The worldwide decline of top mammalian carnivores is a key ecological and conservation concern, not least because this guild can disproportionately affect ecosystem structure and function via trophic cascades. These indirect effects of apex carnivores are mediated by either consumptive reductions in prey abundance, or through “landscapes of fear”, in which the fear of being eaten alters prey behavior in space and time. Previous work indicates that risk effects can be substantial, suggesting that the loss of top carnivores, conversely, should create a “landscape of fearlessness” and relax risk-sensitive behaviors among prey. Experiments that simulate risk have demonstrated the consequences of fearlessness in several small-bodied species. However, fear-driven effects on prey remain hard to experimentally quantify in large mammalian systems, and empirical evidence of their ecosystem-level consequences remains controversial. In this study, we used a series of manipulative experiments to (i) identify the drivers behind an apparent expansion of prey habitat use and relaxation of risk-sensitive behaviors in an antelope (Tragelaphus sylvaticus), (ii) explore the consequences of this shift for prey resource acquisition and body condition, and (iii) quantify the cascading effects on the basal trophic level, following the war-induced collapse of apex carnivores in Mozambique's Parque Nacional da Gorongosa.
Results/Conclusions
During predator simulation experiments, all bushbuck avoided predator cues. Bushbuck that had previously expanded their habitat selection to more open, typically risky, areas showed a stronger response to the predator cues by also increasing their use of tree cover. Using DNA meta-barcoding of fecal samples, we found evidence that the relaxation of risk and attendant habitat shift influenced prey diet: bushbuck in open habitat showed pronounced dietary differentiation compared to bushbuck that remained in woodland habitat. This change in diet was accompanied by increased diet quality and body condition in floodplain-grassland individuals. Through an herbivore exclusion experiment, we also demonstrate how the expansion of prey habitat use has induced measurable effects on the plant community. Collectively, our results provide evidence of the disruption of a behaviorally-mediated trophic cascade in a large mammalian system: large carnivore removal in PNG decreased risk effects, causing a shift in habitat use and diet by an ungulate prey species. Yet, our results suggest that risk effects are highly dependent on the ecology of the species involved. We recommend further studies across a wide range of prey taxa and lower tropic levels to improve understand and predict the consequences of large carnivore declines across ecosystems.