Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers
Article first published online: 29 JUL 2011
© 2011 The Authors. Journal of Animal Ecology © 2011 British Ecological Society
Journal of Animal Ecology
Volume 81, Issue 1, pages 201–213, January 2012
How to Cite
Hopcraft, J. G. C., Anderson, T. M., Pérez-Vila, S., Mayemba, E. and Olff, H. (2012), Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers. Journal of Animal Ecology, 81: 201–213. doi: 10.1111/j.1365-2656.2011.01885.x
- Issue published online: 8 DEC 2011
- Article first published online: 29 JUL 2011
- Received 5 March 2010; accepted 9 June 2011 Handling Editor: Atle Mysterud
- consumer resource interaction;
- herbivore distribution;
- predator-safe refugia;
- resource selection;
- risk-forage trade-off;
- trophic structure;
1. 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.