Quaternary megaherbivore extinctions altered global fire activity, with lessons for future fire models

The loss of the large-bodied grazers following the late Quaternary thousands of years ago drastically altered fire regimes worldwide, increasing grassy ecosystem fire activity, particularly where extinctions were most severe. These findings from a new study suggest that herbivore-fire impacts operate at continental scales and need to be considered when predicting global fire activity in the past, present, and future. Wildfires provide crucial ecological services and understanding their drivers and frequency is central to understanding ecosystems, climate regimes and biogeochemical processes over geologic time scales. Large grassland grazers limit fire activity by consuming plants that would otherwise provide fuel for wildfires. However, while herbivore-fire interactions are increasingly recognized as important influences on large-scale ecosystem functioning, it remains unclear whether these effects remain locally restricted or if they have far-reaching effects on fire at continental scales. According to the authors, the megafaunal extinctions of the late Quaternary (~50 to 7 thousand years ago) – which resulted in continental-scale depopulation of large-bodied herbivores – present a unique opportunity to test the hypothesis that herbivore effects on wildfires are far-reaching. Allison Karp and colleagues used the late Quaternary extinction as a global exclusion experiment to examine the responses of grassy ecosystem wildfire activity to megaherbivore loss. To do this, Karp et al. compared the severity of extinction in different grassy biomes around the world with changes in paleofire activity derived from sedimentary charcoal data. They found that fire frequency increased more where megaherbivore extinctions were greatest, like South America, and least where fewer extinctions occurred, such as Africa. The results suggest that the loss of large-bodied grazers across the late Quaternary greatly altered wildfire regimes worldwide. The authors note that despite major shifts in extant herbivore communities, few global fire models incorporate herbivore-fire interactions. “Our results highlight the need to incorporate herbivore controls into our understanding of fire activity and explicitly into fire models, which may help reconcile mismatches between observations and modeled scenarios for grassland fire activity.”