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.

Background and aims The diverse flora of the Brazilian Cerrado is threatened by agricultural expansion, nutrient enrichment and invasion of alien plants. We performed a fertilization experiment to investigate the nature of nutrient limitation in Cerrado vegetation, and evaluate whether native and alien invasive species are limited by the same or different nutrients. Methods We applied various combinations of nutrients (phosphorus (P), nitrogen (N), and a mixture of other macro-and micro-nutrients ('cations treatment')) to six types of Cerrado vegetation. We then studied over a 3year period how these treatments affected the aboveground biomass of native forbs, native C 3 and C 4 grasses, and invasive C 4 grasses. ResultsThe full nutrient treatment (N+P+ 'cations') significantly increased total community biomass across our sites, but P alone had no effect. The nutrient treatments also affected the relative abundance of functional plant groups in the six vegetation types. P addition, either alone or in combination with other nutrients, increased the biomass of alien C 4 grasses, where present, whereas the cations treatment stimulated growth of the native C 4 grasses. Addition of N+P reduced the biomass of native C 3 grasses. Conclusions Our results indicate co-limitation by several nutrients, including P, perhaps N, and at least one other nutrient. Further research is needed to determine what the other nutrient (or nutrients) may be. Native and invasive species appear to be limited by different nutrients, with P alone stimulating growth of African C 4 grasses. This should be considered in managing both natural and invaded communities.

SummaryAlthough endangered and alien invasive plants are commonly assumed to persist under different environmental conditions, surprisingly few studies have investigated whether this is the case. We examined how endangered and alien species are distributed in relation to community biomass and N : P ratio in the above-ground community biomass in savanna vegetation in the Brazilian Cerrado.For 60 plots, we related the occurrence of endangered (Red List) and alien invasive species to plant species richness, vegetation biomass and N : P ratio, and soil variables.Endangered plants occurred mainly in plots with relatively low above-ground biomass and high N : P ratios, whereas alien invasive species occurred in plots with intermediate to high biomass and low N : P ratios. Occurrences of endangered or alien plants were unrelated to extractable N and P concentrations in the soil.These contrasting distributions in the Cerrado imply that alien species only pose a threat to endangered species if they are able to invade sites occupied by these species and increase the above-ground biomass and/or decrease the N : P ratio of the vegetation. We found some evidence that alien species do increase above-ground community biomass in the Cerrado, but their possible effect on N : P stoichiometry requires further study.

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