Trophic processes constrain seasonal ungulate distributions at two scales in an East African savanna

Abstract

Habitat selection is a dynamic biological process where species respond to spatiotemporal variation in resource availability. The resulting distribution patterns can be detected as presence–absence or heterogeneity in abundance and indicate habitat preferences based on environmental correlations at multiple scales. Variation in habitat selection by ungulates is constrained by trade-offs in top-down and bottom-up trophic processes arising from differences in forage requirements, water dependency, anthropogenic effects, and predation avoidance, and mediated by physiological (feeding guild) and morphological (body size) factors. We conducted distance sampling over 7 years in the Tarangire Ecosystem (TE) of northern Tanzania for six resident ungulate species: Kirk’s dik-dik (Madoqua kirkii), Grant’s gazelle (Nanger granti), Thomson’s gazelle (Eudorcas thomsonii), Masai giraffe (Giraffa camelopardalis tippelskirchi), impala (Aepyceros melampus), and common waterbuck (Kobus ellipsiprymnus), and tested hypotheses related to effects of top-down and bottom-up processes on ungulate presence and abundance. We modeled ecological correlates against two distributional responses to understand which environmental factors constrained these ungulate species at different scales; (i) presence–absence observations modeled in a logistic regression to assess habitat selection at an ecosystem scale; (ii) local abundances from presence-only observations modeled using a negative binomial distribution for finer-scale selection. Browser and grazer species in the TE selected suitable habitat proximal to rivers and avoided the Combretum–Azanza woody plant assemblage. Browsers and grazers also showed strong preference for habitat with more dense cover of preferred forage species, and abundance was influenced by the presence of specific forage species with significant seasonal variation. Mixed feeders were more heterogeneous in habitat suitability implying that broader diets allow avoidance of areas with high human activity. Small-bodied and dehydration-sensitive species selected areas near rivers and seasonal tributaries. Seasonal habitat selection was more pronounced among mixed feeders. Conservation strategies based on spatially and seasonally explicit resource selection studies such as ours can minimize impacts to biodiversity by protecting vital resources to ungulates through all seasons of the year.