The global decline in biodiversity induced by human activities has prompted scientists to investigate the potential value of biodiversity for ecosystem services (Chapin et al. 2000; Schröter et al. 2005; Diaz et al. 2007). A large number of experiments have been conducted in the last decade, in particular in herbaceous communities, in order to assess the potential roles of different components of biodiversity for community productivity, stability, and invasibility (see Hooper et al. 2005). Because most results of these experiments were considered to be opposite to the natural patterns of biodiversity along productivity gradients (Loreau et al. 2001), this fueled substantial debate about the underlying mechanisms of the effect of biodiversity on ecosystem functions (e.g., Huston et al. 2000). Furthermore, this created a new interest in the ecological drivers of biodiversity in natural environments, a topic of research that had interested plant ecologists in the past (Whittaker 1972; Grime 1973; Connell 1978; Huston 1979).