Ecology

A Leap for Lion Populations

Esa Ranta and Veijo Kaitala

There are substantial populations of lions in Africa and Asia. Despite being known as "the king of the jungle," the African lion is principally found on open savannas, with smaller numbers in woodland areas. Since the 1960s, the ecology of lions has been intensively studied in the Serengeti National Park of Tanzania in East Africa (1-3) (see the figure). Lions live in family groups known as prides. Both woodlands and plains prides of the Serengeti are typically composed of six related females, their cubs, and a few unrelated males who mate with the adult females. The females do most of the hunting for the pride (1), and they must hunt in

The authors are in the Integrative Ecology Unit of the Department of Biological and Environmental Sciences, University of Helsinki, FIN-00014 Helsinki, Finland. E-mail: [email protected] helsinki.fi

groups to be successful. Most of the daily activity of males concerns maintenance of territory. Packer et al. (4) have completed a detailed analysis of long-term records of lion populations in a 2000-km2 area of the Serengeti National Park. As they report on page 390 of this issue, lion population size has remained remarkably stable for long periods (10 to 20 years) punctuated by sudden increases that do not seem to reflect numbers of available prey.

The population dynamics (changes in the population size) of animals obey several key rules (5, 6). Population increases are due to births of new individuals and arrival of immigrants from nearby populations, whereas any decreases are due to individuals dying or leaving their natal population. Often births and deaths are dependent on each other either directly or with an intervening time lag. Population increases also depend on the amount of resources that mature females are able to monopolize. However, Packer et al. now reveal an unusual feature of the population dynamics of Serengeti lions (4). They discovered that lion numbers, both in the woodlands and on the plains, undergo long periods of stability interrupted by abrupt changes. This is intriguing because no such saltatory changes in population dynamics are reported for lion prey such as wildebeest, Cape buffalo, and gazelle (4).

Serengeti National Park \

Tanzania

Rather, the populations of these Serengeti prey species have changed without evidence of the dramatic leaps characterizing lion populations since the 1960s, when quantitative censuses began.

A straightforward expectation of predator-prey population dynamics is that a gradual change in resources (prey) would support a corresponding smooth change in predator population size (6). However, in Serengeti lions, the population increases have been abrupt often constituting an increase of 20 to 50% of the extant population size. The population records since the end of the 1960s include three saltatory changes (1973, 1983, and 1999) for lions inhabiting Serengeti woodlands and one (1997) for lions inhabiting the plains. In between these leaps, the lion population remained stable, hovering around the number reached during the previous increase.

Packer and his research team (4) now provide an explanation for these jerky dynamics by incorporating into their simulation model the social grouping behavior of lions within their prides. Serengeti lion prides have an optimal size: In the smallest prides, females have very low reproductive success, because hunting of their major ungulate prey calls for the cooperation of several individuals. In contrast, in the largest prides, females suffer from intense competition for the prey they have subdued jointly. When resources increase sufficiently to support more offspring and when the size of an extant pride is sufficiently large, the pride splits; several related females disperse together to form a new pride. Packer et al. suggest that it is optimal from an evolutionarily standpoint for a group of females to leave their pride and to establish a new pride with a different territory because this behavior ensures greater breeding potential. In this way, a gradual increase in prey population size supports a sequence of continued stability interrupted by abrupt increases in the abundance of lion populations.

The only major decrease (since the 1960s) in the population size of Serengeti lions occurred in 1994. The rapid decline (to 50% of the pre-1994 level) of woodland lions coincided with a decrease in the numbers of wildebeest and Cape buffalo due to a severe drought in that habitat. It is tempting to suggest that the collapse in lion numbers represented a reversal of the social mechanisms that result in punctuated increases in lion population size. A decrease in resources could have affected lion population size by reducing the number of prides in the overall population. However, this possibility cannot be verified because the 1994 decline in lion numbers was in fact due to an epidemic of canine distemper (4).

It is worth contrasting the population dynamics of Serengeti lions with the known dynamics of other species where long-term data are available (6). Whereas Serengeti lion population dynamics show saltatory changes in size, all other known predator-prey populations follow—with a time lag— a model in which smooth increases in prey availability result in smooth increases in predator population size. The best documented predator-prey populations display characteristic periodic fluctuations (5-7) that obey the standard models found in any population ecology textbook (6, 7). One feature shared by traditional predator-prey models (6, 7) and the Serengeti lion model of Packer et al. (4) is the presence of a time lag between changes in the population size of prey and that of predator.

The new data about Serengeti lion population dynamics (4) enriches our existing palette of predator-prey population patterns. One recent theory takes as a starting point the idea that individual behavior eventually has repercussions on long-term population ecology (8). The findings of Packer and colleagues support this view, although traditional population theories (5-7) have yet to incorporate the notion of social behavior into their models. The population dynamics of woodland and plains Serengeti lions are largely driven by the social structure of prides: To breed successfully, lionesses must belong to a pride that is suf-

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