S

Year

Fig. 1. Lion population sizes each month: (A) woodlands, (B) plains. Horizontal lines indicate periods where population sizes were statistically homogeneous but different from adjacent periods. Blue lines include all individuals; black lines indicate lions >2 years. Diamonds designate change points. Pale green blocks highlight times when the populations were below local equilibrium density; dark green lines demarcate years within these periods with favorable rainfall. Red line shows the CDV die-off in 1994. (C) Serengeti herbivore population sizes. Vertical bars show SE. Green box highlights recovery from rinderpest; brown box highlights drought-related die-off in the wildebeest.

Fig. 2. Wildebeest, fire, and the regeneration of woody vegetation in the Serengeti woodlands. (A) The extent of wildfire is inversely related to the size of the wildebeest population. (B) Wildfire reached a low point in the late 1970s and early 1980s. (C) Population growth rates of acacias in the Serengeti woodlands as measured from fixed-point photography; woodland recovery peaked in the early 1980s. Green band indicates time period when the woodlands lions experienced the greatest increase in prey accessibility.

(16)], 1983 and 1999 were followed by the two driest wet seasons in more than 40 years, and the increase in the plains population occurred during the extreme El Nino rainfalls of 19971998, which were the heaviest since 1962. (Migrant herbivores spend less time on the plains in ''dry'' wet seasons and more time on the plains in ''wet'' wet seasons.)

Thus, the background of long-term change in prey availability is overlain with a stochastic year-to-year pattern of prey distribution, and the first ''good year'' permitted rapid recruitment in the lion population. Across all significant population increases, the primary demographic response was increased cub survival (P < 0.01) rather than larger litter size or shorter interbirth intervals. All the population ''leaps'' involved successful reproduction in an exceptional num ber of prides. Five of six woodlands prides successfully raised cohorts of cubs in 1973 and 1983 (four of six was the prior record) and six of seven in 1999. There had never been more than six successful prides in any single year on the plains until 1997, when 11 of 12 prides successfully fledged offspring.

Our data clearly reveal the impact of the wildebeest on the Serengeti lions. Buffalo and gazelle both returned to 1960s levels by 2002 (Fig. 1C) without a concomitant decline in lion numbers, whereas the wildebeest population has remained at about 1.2 million for the past 25 years. The wildebeest were also responsible for two indirect effects on the lions. Increased levels of grazing led to extensive regeneration of woody vegetation, permitting an increase in the woodlands lion population,

Fig. 3. Long-term changes in grass height. (A) Grasslands map for the Serengeti plains. (B) Bar graphs indicate percentage of each grass type along the three transects in (A). The extent of tall grass has increased since 1994 (P < 0.01).

whereas a temporary decline in the wildebeest population increased the average height of grasses in the intermediate grass community, enabling an expansion of the lion population on the plains. The first significant improvement in local wildebeest abundance during a period of persistent ecological change also permits the simultaneous establishment of viable new prides (with >4 females), thus triggering the sudden increase of the population as a whole (13). In contrast, the herbivore community in the nearby Ngorongoro Crater is nonmigratory, and the Crater lion population fell to one-eighth of its local equilibrium density after a disease outbreak in 1962 (19) but subsequently showed a continuous period of exponential growth, doubling every 4 years for 12 years (20).

Impact of social structure. To evaluate the importance of group living on population changes in the Serengeti, we developed a detailed simulation model that incorporated long-term data on cub productivity, pride splitting, and adult survival as functions of annual rainfall, pride size, and dispersal status. We modeled the impact of large-scale ecological change as an increase in the number of potential territories in each study area (the magnitude being set by the observed change in equilibrial population size); rainfall followed the observed sequence over the past 40 years, and the simulated population suffered the observed level of disease mortality in 1994. Pride formation was a stochastic process that depended on the number of available territories, the size of the maternal pride, and cub recruitment. Key parameters were varied first to mimic an asocial species. In this initial case, all offspring dispersed and females were solitary (thus the model was deterministic rather than stochastic). In the second scenario, lions lived in stochastically created prides and new prides were only viable if they contained >4 females, but there was no within-group den-

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