How Plant Population Processes Mediate Biodiversity Effects on Ecosystem Functioning

In a large integrated biodiversity project (“The Jena Experiment” in Germany) we established two experiments, one with a pool of 60 plant species that ranged broadly from dominant to subordinate competitors on large 20 × 20 m and small 3.5 × 3.5 m plots (= main experiment), and one with a pool of nine potentially dominant species on small 3.5 × 3.5 m plots (= dominance experiment). We found identical positive species richness–aboveground productivity relationships in the main experiment at both scales. This result suggests that scaling up, at least over the short term, is appropriate in interpreting the implications of such experiments for larger-scale patterns. The species richness–productivity relationship was more pronounced in the experiment with dominant species (46.7 % and 82.6 % yield increase compared to mean monoculture, respectively). Additionally, transgressive overyielding occurred more frequently in the dominance experiment (67.7 % of cases) than in the main experiment (23.4 % of cases). Additive partitioning and relative yield total analyses showed that both complementarity and selection effects contributed to the positive net biodiversity effect. Overyielding in experimental grassland communities 13 INTRODUCTION A central issue in current ecological research is the potential influence of biodiversity on ecosystem functioning (Chapin et al. 1998; Loreau et al. 2001). Environmental conditions, species interactions and the available species pool all influence species diversity and ecosystem properties (Lepš 2004). As such, observational studies cannot provide the same insights as biodiversity experiments, where either diversity or ecosystem properties are experimentally manipulated (Pfisterer et al. 2004; Schmid & Hector 2004). For example, clear causal relationships between species richness and ecosystem productivity can be examined only with experimental approaches, keeping environmental conditions constant among treatments (“within-site comparisons”). Such experiments can best be done in ecosystems that allow easy manipulation of species richness and rapid measurement of productivity, such as perennial grassland communities (Loreau et al. 2002). Experiments with this system have often, but not always found a positive, asymptotic relationship between plant species richness on the x-axis and aboveground plant biomass production on the y-axis (e.g., Tilman et al. 1997; Hector et al. 1999; further references in Schmid et al. 2002b). Initial disputes about the possible explanations for the positive relationship have been addressed with the newly developed additive partitioning analysis method that allows separating a net biodiversity effect into contributions of complementarity and selection effects (Loreau & Hector 2001; Hector et al. 2002a). Complementarity occurs if performance of species in mixture is on average higher than expected from their monoculture yields, while the selection effect explains higher productivity of mixtures by the dominance of individual, highlyproductive species. Despite the advances in experimental design and statistical analysis (Schmid et al. 2002a), major questions remain about the ability to extrapolate the relationship between plant species richness and biomass production across time, space, environments, species pools and other Overyielding in experimental grassland communities 14 factors. Analyses of data from the Cedar Creek experiment in Minnesota and theoretical considerations suggest that complementarity effects increase over time whereas positive selection effects decrease (Tilman et al. 2001; Pacala & Tilman 2002). The BIODEPTH multi-site experiment has shown that diversity effects on biomass production can vary to some extent across localities, but a general pattern was found if all sites were analysed together (Hector et al. 2002b). Several studies found that increasing CO2 or nutrient levels can accentuate diversity effects (Stocker et al. 1999; Reich et al. 2001; He et al. 2002; Fridley 2002, 2003). Experimental studies often simulate random species loss, but in natural communities, rare and uncommon species are subject to higher risk of extinction because of smaller population sizes (MacArthur & Wilson 1967; Pimm et al. 1988). Recent results of removal experiments reducing species number in a non-random fashion provided evidence that dominant species can sometimes control ecosystem functioning (Smith & Knapp 2003; Zavaleta & Hulvey 2004). It may be that the drop in productivity with random species loss, observed in experiments, would be less strong if subordinate species were not included in the species pool from the beginning (equivalent to subordinate species going extinct first). If selection effects in such a pool of only dominant species are the major cause for the relationship, then productivity would be expected to increase less with species richness in this case, because averaging only across highly productive species in monocultures or low-diversity mixtures will not yield the low values expected with averaging across monocultures or low-diversity mixtures of both dominant and subordinate species. In addition to using different species pools, the spatial scale of previous biodiversity experiments varied considerably, with plot sizes from about one to more than 100 m, such that the experimental effects might have been confounded with scale effects (Schmid et al. 2002a). Scale effects may be caused by abiotic factors (e.g. different intensity of exchange Overyielding in experimental grassland communities 15 processes with the surrounding environment of large versus small stands), biotic factors (e.g. smaller population sizes and reduced fitness, higher or lower visitation rates of plants by herbivores or pollinators in small plots), or increasing environmental heterogeneity with increasing plot size and other, presently unknown factors. In the literature these effects are often related to an increasing amount of edge relative to central parts in smaller plots (see e.g. Groppe et al. 2001; Fahrig 2003 and references in these). To examine these issues, we manipulated two factors. First, we manipulated plot size to examine scale effects. Second, we manipulated the relative abundance of dominant species in the species pool to determine the extent to which the exclusion of minor species changes the richness–productivity relationships. The experimental set-up of this integrated project (“The Jena Experiment”), carried out near the Saale river in Jena (Germany), comprises a total of more than 480 plots, arranged on a single large field with four blocks (Roscher et al. 2004). The main experiment uses a large species pool of 60 species, on either plots of 20 × 20 m or of 3.5 × 3.5 m. A second experiment (dominance experiment) uses a species pool of nine dominant species and plots of 3.5 × 3.5 m. These experiments serve as a platform for a number of studies evaluating biodiversity effects on ecosystem functioning. In this paper, we present the first results of The Jena Experiment obtained by measuring peak biomass at the end of May in the second year of the project (2003), comparable to the main measurements analysed in the European BIODEPTH project (Hector et al. 1999, 2002a, 2002b). First, we ask if a positive relationship between plant species richness and productivity (henceforth species richness-productivity relationship) also applies in The Jena Experiment. Second, we test if the relationship differs between large and small plots of the main experiment. If the relationship is independent of plot size, this may indicate that scaling up from small-plot studies is possible, at least in the short term. Third, we compare the relationship from the main experiment with the large species pool with the dominance Overyielding in experimental grassland communities 16