The Serengeti wildebeest [Connochaetes taurinus (Burchell)] population is perhaps the best known of the large migratory grazing systems of the world. It is the keystone that determines the rest of the Serengeti ecosystem (Sinclair 1979a; Sinclair & Arcese 1995a) and consequently its viability is the major management goal for conservation in this region of Tanzania. Information on its population ecology has accumulated for 40 years, and although analyses have been conducted at intervals, a complete synthesis has yet to be published. In this paper we present a synthesis of the available information on its population dynamics.
The migratory wildebeest have been monitored since 1958 (Grzimek & Grzimek 1960) with censuses every few years (Fig. 1) and estimates of reproduction, recruitment and mortality at various intervals (earlier publications are cited below). Prior to 1963 the population experienced recurrent annual mortality of calves from the exotic viral disease, rinderpest (Talbot & Talbot 1963). However, in 1963 the disease died out, probably as an indirect result of vaccination campaigns with the surrounding cattle herds. The evidence for the disappearance of this disease comes from serum antibody samples collected in the late 1950s, throughout the 1960s, and at intervals subsequently (Sinclair 1977a,b; Plowright 1982; Dobson 1995 and references therein). Following the disappearance of rinderpest the population, at nearly 0·24 million, increased steadily until 1977 at which time it reached 1·4 million (Sinclair & Norton-Griffiths 1982). Thereafter the population fluctuated between 1·1 and 1·4 million until at least 1991 (Dublin et al. 1990; Campbell & Borner 1995; Sinclair 1995). In 1993 a drought, more severe than any since records began in 1938 (Mduma 1996), occurred in the Serengeti region and the wildebeest population dropped in that year to 0·9 million and has remained at that level subsequently (Farm & Woodworth 1994; Mduma, Hilborn & Sinclair 1998; Tanzania Wildlife Conservation Monitoring, personal communication).
These long-term records, the dramatic change in abundance, and the events which affected the wildebeest numbers, have allowed a determination of the life stages where survival fluctuates the most and where density dependence occurs (the regulating stage). We analyse first the life stages at which the main mortalities occur to identify where fluctuation and regulation take place (Sinclair 1989). We then present the evidence for the causes of these mortalities.
The life stages for which we obtained data were fertility loss, newborn calf mortality occurring in the wet season (March–June, ages 0–4 months), dry season calf mortality (July–December, 5–10 months), yearling mortality (ages 11–24 months) and adult mortality (> 24 months). Causes of mortality were lumped into predation and non-predation, the latter representing undernutrition and associated diseases (Sinclair 1977a, 1979b; Sinclair, Dublin & Borner 1985; Sinclair & Arcese 1995b).