The wildebeest is one of the most numerous large mammals on Earth. Its annual “Great Migration” across the Serengeti-Mara ecosystem in East Africa is the largest overland animal movement remaining globally, involving approximately 1.2 to 1.5 million individuals. A keystone species is defined by its disproportionately large impact on its environment relative to its biomass, and the wildebeest fits this description by shaping the structure and function of the savanna. Their persistent presence and movement affect plant communities, soil chemistry, and the population stability of other herbivores and predators. A significant decline in their numbers would trigger a cascading collapse in the region’s biodiversity and ecological processes.
Landscape Transformation Through Grazing
The sheer volume of wildebeest grazing physically restructures the vegetation of the Serengeti-Mara ecosystem. By consuming over 4,500 tons of grass daily, the herds prevent the dominance of certain fast-growing, unpalatable grass species. This high-density grazing limits competitive exclusion, where one plant species eliminates others. The herds remove approximately 50% of the grass biomass, leaving behind a short stubble of 10 to 25 centimeters.
This alteration maintains the short-grass plains vital for many other smaller herbivores. Thomson’s gazelles, for instance, prefer these shorter grasses, which allow them to scan the horizon for predators. The intensive grazing also stimulates new growth, ensuring the remaining vegetation is high in nutrients, such as nitrogen and phosphorus, benefiting subsequent grazers. By regulating the height and composition of the grass layer, the wildebeest engineer the habitat for a wide array of other species. This cycle of consumption and regrowth drives the savanna’s biodiversity.
Maintaining Predator Populations
The immense biomass of the migratory wildebeest population provides a predictable food source that buffers predator populations in the Serengeti-Mara. The annual migration ensures that apex predators, including lions, spotted hyenas, and Nile crocodiles, have a year-round supply of meat. Lions rely heavily on wildebeest, which make up a significant portion of their diet. This stable food base allows predator numbers to remain high, which in turn regulates the populations of other medium-sized herbivores.
Without the migration, the predator population would decline due to starvation, particularly in the dry season when other prey species are scarce. This ‘prey buffering’ prevents the top trophic layer from fluctuating wildly with seasonal changes. The removal of the wildebeest would trigger a trophic cascade: the decline of apex predators would lead to the overpopulation of other herbivores, resulting in landscape degradation. The wildebeest serves as the energetic foundation supporting the carnivore guild and ensuring ecological balance.
Regulation of Fire Frequency
Wildebeest grazing acts as a natural fire management system by consuming the dry material that would otherwise fuel large wildfires. The herds reduce the dry grass load, which is the primary source of combustible material. During the mid-twentieth century, when the wildebeest population was low due to disease, uncontrolled fires consumed up to 80% of the ecosystem annually. Since the recovery of the herds, the extent of the area burned each year has dropped significantly.
The reduction in fuel load limits the intensity, size, and frequency of seasonal wildfires. Less intense fires allow for the regeneration of woody vegetation, such as acacia seedlings, which would otherwise be consumed by flames. This shift from a fire-driven ecosystem to a herbivore-driven one alters the balance between grassland and woodland. By controlling the fire regime, the wildebeest indirectly promotes the growth of trees, providing habitat and cover for other species.
Accelerating Nutrient Cycling
The movement and waste products of the wildebeest herds accelerate the nutrient cycle in the savanna’s soils. The deposition of dung and urine rapidly transfers essential nutrients, such as nitrogen and phosphorus, from vegetation back into the ground. This transfer is spatially concentrated and distributed across the landscape due to the migratory pattern. Continuous trampling by millions of hooves also aerates the soil, enhancing water infiltration and mixing the dung into the soil layers.
This input of organic matter supports soil microorganisms and decomposers, which accelerates the breakdown of waste and the release of nutrients for new plant growth. The migratory pattern ensures that nutrients are constantly moved and deposited across the entire range, preventing depletion in one area. The annual mass drownings during river crossings introduce a pulse of nutrients, including 1,100 tons of biomass on average, into the Mara River ecosystem each year, affecting aquatic food webs. This movement and deposition of chemical resources underpins the high productivity of the Serengeti-Mara ecosystem.