Research Article

Ecological Change, Group Territoriality, and Population Dynamics in Serengeti Lions

Craig Packer,1* Ray Hilborn,2 Anna Mosser,1 Bernard Kissui,1 Markus Borner,3 Grant Hopcraft,3 John Wilmshurst,4 Simon Mduma,5 Anthony R. E. Sinclair6

Territorial behavior is expected to buffer populations against short-term environmental perturbations, but we have found that group living in African lions causes a complex response to long-term ecological change. Despite numerous gradual changes in prey availability and vegetative cover, regional populations of Serengeti lions remained stable for 10- to 20-year periods and only shifted to new equilibria in sudden leaps. Although gradually improving environmental conditions provided sufficient resources to permit the subdivision of preexisting territories, regional lion populations did not expand until short-term conditions supplied enough prey to generate large cohorts of surviving young. The results of a simulation model show that the observed pattern of "saltatory equilibria" results from the lions' grouping behavior.

To test the effects of ecological changes on population dynamics (1-3), we rely on long-term records available from the Serengeti National Park, Tanzania (4). Lions in a 2,000 km2 area of the Serengeti have been studied continuously since 1966 (5). "Woodlands" prides reside in regions dominated by Acacia and Commiphora trees, with resident herds of hartebeest, topi, and buffalo. "Plains" prides occupy grasslands consisting primarily of Sporobo/us, Themeda, PennisetMm, and Cyno-don spp., with low densities of resident warthog and Grant's gazelle. Large numbers of migratory wildebeest, zebra, and Thompson's gazelle move through both habitats in response to seasonal rainfall patterns each year. Lion prides consist of 1 to 18 adult females, their dependent offspring, and a resident coalition of 1 to 9 males. Females defend joint territories, and larger prides dominate smaller ones (6, 7). As a result, prides of <2 females suffer very low reproductive success; prides of >10 females also fare poorly because of high levels of within-group competition (5, 8). Prides persist for generations, and new prides consist of related females that disperse together from preexist

1Department of Ecology, Evolution, and Behavior,

University of Minnesota, 1987 Upper Buford Circle, Saint Paul, MN 55108, USA. 2School of Aquatics and

Fishery Science, University of Washington, Seattle, WA 98105, USA. 3Frankfurt Zoological Society, Post Office Box 14935, Arusha, Tanzania. 4Parks Canada,

Ecosystem Services, Winnipeg, MB R3B OR9, Canada.

5Tanzanian Wildlife Research Institute, Box 661, Arusha, Tanzania. 6Center for Biodiversity Research, University of British Columbia, Vancouver, BC VLT 1Z4, Canada.

*To whom correspondence should be addressed. E-mail: [email protected]

ing prides (9). Lion territory size varies with overall food availability (10) but not with the number of females in the pride (11), and a pride territory must also contain permanent water and adequate denning sites (5, 12).

Population stasis and transition. To breed successfully, female lions must have sufficient companions to defend adequate food, water, and denning sites. Thus, an expanding food supply can only cause lion populations to grow when preexisting prides can split to form descendant prides that are large enough to establish themselves successfully. Population size should therefore increase as a direct function of the number of breeding groups in the population. The month-by-month change in population size between the 1960s and 2002 was highly correlated with the corresponding change in the number of prides in that habitat, regardless of whether we defined a pride as containing a minimum of two, three, or four adult females (and whether females were defined as "adults" after reaching 2, 3, or 4 years of age). However, the best statistical fit defined prides as groups containing a minimum of four females of at least 2 years of age (13), the age at which young females first participate in territorial defense (14).

Figure 1, A and B, shows the monthly population sizes in each study area. Totals fluctuated slightly from month to month; but at a broad time scale, each population showed long periods of stasis ended by a sudden transition to a new equilibrium (15), and each change point was associated with a specific ecological change in that habitat. Figure 1C shows the population sizes of the major Serengeti herbivore species over the past 40 years. Wildebeest and buffalo numbers increased dramatically from the early 1960s until the late 1970s because of their release from rinderpest infection in 1963 (4). This was by far the most substantial change in prey abundance over the entire study period, and the lion population showed a clear increase over this same span [also see (16)]. Most striking, however, is that the woodlands lion population suddenly increased to a new equilibrium in 1973. The plains lions were not monitored between 1969 and 1974, so the precise timing and tempo of its increase are not known.

Lions enjoy higher feeding success in areas with greater vegetative cover (17), and each Serengeti habitat has undergone a large-scale increase in cover in the past 20 years. The grazing wildebeest herds remove vast quantities of grass that would fuel wildfires if left to se-nesce, and the enormous increase in wildebeest numbers led to a striking decrease in grassfires that, in turn, stimulated a regeneration of the Serengeti woodlands during the 1980s (Fig. 2). In November 1983, the woodlands lion population suddenly increased to a new plateau after several years of unprecedented growth of woody vegetation (Fig. 1A). The wildebeest population declined in 1994 (as a result of severe drought); the migration ''skipped'' the intermediate grass plains in 1995, enabling the tallest species in this community to dominate, and this pattern persisted for the following 5 to 6 years (Fig. 3). The tall grass provided improved cover for the plains lions, and the plains population suddenly increased in February 1997 after remaining at a persistent equilibrium since at least 1975.

The woodland lion population dropped significantly in 1994 because of the canine distemper virus (CDV) epizootic that killed approximately one-third of the Serengeti lions (18). Although the die-off caused the lions to drop well below their equilibrium density, the population remained relatively constant for 5 years until suddenly returning to its previous plateau in May 1999.

Determinants of population change. The irruption of the herbivores, the regeneration of the woodlands, and the expansion of the tall grass plains were all processes that continued over several years, yet the lion populations always reached a new equilibrium in a single year. Similarly, the woodland population recovered suddenly but not until 5 years after the CDV outbreak. What determined the precise timing of these changes? The migratory patterns of the dominant herbivores (wildebeest, zebra, and gazelle) are primarily driven by seasonal rains in the Serengeti, and all of the sudden changes in lion population size coincided with years of unusual rainfall: 1973 was the first in a series of unusually "wet" dry seasons [which attracted the migratory herds to the woodlands study area

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.

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