Introduction

The various drivers of environmental global change, namely changes in land-use, climate and biogeochemical cycles, or the spread of invasive species, can have major impacts on the biological diversity of different ecosystems, including forests (Sala et al. 2000). Ongoing biodiversity loss has prompted concerns that the functioning of ecosystems and the services that humans derive from the environment may be compromised (Daily 1997; Millennium Ecosystem Assessment 2005). These losses and ecosystem degradation have substantial costs for society, as recently shown by The Economics of Ecosystems and Biodiversity study (TEEB 2009). The challenge of understanding the influence of biodiversity on ecosystem functioning (BEF) initiated a new interdisciplinary research area that emerged only two decades ago (Schulze & Mooney 1993), and the science of BEF has become one of the most active fields in ecology since then, as documented by a number of books and reviews (Hooper et al. 2005; Kinzig, Pacala & Tilman 2002; Loreau et al. 2001; Loreau, Naeem & Inchausti 2002; Naeem 2002; Naeem et al. 2009; Scherer-Lorenzen et al. 2005a; Schulze & Mooney 1993), and meta-analyses (Balvanera et al. 2006; Cardinale et al. 2006, 2007, 2011). A recent meta-analysis, for example, listed 574 independent experimental manipulations of species richness, published in 192 peer-reviewed papers, reporting 1417 observations on how species richness affects ecosystem processes (Cardinale et al. 2011).