Unearthed: Why zebra go first in body-size-dependent grazing succession in the Serengeti

Why do Serengeti zebra, wildebeest, and gazelle – all sharing limited food resources – follow the same migratory routes, one after another, in a body-size dependent way? This longstanding question has now been evaluated by researchers who used novel data to show how a balance of species interactions and ecological factors regulate this process. They say competition pushes zebra ahead of the wildebeest, and wildebeest then eat plants in a way that facilitates development of newer growth the trailing gazelle like. “Our results highlight a balance between facilitative and competitive forces,” the authors say. Large seasonal migrations are a defining feature of many marine and terrestrial ecosystems. The migration of large herbivores in Serengeti National Park – an annual event involving nearly 2 million animals – is a classic example of body-size-dependent “grazing succession,” in which zebra (~230 kilograms), wildebeest (~180 kilograms), and gazelle (~20 kilograms) sequentially follow the same migratory routes one after another. Although migration is common in large mammalian herbivore species, the ecological dynamics underlying large migrations involving several species all sharing limited food resources remain poorly understood. Several explanations of this sequential pattern have been proposed, including competition, facilitation, and predation. However, disentangling the potential mechanisms that drive these migratory grazers’ movement and foraging behaviors has been challenged by a lack of long-term, detailed data.

Michael Anderson and colleagues conducted an analysis of animal movements in the Serengeti using data from an 8-year camera-trap survey, GPS-collared animal tracking data, and fecal DNA metabarcoding to characterize the timing, arrival, and interactions between zebra, wildebeest, and gazelle. Anderson et al. discovered that the observed patterns of grazing succession were driven by both competition and facilitation and characterized by a “push-and-pull” dynamic centered on the dominant grazer, wildebeest. According to the findings, competition for food “pushes” zebra ahead of the larger herds of wildebeest during migratory succession. As the wildebeest move along, they reduce grass biomass and facilitate the development of newer growth, “pulling” along gazelle who take advantage of the new food as they trail the other two species. There was no evidence that predation played any role in grazing succession. What’s more, Anderson et al. leveraged observations of intense wildfires and rainfall in the region and found that these events, respectively, strengthen successional grazing patterns.