Factors limiting the abundance abd distribution of hirola (Beatragus hunteri) in Kenya

Abstract

This study investigates the factors limiting the abundance and distribution of hirola, or Hunter's antelope, (Beatragus hunteri), in Kenya's Tsavo East National Park (ex-situ population) and Garissa (in-situ popUlation). The hirola is widely recognized as the most severely threatened monotypic species of antelope in sub-Saharan Africa. Data were collected between 1996 and 2000 on the ex-situ population, with occasional comparative sampling of the in-situ population. The study was carried out almost entirely in the field. Hirola were located by ground searching, by radio tracking and occasionally using aircraft; animals were observed from a landrover. In Tsavo NP the population was found to be static (71.1±9.3 in 844 km2 range), while the Garissa population was declining with about 672 animals remaining in a 5,171 km2 range. Calving in Tsavo took place between August and March, with the peak occurring in late October and early November. About 69.8% of calves died or disappeared within the first 6 months of life and about 18.0% survived to the age of 2 years. Mortality in Tsavo was predominantly associated with predation, while in Garissa, mortality was associated with disease and poaching. Hirola occurred in 8 distinct family groups in Tsavo, each with an adult male and a number of females and their offspring; temporary separations involving the adult male, or females with young, were also recorded. Remarkably, offspring left their natal groups at about 6-15 months of age and spent a period isolated or with other young animals; females eventually rejoined a family group or joined a lone male to form a new group. Young males generally formed bachelor groups and eventually separated as adults to become solitary. Hirola were most active in the early morning hours and late evening when it was cool; hot times of the day were spent resting in the shade of trees and bushes. Densities of other ungulate species within the hirola range varied seasonally, being more abundant in the wet season. Controlling for the relative abundance of other herbivore species, hirola in Tsavo were found to associate predominantly with Grant's gazelle (Ga~ella granfii), while in Garissa they associated mainly with topi (Damaliscus korrigwn). The presence of fewer associates and fewer alternative prey in the dry season appears to have increased the risk of predation to hirola, perhaps due to reduced detection and dilution effects. Home ranges occurred on generally elevated areas associated with red soils (in Tsavo) or sandy soils (in Garissa), and scattered seasonal waterholes. Vegetation in these areas was composed of tall, fairly open bushes interspersed with short green grass patches, that offered preferred food, shade and cover. In Tsavo, hirola fed on a total of 56 plant species (grasses, 74.9%; forbs, 24.2; and sedges, 0.9%) while in Garissa, 27 plant species were eaten (grasses 85.2% and forbs 14.8%). Hirola selected short green grasses and forbs, eating an entire plant (leaf, stem and inflorescence) at a mean bite height of 7.4±3.93SD cm. The nutrient content of the preferred plant species were measured on a seasonal basis. The Garissa population has declined as a result of range reduction, caused by human encroachment. In addition, it has been further affected by poaching and spread of diseases from cattle. The factors limiting the Tsavo population were investigated by multivariate analysis of the relationship between the main candidate factors (estimates of predation pressure, food abundance, shade tree densities, competitors, etc) and variation in the size, reproduction and mortality rates across the hirola family groups studied. The main limiting factors in the Tsavo population appeared to be predation pressure and the amount of specific microhabitats available for shade, cover against predators, and food. Finally, I discuss how these results may shape the development of conservation management strategies for hirola in the future. Recommendations include the restoration of protected areas in Garissa and the establishment of further ex-situ populations, preferably in predator free environments, to allow a rapid increase in numbers and to spread the risk of species extinction.