There are numerous studies demonstrating that biodiversity increases the strength of ecosystem functions (e.g., biomass production or pollination services). However, most studies manipulate species diversity by randomly assembling communities within small homogeneous environments. Although effects are generally positive, there is considerable variation in the strength of biodiversity-ecosystem function relationships (biodiversity effect) across studies. As such, there is debate regarding the extent to which these biodiversity-effects scale up to natural, heterogeneous landscapes where most management actions are targeted. We evaluated how landscape-scale environmental variation and species niche differences alter the relationship between herbaceous plant diversity and productivity in a temperate forest understory. First, we censused plant species composition, biomass production, and six functional traits in 90 1-m2 plots distributed across strong soil and light resource gradients in the forest. Next, we quantified niche differences based on functional trait diversity using three independent metrics: functional dispersion, functional divergence and functional evenness. Finally, we used general linear models to test the effects of environment and functional diversity on the strength of the diversity-productivity relationship.
Consistent with previous studies, we found species diversity increased productivity across all environments. However, both environmental differences and trait diversity altered the biodiversity effect. As environmental harshness increased, the relationship between diversity and productivity became significantly weaker. Additionally, two components of functional diversity mediated the strength of this relationship. Functional divergence (the degree of niche differentiation among dominant species) increased the biodiversity effect whereas functional evenness (the degree to which species abundances are distributed in niche space) decreased the biodiversity effect. Ultimately, we make three conclusions from this study. First, the importance of biodiversity for ecosystem functions demonstrated in homogenous, random assembly experiments can be extended to heterogeneous, naturally assembled communities. Second, the total production that can be realized by biodiversity is constrained or facilitated by environmental context. Third, greater differences between niches of the most common species, not the total niche space occupied, increased the impact of biodiversity on ecosystem function. These results have important implications for extending current theory to better predict how biotic homogenization and habitat loss will alter the functions of real-world ecosystems.