Effects of fragmentation on cattle in African savannas under variable precipitation

Abstract

Cattle move to access patches that vary in forage quantity and quality. Fragmentation can prevent animals from reaching patches. I used an integrative ecosystem model applied to three African landscapes to explore the sensitivity of cattle populations to fragmentation (here, changes in populations as parcel areas decreased) under different precipitation patterns. I hypothesized that low and high precipitation would yield populations relatively insensitive to fragmentation, intermediate precipitation would yield more sensitive populations, and more variable inter-annual precipitation would reduce sensitivity to fragmentation. Precipitation data were altered to yield averages of 100–1,000 mm year−1 and inter-annual coefficients of variation of 0–60%. A 1,000 km2 landscape in each area was divided into progressively smaller parcels and simulations conducted for each parcel. Rainfall at 100 mm year−1 supported low populations that were insensitive to fragmentation. Populations peaked at rainfall levels similar to those observed, and declined under higher precipitation, due in-part to shrub expansion. Fragmenting landscapes caused up to a 62% decline in cattle. High inter-annual variation in precipitation reduced sensitivity to fragmentation when precipitation was above that observed. The pattern was opposite when precipitation was below what was observed. Cattle on the landscape with fine-scale heterogeneity were relatively insensitive to fragmentation, and those on the heterogeneous but coarse-grained landscape were extremely sensitive. Fragmentation in landscapes where populations are sensitive will require more intensive inputs to offset losses, and changes in the frequency of extreme weather associated with climate change will alter the sensitivity of some populations to fragmentation.