Fear or food? Climate alters the landscape of fear in an African savanna

Background/Question/Methods

Herbivores are said to exist in a “landscape of fear” – avoiding areas of high predation risk even if the food opportunities in those areas are greater than in less risky areas. A number of studies have suggested strong and cascading effects of fear among terrestrial large herbivores, both in the western United States (wolves and elk) and Africa (lions and a variety of ungulates). However, these studies have relied on observational data and comparisons between predator-free and predator-present areas. To date, no study has manipulated the actual landscape in which a putative fear-based cascade is operating. I experimentally manipulated tree density (previously shown to correlate negatively with herbivore use of the landscape) in an African savanna system. Specifically, I cleared, thinned, or left intact replicated, large-scale plots dominated by the whistling thorn acacia (A. drepanolobium) and measured the response among several species of ungulates (primarily plains zebra, Grant’s gazelle, hartebeest, oryx, and elephant) over five years. This time window included years of above average rainfall as well as a decadal drought.

Results/Conclusions

 In years of average to above-average rainfall, meso-herbivores overwhelmingly favored sites with few or no trees, while elephants (too large to be vulnerable to predation) favored sites with many trees. Meso-herbivore responses were strongly correlated with visibility (R²=0.64 , p<0.0001) but not grass production (R²=0.01, p=0.61) – suggesting that these herbivores favored sites where their ability to detect and avoid predators was enhanced. During a ten-year drought in 2009, however, meso-herbivores favored sites that had high pre-drought grass production (high standing grass biomass) (R²=0.19 , p=0.03)  – but not high visibility (R²<0.01 , p=0.95). Thus, during the drought, herbivores sought areas where food was more abundant, despite probable higher risk of predation. These results provide some of the first experimental evidence that large terrestrial herbivores trade off their need to find food with their need to avoid predation. Moreover, these results illustrate that the “landscape of fear” is not static, but rather shifts markedly under different conditions. Future climatic changes thus have the potential to alter the strength, importance, and spatial dynamics of behaviorally-mediated trophic cascades in terrestrial large herbivore systems.