IDEA AND PERSPECTIVE Coexistence, niches and biodiversity effects on ecosystem functioning Lindsay Ann Turnbull, 1 * Jonathan M. Levine, 2,3 Michel Loreau 4 and Andy Hector 1 Abstract General principles from coexistence theory are often invoked to explain how and why mixtures of species outperform monocultures. However, the complementarity and selection effects commonly measured in bio- diversity experiments do not precisely quantify the niche and relative fitness differences that govern species coexistence. Given this lack of direct correspondence, how can we know whether species-rich mixtures are stable and that the benefits of diversity will therefore persist? We develop a resource-based included-niche model in which plant species have asymmetric access to a nested set of belowground resource pools. We use the model to show that positive complementarity effects arise from stabilising niche differences, but do not necessarily lead to stable coexistence and hence can be transient. In addition, these transient comple- mentarity effects occur in the model when there is no complementary resource use among species. Includ- ing a trade-off between uptake rates and the size of the resource pool stabilised interactions and led to persistent complementarity coupled with weak or negative selection effects, consistent with results from the longest-running field biodiversity experiments. We suggest that future progress requires a greater mechanis- tic understanding of the links between ecosystem functions and their underlying biological processes. Keywords Additive partition, biodiversity, coexistence, complementarity effects, niches, selection effects, transient complementarity. Ecology Letters (2013) 16: 116–127 INTRODUCTION Current rates of biodiversity loss may be as high as those observed during the major extinction events known from the fossil record (Barnosky et al. 2011). Unfortunately, efforts during the last two decades to slow the decline of biodiversity have largely failed, and this general degradation of the environment is set to continue for the foreseeable future (Butchart et al. 2010; Perrings et al. 2011). This lack of success is disappointing and worrying not only because of the loss of diversity itself but also because of potentially negative consequences for the functioning and stability of ecosystem pro- cesses and the environmental services provided to humanity. Recent reviews and meta-analyses of research from the last 20 years have confirmed that when compared with depauperate versions, more diverse ecosystems appear to utilise resources more effectively and are consequently more productive and stable (Balvanera et al. 2006; Duffy 2009; Cardinale et al. 2011, 2012; Hooper et al. 2012; Naeem et al. 2012). There is therefore a general consensus that biodiversity loss will have undesirable consequences for ecosystem functioning. While the empirical results are clear, the cause of the positive relationship between diversity and functioning has been rather more contentious. Over the last 20 years, biodiversity researchers have repeatedly drawn on the coexistence literature to help explain observed patterns, for example, suggesting that niche differences allow species to complement one another in mixture and hence bet- ter utilise the available resources (Gross et al. 2007; Northfield et al. 2010; Cardinale 2011). Coexistence theory is clearly relevant for bio- diversity research, as only in stable communities can diversity persist and hence continue to deliver whatever benefits it confers. How- ever, ecological niches and the coexistence they enable are notori- ously slippery concepts, perhaps explaining why most references to niches within the biodiversity literature remain vague. Here, we first review past and current attempts to synthesise coexistence theory with the biodiversity literature and use a new model developed for plant communities to improve our interpretation of the complemen- tarity and selection effects and how they relate to concepts from coexistence theory. Conceptual development A Darwinian synthesis? The idea that the mechanisms underpinning species coexistence are the same as those that link biodiversity with ecosystem functioning can be traced all the way back to Darwin’s principle of divergence (McNaughton 1993; Hector & Hooper 2002; Hector 2009) whereby natural selection drives the evolution of species into different and complementary niches. Darwin explicitly states that more diverse ecosystems should have higher rates of ‘chemical composition and decomposition’. He even talks of the ‘division of labour’ arising from the divergence of species into different niches, using Adam 1 Institute of Evolutionary Biology and Environmental Studies, University of Zurich, 8057, Zurich, Switzerland 2 Institute of Integrative Biology, ETH Zurich, 8092, Zurich, Switzerland 3 Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, USA 4 Centre for Biodiversity Theory and Modelling, Station d’Ecologie Exp erimentale du CNRS, 09200, Moulis, France *Correspondence: E-mail: lindsay.turnbull@ieu.uzh.ch © 2012 John Wiley & Sons Ltd/CNRS Ecology Letters, (2013) 16: 116–127 doi: 10.1111/ele.12056