Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers

Abstract

Summary1. Theory predicts that small grazers are regulated by the digestive quality of grass, while large grazers extract sufficient nutrients from low-quality forage and are regulated by its abundance instead. In addition, predation potentially affects populations of small grazers more than large grazers, because predators have difficulty capturing and handling large prey. 2. We analyse the spatial distribution of five grazer species of different body size in relation to gradients of food availability and predation risk. Specifically, we investigate how the quality of grass, the abundance of grass biomass and the associated risks of predation affect the habitat use of small, intermediate and large savanna grazers at a landscape level. 3. Resource selection functions of five mammalian grazer species surveyed over a 21-year period in Serengeti are calculated using logistic regressions. Variables included in the analyses are grass nitrogen, rainfall, topographic wetness index, woody cover, drainage lines, landscape curvature, water and human habitation. Structural equation modelling (SEM) is used to aggregate predictor variables into 'composites' representing food quality, food abundance and predation risk. Subsequently, SEM is used to investigate species' habitat use, defined as their recurrence in 5 · 5 km cells across repeated censuses. 4. The distribution of small grazers is constrained by predation and food quality, whereas the distribution of large grazers is relatively unconstrained. The distribution of the largest grazer (African buffalo) is primarily associated with forage abundance but not predation risk, while the distributions of the smallest grazers (Thomson's gazelle and Grant's gazelle) are associated with high grass quality and negatively with the risk of predation. The distributions of intermediate sized grazers (Coke's hartebeest and topi) suggest they optimize access to grass biomass of sufficient quality in relatively predator-safe areas. 5. The results illustrate how top-down (vegetation-mediated predation risk) and bottom-up factors (biomass and nutrient content of vegetation) predictably contribute to the division of niche space for herbivores that vary in body size. Furthermore, diverse grazing assemblages are composed of herbivores of many body sizes (rather than similar body sizes), because these herbivores best exploit the resources of different habitat types.

The spatial distribution of African savannah herbivores: species associations and habitat occupancy in a landscape context

“…We acknowledge potential difficulties linking satellite-derived data to the forage quantity/quality trade-off, but note that our results are consistent with ground-truthed vegetation data that demonstrate the greater quality (low C : N ratios) of low biomass vegetation in regions of the Serengeti that map as low NDVI (e.g. [50,54]). Smaller herbivores also associated with other habitat features, such as recent burns and termite mounds, which improve forage quality relative to the surrounding matrix.…”

“…R. Soc. B 371: 20150314 such as cheetahs and wild dogs ( [50,54]; figure 6). In fact, when the landscape occurrence probabilities of grazers are viewed as frequency distributions (electronic supplementary material, figure S2), three distinct groups appear: mobile mega-herbivores (buffalo); large-bodied migrants (zebra and wildebeest) and small-bodied (Thomson's gazelle) or resident species (topi and hartebeest).…”

Comparative changes in density and demography of large herbivores in the Masai Mara Reserve and its surrounding human-dominated pastoral ranches in Kenya

“…In the wet season less heavily grazed grasses, such as occur in most parts of the Mara reserve, become tall and therefore allocate more energy to developing structural fibers with higher carbon to nitrogen ratios, thereby diluting the concentration of nitrogen and phosphorous available to herbivores (Anderson et al 2007). From an herbivore's perspective, the digestibility of grasses is therefore inversely related to rainfall amount (Hopcraft et al 2011). Mature grasses of tall stature are thus particularly unfavourable for small and medium herbivores due to their low digestibility and nutritional quality (Fritz and Duncan 1994;Olff et al 2002).…”

“…However, these seasonal dispersal movements might be constrained by body size (Hopcraft et al 2011) through its influence on food quantity and quality requirements as well as vulnerability to predation.…”

Spatially nested niche partitioning between syntopic grazers at foraging arena scale within overlapping home ranges

“…Whether the concentration of wildebeest in grazing lawn grasslands is an outcome of narrow food requirements, or increased security from predation due to little vegetation cover for stalking lions, has still to be resolved (Smuts 1978, Yoganand and Owen-Smith 2014. In the Serengeti ecosystem, both food availability and predation risk influenced the regional distribution patterns of grazing ungulates (Hopcraft et al 2012(Hopcraft et al , 2014. Spatial separation among large mammalian herbivores based on distinct responses to the risk of predation is likely to be expressed on a larger scale than foraging arenas based on dietary distinctions.…”