Interactions Between Fire and Herbivory: Current Understanding and Management Implications

Abstract

In this Chapter, we describe the interaction between fire and herbivory, as a single disturbance (fire × herbivory) with more disparate effects on ecosystems than either process will have separately. For much of human history (since about the end of the Pleistocene), herbivores and fires have been in dynamic interplay with fire the dominant process when herbivores decrease (and vice versa) and with changes in herbivore densities often anthropogenically determined. We propose a conceptual model that summarise the consequences for processes involved in fire × herbivory for systems that are either grass- or tree-dominated, as well as mixed tree/grass systems. In grass-dominated systems, high levels of grazing can reduce grass biomass and change the grass layer structure and composition, resulting in patches or landscapes with reduced flammability, with feedbacks to fire and grazing behaviour. In tree-dominated systems, the direct effects of fire × browsing include altered fuel accumulation and vertical fuel structure, while indirect effects are often through herbivory influencing forest regeneration trajectories post-fire, with implications for the future flammability of the system. In mixed tree/grass and grazer/browser systems, increased grazing pressure can reduce fire frequency and intensity directly, through fuel removal and changes to the herbaceous layer. Increased browsing may increase fire indirectly through reducing woody recruitment and hence reducing woody competition with the herbaceous layer. However, there may also be negative feedbacks resulting in variable net effects. Ecosystem patterns and processes that are influenced through fire × herbivory, will influence grazing and browsing behaviour through various feedbacks, and may ultimately have consequences for productivity and biodiversity. Managers may achieve objectives e.g. in reducing fire risk or curbing woody thickening, more readily if they consider the interaction and feedbacks between fire and herbivory. Actions like rewilding, and mimicking or manipulating fire and herbivore spatio-temporal patterns may be important tools to achieve these objectives.