Grazing herbivores reduce herbaceous biomass and fire activity across African savannas

Authors: KARP, ALLISON - Yale University; KOERNER, SALLY - University Of North Carolina Greensboro; HEMPSON, GARETH - University Of Witwatersrand; ABRAHAM, JOEL - Princeton University; ANDERSON, T - Wake Forest University; BOND, WILLIAM - University Of Cape Town; BURKEPILE, DERON - University Of California; GOHEEN, JACOB - University Of Wyoming; GUYTON, JENNIFER - University Of Wyoming; KARTZINEL, TYLER - Brown University; KIMUYU, DUNCAN - Karatina University; MOHANBABU, NEHA - Syracuse University; PALMER, TODD - University Of Florida; Porensky, Lauren

Submitted to: Ecology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2024
Publication Date: 6/10/2024
Citation: Karp, A.T., Koerner, S.I., Hempson, G.P., Abraham, J.O., Anderson, T.M., Bond, W.J., Burkepile, D.E., Goheen, J.R., Guyton, J.A., Kartzinel, T.R., Kimuyu, D.M., Mohanbabu, N., Palmer, T.M., Porensky, L.M. 2024. Grazing herbivores reduce herbaceous biomass and fire activity across African savannas. Ecology Letters. 27. Article e14450. https://doi.org/10.1111/ele.14450.
DOI: https://doi.org/10.1111/ele.14450

Interpretive Summary: Fire and herbivory both shape ecosystems and carbon cycling, and these processes interact. In savannas, herbivores can reduce fire through the removal of grass biomass, but the size and variability of these effects remains uncertain. We quantified grazing reductions of fuel and burned area under different environmental conditions across African savannas using a combination of herbivore exclosure experiments and remotely sensed estimates of fire activity and grazing herbivore density. We found that grazing herbivores affect both herbaceous biomass and fire activity. The addition of 100 kg/km2 grazing herbivore biomass (~ one zebra) resulted in a ~30 kg/ha reduction in herbaceous biomass and a ~0.35% reduction in burned area, on average. Effects are strongly tied to grazing herbivore densities and consistent across different environments. Our results indicate that burned area models may be improved by including grazing effects on grass biomass.

Technical Abstract: Grazing herbivores have strong impacts on herbaceous biomass in savannas. The magnitude of grazer impacts is known to increase with grazing herbivore abundance, but it remains uncertain whether they also vary with environmental conditions (e.g. rainfall and soil properties). These variations could have knock-on consequences for other ecosystem processes, and it also remains unclear how variation in grazing impacts scale up to influence savanna fire regimes. To examine grazing effects on herbaceous biomass and fire regimes across savannas, we combined herbivore exclosure experiment data (6 reserves) with data on fire activity and herbivore density (31 reserves) in sub-Saharan Africa. Across exclosures, grazing herbivore metabolic density strongly reduced herbaceous biomass. Moreover, grazing herbivore density directly affected the absolute amount of herbaceous biomass removed, whereas rainfall and soil conditions only affected proportional reductions in herbaceous biomass relative to primary production. At the landscape level, these grazing effects translated into reductions in burned area, with larger reductions in burned area in reserves with more grazing herbivores. The addition of 100 kg/km2 grazing herbivore biomass (~ one zebra) resulted in a ~30 kg/ha reduction in herbaceous biomass and a ~0.35% reduction in burned area, on average. Grazing herbivore effects on herbaceous biomass and fire activity are clearly substantial, strongly tied to grazing herbivore densities, and generally consistent across different environments.