A population can be regulated by limits in the food supply (a “bottom-up” process) or by the harmful effects of parasites, predators or competitors (“top down” processes). Lions are subject to both bottom-up and top-down regulation. In migratory ecosystems, lions face food shortages in the lean season each year, resulting in widespread cub starvation and an associated impact on population growth. Lions are also sensitive to within-group food competition (lions in very large prides suffer lower feed intake rates than lions in moderate-sized prides) and between-group competition (larger prides are able to annex food-rich areas from their neighbors’ territories). Although lions are the top predator in Africa, leopards and hyenas are known to kill lion cubs, and disease outbreaks in the Serengeti and Ngorongoro have killed 35% of each lion population.
Between 1962 and 1993, the Ngorongoro Crater lion population showed clear signs of density dependence at about 100 individuals. The population usually grew when the total was below 100 but almost always declined once it exceeded 100. However, the Crater population has remained below 60 individuals since 1994 even though prey abundance has remained virtually unchanged, and the remaining lions have enjoyed increased levels of food-intake and high cub recruitment. The Crater lions have suffered from a number of severe disease outbreaks over the past 50 years, but, whereas the population recovered exponentially from a severe epizootic in 1963 (that reduced the population from ~75 to 12 individuals), three outbreaks between 1994 and 2001 occurred in such rapid succession that the population was unable to return to the carrying capacity. The Crater population may have become unusually vulnerable to infectious disease in recent years owing to its close proximity to a growing human population and a history of close inbreeding. Unvaccinated domestic dogs are the most important reservoir for canine distemper virus; inbreeding reduces the genetic diversity and efficacy of the mammalian immune response. Thus the top predator of the Ngorongoro Crater appears to have been regulated by top-down processes over the past 16 years.
The Serengeti lions have shown a complex response to long-term changes in prey availability and vegetative cover over the past 40 years. The Serengeti wildebeest, gazelle and buffalo populations grew exponentially following the eradication of rinderpest in 1962. Increased grazing pressure from these growing herds reduced the frequency of grass fires thereby allowing an expansion of woodland habitat in the 1980s. Higher prey densities support larger lion populations, and lions can better capture prey in habitats with more cover. While the carrying capacity increased gradually and continuously in the Serengeti, lion numbers in the plains and woodlands remained constant for 10- to 20-year periods and only shifted to new levels in sudden leaps. Years with favorable rainfall allow migratory Serengeti prey species to remain within reach of local lion prides long enough to rear large cohorts of surviving young. Lion populations are ultimately limited by the number of territories in a given area, and new territories can only be established by newly-formed groups that are strong enough to compete successfully against established neighbors. Thus the Serengeti lion population did not track immediate changes in food abundance or prey accessibility but could only catch up in a sporadic pattern.