The Longevity of the Magnet Effect: Fire-Herbivory Interactions in Central Kenya

Abstract

The coexistence of grass and trees that typifies savanna ecosystems is maintained in part by top-down forces including herbivory and fire. Understanding fire-herbivore interactions is an integral step in determining the effect of these forces individually and informs decisions about the use of fire as a livestock management technique. The magnet effect, in which grazers are drawn to an area after it is burned, is one such interaction between fire and herbivores. While the process of the magnet effect has been described, neither its longevity nor the manner through which fire intensity affects its strength is understood. In this study, we explored whether the increase in herbivory pressure predicted by the magnet effect is maintained over long periods after a fire, how fire intensity influences the strength of the magnet effect, and which feeder types are most influenced by the magnet effect. We established five field sites at the Mpala Research Centre in Laikipia, Kenya that ranged in fire intensity and elapsed time since last the burn. We found that the magnet effect is transient on a seven-year scale and affects grazers more than browsers or mixed feeders, though herbivory and grass biomass responses to fire intensity were conflicting. This research clarifies the role of the magnet effect in maintaining savanna ecosystems and can aid agriculturalists seeking to use fire as a management technique by indicating how long after a burn increased palatability of grass can be expected. Author’s Note The authors completed this research while studying in Columbia and Princeton’s joint program in Tropical Biology and Sustainability in Kenya. We would like to thank Dr. Carla Staver for her helpful comments on earlier versions of this manuscript. We would also like to thank Josphat Mwangi, Agnes Wachu, and the rest of the staff at the Mpala Research Centre, all of whom were so kind to us during our stay with them.