based on the variation in species richness. The effect of species turnover was unrelated to environmental heterogeneity.

Main Conclusions
Our proposed framework enables the quantification of BEF relationships at large spatial scales and across various ecosystem types. It expands on previous studies by allowing comparisons among naturally assembled communities along environmental gradients. By applying our framework to two tropical systems, we show that changes in species richness and turnover contribute similarly to variations in ecosystem functioning across both elevational gradients. However, species richness is particularly important in mediating the effects of environmental heterogeneity on ecosystem functioning.