What limits the Serengeti zebra population?

Abstract

The populations of the ecologically dominant ungulates in the Serengeti ecosystem (zebra, wildebeest and buffalo) have shown markedly different trends since the 1960s: the two ruminants both irrupted after the elimination of rinderpest in 1960, while the zebras have remained stable. The ruminants are resource limited (though parts of the buffalo population have been limited by poaching since the 1980s). The zebras' resource acquisition tactics should allow them to outcompete the ruminants, but their greater spatial dispersion makes them more available to predators, and it has been suggested that this population is limited by predation. To investigate the mechanisms involved in the population dynamics of Serengeti zebra, we compared population dynamics among the three species using demographic models based on age-class-specific survival and fecundity. The only major difference between zebra and the two ruminants occurred in the first-year survival. We show that wildebeest have a higher reproductive potential than zebra (younger age at first breeding and shorter generation time). Nevertheless, these differences in reproduction cannot account for the observed differences in the population trends between the zebra and the ruminants. On the other hand, among-species differences in first-year survival are great enough to account for the constancy of zebra population size. We conclude that the very low first-year survival of zebra limits this population. We provide new data on predation in the Serengeti and show that, as in other ecosystems, predation rates on zebras are high, so predation could hold the population in a "predator pit". However, lion and hyena feed principally on adult zebras, and further work is required to discover the process involved in the high mortality of foals.

Competition, predation, and migration: individual choice patterns of Serengeti migrants captured by hierarchical models

“…Hindgut fermenters offset their lower digestive efficiency by processing greater quantities of forage faster, which enables species such as zebra to gain sufficient energy from low-quality grass (Bell 1970, Ben-Shahar andCoe 1992). The maximum abundance of wildebeest in the Serengeti seems to be regulated by the availability of dry-season forage rather than predation (Mduma et al 1999), whereas evidence suggests the Serengeti zebra population might be limited by predation, especially on juvenile age classes, rather than by the overall food supply (Sinclair 1985, Grange et al 2004). Therefore, it is possible that zebra might be choosing where and how to move during the migration based on factors related to predation, whereas wildebeest might make choices based on food availability.…”

Social structure, vigilance and behaviour of plains zebra (Equus burchellii): a 5-year case study of individuals living on a managed wildlife reserve

“…Much has been learnt about the social organisation and population ecology of plains zebra (Equus burchellii) in the wild (Fischhoff et al 2007a;Fischhoff et al 2007b;Fischhoff et al 2009;Gasaway et al 1995;Georgiadis et al 2003;Grange et al 2004;Klingel 1969a;Klingel 1969b;Rubenstein 1994;Rubenstein and Hack 2004;Smuts 1976a). The highly flexible and dynamic social organisation of this species may be a response to solving challenging and unpredictable social and ecological problems such as resource availability and risk of predation (Rubenstein and Hack 2004).…”

Inter-birth interval in zebras is longer following the birth of male foals than after female foals

“…1). Grange et al, 2004), but this may be due to a biased sample, since unlike in the Kruger study there was no specific procedure to ensure that animals were chosen at random (see Grange et al, 2004). The use of individually based data therefore produces results which are consistent with previous studies; it can however be noted that due to the method used, some very long inter-birth intervals can be missed and lead to an over-estimation of the foaling rate.…”

“…Further, older females can be expected to have different intervals compared to prime-aged animals: the intervals could be lengthened through the process of senescence. This was true in the Serengeti, as senescent females had lower reproductive rates than prime age females (Grange et al, 2004).…”

“…Reproductive rates in Plains zebra populations have been determined in only two previous studies, and in both cases by examining shot animals: in the Kruger National Park the pregnancy rate was 79% (Smuts, 1976) and in the Serengeti 88% (Grange et al, 2004). In this paper we describe a non-invasive approach for measuring reproductive rates in a population of wild Plains zebras, by calculating the mean foaling rate of the population, and subsequently testing for sources of variation in the inter-birth intervals of individually observed females.…”