The impact of burning on herbivore distributions: Examining the roles of bottom-up and top-down processes

Background/Question/Methods

Numerous studies have found that certain herbivore species prefer burned areas, but the reason for this preference is unclear. Burning causes increases in plant nutrients which may explain the preference, but it also reduces vegetation height and increases visibility, thereby potentially reducing predation risk. Therefore, the bottom-up benefit of increased plant nutrients and/or the top-down benefit of decreased predation could be driving the preference for burned areas. Additionally, the reason(s) for burned area preference might differ between herbivore species.

We tested how vegetation height, biomass to necromass ratio, and leaf nutrients influenced the preference for burned areas of different herbivore species in Serengeti National Park, Tanzania. Herbivores were counted bi-weekly for a year on 14 paired burned and unburned sites. Vegetation height, to be used as a proxy for predation pressure, phytomass, and forage nutrient content were collected bi-monthly. Additionally, we looked at lion (Panthera leo) distributions in relation to burned areas by overlaying six years of lion GPS locations on burned area distribution maps. The Serengeti is an ideal place in which to conduct this study due to its large abundance and diversity of herbivores and carnivores and the annual setting of fires by park management.

Results/Conclusions

Burning caused a six month increase in leaf nitrogen (N), aluminum (Al), calcium (Ca), cooper (Cu), potassium (K), magnesium (Mg), and manganese (Mn), and a decrease in vegetation height and the biomass to necromass ratio. The preference for burned areas by Thomson’s gazelles (Gazella thomsoni) in the six months after burning was best explained by decreased vegetation height, which was associated with increased sighting distances and potentially decreased predation. However, the difference in impala (Aepyceros melampus) abundance between burned and unburned habitats was positively associated only with differences in leaf N. Individual species differences explained why preference for burned areas by all herbivores was best explained by both decreased vegetation height and increased leaf N. Thus our results show that both bottom-up and top-down processes explain preference for burned areas, but their influence changes depending on the herbivore species. We also found that lions avoid burned areas, which adds further support to the role of decreased predation in burned area preference.