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Grassland species loss resulting from reduced niche dimension


Intact ecosystems contain large numbers of competing but coexisting species. Although numerous alternative theories have provided potential explanations for this high biodiversity, there have been few field experiments testing between these theories. In particular, theory predicts that higher diversity of coexisting competitors could result from greater niche dimensionality1, for example larger numbers of limiting resources or factors. Alternatively, diversity could be independent of niche dimensionality because large numbers of species can coexist when limited by just one or two factors if species have appropriate trade-offs2. Here we show that plant coexistence and diversity result from the ‘niche dimensionality’ of a habitat. Plant species numbers decreased with increasing numbers of added limiting soil resources (soil moisture, nitrogen, phosphorus and base cations), which is consistent with theoretical predictions that an increased supply of multiple limiting resources can reduce niche dimension. An observational field study gave similar results. The niche dimension hypothesis also explained diversity changes in the classic Park Grass Experiment at Rothamsted. Our results provide an alternative mechanistic explanation for the effects of nutrient eutrophication on the diversity of terrestrial, freshwater and marine ecosystems.

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Figure 1: Multiple resource limitation of plant productivity.
Figure 2: Species loss with reduction of niche dimension.
Figure 3: Effects of multiple resource supply on diversity and productivity across natural and experimental productivity gradients.


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We thank J. Chase, J. HilleRisLambers, K. Suding, S. Hobbie, R. Sterner, E. Lonsdorf and G. Oehlert for comments on early drafts of the manuscript, and S. Watts, H. Whitesides and R. Isip for field assistance. We also thank Sedgwick Reserve for research support, and P. Poulton and Rothamsted Research for assistance with data. This work was supported by the Andrew W. Mellon Foundation.

Author Contributions W.S.H. planned and executed the experiment and wrote the manuscript. D.T. contributed to planning and writing.

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Correspondence to W. Stanley Harpole.

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This file contains Supplementary Methods, Supplementary Tables S1-S4 with summary statistics and additional references. (PDF 893 kb)

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Harpole, W., Tilman, D. Grassland species loss resulting from reduced niche dimension. Nature 446, 791–793 (2007).

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