Livestock Herbivory Shapes Fire Regimes and Vegetation Structure Across the Global Tropics

Abstract

Livestock grazing is the most extensive human land use and one of the key drivers of the conversion of tropical forests into grasslands. Livestock effects on vegetation structure are complex, as they can prevent tree recruitment and growth through browsing and trampling, but they can also affect vegetation indirectly through fire interactions. However, a systematic analysis of the overall effects of livestock across the global tropics is lacking. We analyzed remote sensing data on vegetation height and cover, climate, and fire as well as ground data on livestock density. We used generalized linear models and structural equation models to analyze the effects of livestock on fire regimes and vegetation structure. Across the global tropics, higher livestock densities are associated to lower fire frequency and a higher cover of shrubs and dwarf trees. This pattern occurs across continents, and is particularly pronounced at intermediate precipitation levels (1000–1500 mm y−1) where fire frequency is highest. In those regions, fire frequency is on average 49% lower in areas with high versus low livestock densities. South America has much higher livestock density and lower fire frequency than Africa and Asia–Australia across the whole precipitation gradient. Our findings suggest that livestock grazing reduces fire incidence through grass consumption and favors shrubs and a sparse cover of trees in regions where forests could potentially exist. Livestock can thus be a strong modifier of the climatic effect on vegetation structure, and livestock management changes can impact the structure and functioning of savannas and grasslands throughout the global tropics.