Group Dynamics of Zebra and Wildebeest in a Woodland Savanna: Effects of Predation Risk and Habitat Density

Abstract

BackgroundGroup dynamics of gregarious ungulates in the grasslands of the African savanna have been well studied, but the trade-offs that affect grouping of these ungulates in woodland habitats or dense vegetation are less well understood. We examined the landscape-level distribution of groups of blue wildebeest, Connochaetes taurinus, and Burchell's zebra, Equus burchelli, in a predominantly woodland area (Karongwe Game Reserve, South Africa; KGR) to test the hypothesis that group dynamics are a function of minimizing predation risk from their primary predator, lion, Panthera leo.Methodology/Principal FindingsUsing generalized linear models, we examined the relative importance of habitat type (differing in vegetation density), probability of encountering lion (based on utilization distribution of all individual lions in the reserve), and season in predicting group size and composition. We found that only in open scrub habitat, group size for both ungulate species increased with the probability of encountering lion. Group composition differed between the two species and was driven by habitat selection as well as predation risk. For both species, composition of groups was, however, dominated by males in open scrub habitats, irrespective of the probability of encountering lion.Conclusions/SignificanceDistribution patterns of wildebeest and zebra groups at the landscape level directly support the theoretical and empirical evidence from a range of taxa predicting that grouping is favored in open habitats and when predation risk is high. Group composition reflected species-specific social, physiological and foraging constraints, as well as the importance of predation risk. Avoidance of high resource open scrub habitat by females can lead to loss of foraging opportunities, which can be particularly costly in areas such as KGR, where this resource is limited. Thus, landscape-level grouping dynamics are species specific and particular to the composition of the group, arising from a tradeoff between maximizing resource selection and minimizing predation risk.

Effects of lions on behaviour and endocrine stress in plains zebras

“…Similarly, the presence of lions did not seem to have caused any major effects on zebra group sizes. This result contradicts previous observations which have shown a positive association between predation risk and zebra group sizes (Schmitt, Stears, Wilmers, & Shrader, 2014;Thaker et al, 2010). Both social and environmental factors may constrain optimal group size in addition to effects of predation risk.…”

“…We suggest that complex interactions between resource abundance, available vegetation cover and predator exposure may have caused the observed seasonal influences on the effects of lion presence on zebra vigilance and foraging behaviours and physiology. Such interactions have previously been implied for zebras (Barnier et al, 2014;Fischoff et al, 2007;Thaker et al, 2010). Although there were seasonal influences on the differences between the two reserves in terms of zebra behaviour and physiology, we did not find a stronger effect of lion presence on all aspects of behaviour and physiology.…”

“…, decreased foraging efficiency (Barnier et al, 2014), adjustments of behavioural time budgets (Lima & Bednekoff, 1999;Périquet et al, 2012) and alterations in the level of gregarious behaviour (Thaker, Vanak, Owen, Ogden, & Slotow, 2010). Predation risk has generally been shown to cause increased group sizes in prey (Elgar, 1989), which can be explained by a reduced individual predation risk in larger groups caused by dilution and confusion effects as well as increased collective vigilance (Hamilton, 1971;Lazarus, 1979;Pulliam, 1973;Turner & Pitcher, 1986).…”

Spatiotemporal behavior of a prey–predator system with a group defense for prey

“…A time scale separation thus does not occur. Note also that from the same source, we gather that intermingling for the herbivores does not constitute a "social" problem, other than the standard intraspecific competition for the resources, which in our system (1-2) is modeled via the classical logistic term (Thaker et al, 2010(Thaker et al, , 2011.…”

Effects of males’ presence on female behaviour during the rut

“…By contrast, little is known about female mating strategies (but see BRO-JØRGENSEN 2011). Indeed, female ungulate distribution patterns during the rut may vary according to food distribution (CARRANZA 1995;CARRANZA et al 1995) and predation pressure (THAKER et al 2010), as well as with the availability (quality as well as quantity) of males, as seen in the red deer (CARRANZA & VALENCIA 1999;PÉREZ-GONZÁLEZ & CARRANZA 2011), saiga antelope (MILNER-GULLAND et al 2003) and fallow deer (KOMERS et al 1999).…”