Food limitation and demography of a migratory antelope, the white-eared kob

Abstract

Although it is commonly presumed that many populations of large-herbivores are limited by food availability, supporting evidence is scarce. This hypothesis was investigated in a population of over 800,000 white-eared kob in the Boma National Park region of the southern Sudan. Food availability, body condition, and mortality rates of adults and calves were measured during the dry seasons of 1982 and 1983. Sampled age distributions from both the live population and carcasses were used to calculate age-specific rates of mortality. In 1982, food supplies during the dry season were augmented by substantial rainfall, which produced regrowth of grass in areas that ordinarily had little green forage. As a result, fat reserves declined little, and rates of adult mortality showed no increasing trend. Total adult mortality was 5%. In 1983, there was no rainfall during the dry season and food intake was insufficient to meet the estimated energy requirements of kob. As a result, fat reserves declined and adult mortality rates increased fourfold. Total adult mortality was 10% (equivalent to the recruitment rate of yearling into the population). Calf mortality during the dry season was similar in both years (50%), based on field estimates of mortality rates and calf/female ratios. Lactation throughout the dry season possibly provided a buffer for calves against variations in food availability. The age structure of the live population in 1983 suggests that a drought in 1980 reduced kob numbers by 40%. These results suggest that adult survival is influenced strongly by the availability of food during the dry season. However, the duration of the dry season also plays an important role. During the dry season, declining fat reserves make an increasing proportion of the population vulnerable to mortality. As a result, even moderate droughts may lead to substantial changes in population numbers.