Nutrients cause grassland biomass to outpace herbivory

Abstract

Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs ('consumer-controlled'). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food ('resource-controlled'). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk.

Fig. 1. Predictions for plant biomass with increasing environmental productivity.

Arrows indicate the predicted difference in biomass at ambient (Control) and elevated (Nutrient addition) productivity in the presence (Open) and absence (Fence) of herbivores. Predictions for plant mass with increasing productivity when herbivores a consume all additional biomass produced (see ref. ); d consume a constant proportion of biomass (see ref. ); g consume a constant amount of biomass. Panels b, e, h show these predictions for log(biomass). Panels c, f, i show the difference (arrow lengths (fence and nutrient) and difference of arrow lengths (Nut*Fnc) in b, e, h), and visualize these hypotheses as they would look in the factorial experimental test.

Fig. 2. Treatment effects on aboveground grassland biomass were similar for sites with 2–4 years of response data (n = 58, open circles), at least 5 years (n = 42, gray), and at least 8 years (n = 24, black).

Error bars represent 95% confidence intervals. Main effect values represent model mean differences of the experimental treatment from the control after controlling for site and year as random effects. The nutrient by fence interaction (“Nut*Fnc”) is compared to the sum of the effect of nutrients alone and fencing alone, such that zero indicates additivity.

Fig. 3. Herbivore control of grassland biomass varies with the biotic and abiotic environment.

Plots show the average site-level difference of each treatment from the unfertilized, unfenced control across a gradient of a herbivore impact intensity and frequency, b ambient soil nitrogen, and c mean annual precipitation. Gradients in panels a–c were identified from statistical models presented in Supplementary Tables 4a–c.