Species diversity and productivity: why do results of diversity-manipulation experiments differ from natural patterns?

1. Experiments that directly manipulate species diversity often report a positive diversity effect on productivity, whereas observations of natural communities reveal various productivity-diversity relationships and nutrient addition to natural plant communities generally results in negative productivity-diversity relationships. 2. We hypothesize that this apparent paradox may be potentially explained by the reduced roles of complementarity and positive selection effects, and the increased importance of competitive exclusion in natural communities compared to diversity-manipulation experiments. This hypothesis arises from the difference in species distribution and abundance patterns between immature synthetically assembled communities in diversity-manipulation experiments and more mature natural communities. 3. Our hypothesis applies best to small-scale studies within homogenous habitats and complements the environmental heterogeneity hypothesis that explains diversity-productivity patterns across heterogeneous habitats. 4. Synthesis. Our analysis highlights important differences between synthetic communities in diversity-manipulation experiments and natural communities that may translate into different diversity-productivity patterns, and cautions against indiscriminate extrapolations of results of diversity-manipulation experiments to natural communities.