Fire-Grazer Interactions During the Late Quaternary Extinctions

Grazing herbivores can limit fire activity locally by reducing fuel loads in grassy ecosystems where fires are frequent and fuel-limited. However, while grassy ecosystems drive trends in global fire activity and burned area, it remains unclear if the herbivore-fire interactions documented locally can affect fire activity at these broader spatial scales. Here, we used late Quaternary large-bodied herbivore extinctions as a global exclusion experiment to examine the responses of grassland paleofire activity to continental differences in extinction severity. To reconstruct these processes at a continental scale, we utilized data from two global databases: Herbitraits and the Global Paleofire Database (GPD). Grassy ecosystem fire activity increased in response to grazer extinction, with larger increases on continents that suffered the largest losses of grazers; browser declines had no such effect. The loss of other herbivore functional traits related to grass consumption (body size and digestive strategy) also tracks increased fire activity, but not as well as the loss of grazers alone. Additionally, non-grassy systems lack strong relationships between fire activity and browser extinctions, indicating browser controls on woody fuel loads may not scale up to continents the way that grazer effects do. These shifts suggest that herbivory can have Earth-system scale impacts on fire and that herbivore impacts should be explicitly considered when predicting changes in past and future global fire activity.