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Herbivores and nutrients control grassland plant diversity via light limitation

Abstract

Human alterations to nutrient cycles1,2 and herbivore communities3,4,5,6,7 are affecting global biodiversity dramatically2. Ecological theory predicts these changes should be strongly counteractive: nutrient addition drives plant species loss through intensified competition for light, whereas herbivores prevent competitive exclusion by increasing ground-level light, particularly in productive systems8,9. Here we use experimental data spanning a globally relevant range of conditions to test the hypothesis that herbaceous plant species losses caused by eutrophication may be offset by increased light availability due to herbivory. This experiment, replicated in 40 grasslands on 6 continents, demonstrates that nutrients and herbivores can serve as counteracting forces to control local plant diversity through light limitation, independent of site productivity, soil nitrogen, herbivore type and climate. Nutrient addition consistently reduced local diversity through light limitation, and herbivory rescued diversity at sites where it alleviated light limitation. Thus, species loss from anthropogenic eutrophication can be ameliorated in grasslands where herbivory increases ground-level light.

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Figure 1: Geographic and climatic distribution of experimental sites.
Figure 2: Mixed-effects model parameters showing average response of plots (n = 360) to 3 years of nutrient addition and herbivore exclusion by fencing.
Figure 3: Effects of herbivore exclusion by fencing on mean grassland species richness and the mean proportion of PAR reaching ground level at 29 sites after 3 years of treatment.

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Acknowledgements

This work uses data from the Nutrient Network (http://nutnet.org) experiment, funded at the site scale by individual researchers. Coordination and data management are supported by funding to E. Borer and E. Seabloom from the NSF Research Coordination Network (NSF-DEB-1042132) and Long Term Ecological Research (NSF-DEB-1234162 to Cedar Creek LTER) programs and the UMN Institute on the Environment (DG-0001-13). The Minnesota Supercomputer Institute hosts project data. We are grateful to F. Isbell for suggestions that improved the manuscript. Any use of trade names is for descriptive purposes only and does not imply endorsement by the US Government.

Author information

Authors and Affiliations

Authors

Contributions

E.T.B., E.W.S., W.S.H. and E.M.L. are Nutrient Network coordinators. E.T.B., W.S.H., H.H. and D.S.G. developed and framed the research questions in this paper. All authors contributed data from this experiment. E.T.B. and E.W.S. analysed the data. D.S.G., W.S.H. and E.M.L. contributed to data analyses. E.T.B. wrote the paper with input from all authors. Supplementary Information Appendix S2 provides further information on author contributions.

Corresponding author

Correspondence to Elizabeth T. Borer.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Regression parameters for treatment effects.

ac, All available data are shown for richness (a), total biomass (b) and ambient light reaching ground level (c). Error bars represent ±2 s.e.m. Treatment years and their associated sample sizes are shown in each panel. One- and two-year models represent greater spatial extent and replication, but reduced temporal extent compared to Fig. 2 in the main text. Four-year models represent longer temporal effects, but reduced spatial extent, particularly for light measurements. All models were fitted as in Extended Data Tables 2, 3, 4 and described in the Methods.

Extended Data Figure 2 Fertilization does not alter the relationship between ‘fence’ effects on light and diversity.

The log response ratio (LRR) model of the effect of fences (herbivore exclusion) on richness and light (year 3 data) demonstrates no additional effect of nutrient addition on the relationship shown in Fig. 3. The grey region indicates the 95% confidence interval around the regression. The effect of fences on ground-level light predicts changes in plot-scale species richness (P = 0.00254), whereas fertilization is not included in the final statistical model of this relationship (P > 0.05). Thus, the magnitude of the effect of grazers on richness is dependent on the magnitude of their effect on light regardless of whether a plot has been fertilized.

Extended Data Table 1 Sites contributing experimental data
Extended Data Table 2 Statistical model for treatment effects on richness after 3 years of treatment (n = 29) as a function of fertilization by N, P and K and micronutrients, herbivore exclusion by fencing, and their interaction
Extended Data Table 3 Statistical model for treatment effects on biomass after 3 years of treatment (n = 29) as a function of fertilization by N, P and K and micronutrients, herbivore exclusion by fencing, and their interaction
Extended Data Table 4 Statistical model for treatment effects on proportion of photosynthetically active radiation (PAR) reaching ground level after 3 years of treatment (n = 29) as a function of fertilization by N, P and K and micronutrients, herbivore exclusion by fencing, and their interaction
Extended Data Table 5 Statistical model for biomass effects on ground-level proportion of photosynthetically active radiation (PAR) after 3 years of treatment (n = 29) as a function of total plot-scale biomass
Extended Data Table 6 Effects of climate, nitrogen deposition, soil nitrogen and site productivity on change in ground-level light across experimental fencing treatments after 3 years of treatment
Extended Data Table 7 Effects of climate, nitrogen deposition, soil nitrogen and site productivity on site-level mean biomass change across experimental fencing treatments after 3 years of treatment
Extended Data Table 8 Effects of climate, nitrogen deposition, soil nitrogen, site productivity, and change in light on change in site-level mean plant species richness across experimental fencing treatments after three years of treatments

Supplementary information

Supplementary Information

This file contains Supplementary Appendices S1-S2. (PDF 159 kb)

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Borer, E., Seabloom, E., Gruner, D. et al. Herbivores and nutrients control grassland plant diversity via light limitation. Nature 508, 517–520 (2014). https://doi.org/10.1038/nature13144

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