Why are Migratory Ungulates So Abundant?

Migratory ungulates outnumber residents by an order of magnitude in several savanna ecosystems in Africa, as was apparently the case in other grasslands around the world before the intervention of modern man. Migrants may be more numerous than residents because (1) they use a much larger area, (2) they make more-efficient use of resources, or (3) they are less vulnerable to regulation by predators. These hypotheses were examined using simulation models of migratory and sedentary wildebeest in the Serengeti ecosystem. The larger area used by migrants would not lead inevitably to higher numbers. In seasonal environments, herbivore abundance is probably determined by food availability during periods of resource scarcity. Even though migrants may have access to greater food supplies for most of the year, this would not lead to increased abundance if both morphs have similar food supplies during the leanest period of the year. Rotational grazing could lead to increased numbers of migrants relative to residents only if migrants are able to use mature vegetation that has accumulated while they are foraging elsewhere. This is unlikely for savanna ecosystems in Africa because tropical grasses decline rapidly in quality as they mature. Moreover, our simulations suggest that in the Serengeti such a process would at most produce a twofold difference in abundances of migrants and residents. We conclude that increased efficiency in resource use by migrants is insufficient to explain the order-of-magnitude disparities in abundance seen in some African ecosystems. Our simulations suggest that realistic numbers of predators could regulate resident herbivores at low population densities, whereas such regulation is probably rare for migratory herds. When residents and migrants have overlapping ranges, migrants should always outcompete residents, reducing them to low numbers. These results suggest that differences in the modes of regulation explain the predominance of migratory herbivores in some grassland ecosystems.