Effects of Predation Risk on the Group Size of an Asocial Ungulate in Gorongosa National Park

Abstract

This study investigates the social interactions of bushbuck (Tragelaphus sylvaticus) in Gorongosa National Park following the near-extirpation of large mammal populations during the Mozambican civil war (1977-1992) and the subsequent recovery of wildlife up to 2019. The ecological restoration in Gorongosa has resulted in a large change in community composition, with only some ungulate species recovering rapidly and large carnivores remaining absent or at historically low densities. In 2018, a pack of African wild dogs (Lycaon pictus) was reintroduced to the park, restoring a key component of the large carnivore guild. We collected observational data on bushbuck sociality, as measured by group size, from camera traps and road counts over the time period before and after wild dog reintroduction (2016 to 2019). We hypothesized that (i) in the absence of large carnivores, both mean and maximum bushbuck group size increased above the typical value of 1, and (ii) following the reintroduction of African wild dogs in 2018, both mean and maximum bushbuck group size decreased, owing to the increased predation pressure. As bushbuck are known for their affiliation with wooded areas and habitats that provide high cover, we focused on a gradient of woody cover as represented by two different habitat types—the savanna woodland and the floodplain grasslands. In the camera trap data, we found evidence in support of our hypothesis that bushbuck group sizes had increased in the absence of a dense large carnivore community (based on group sizes in 2016 and 2017), then decreased when wild dogs were reintroduced (based on group sizes in 2019). Both mean group size and maximum group size decreased from 2016 and 2017 to 2019. This pattern was largely driven by bushbuck in the woodland habitat, with no significant shifts in group size occurring on the floodplain. In the road count data, we observed a similar decline in group size over the years, but data limitations prevented any robust conclusions from these results. Our findings demonstrate the importance of predators in influencing the social behaviour of antelope, even among typically asocial species.