THE HISTORIC RANGES OF 3 EQUID SPECIES IN NORTHEAST AFRICA - A QUANTITATIVE COMPARISON OF ENVIRONMENTAL TOLERANCES

The historic ranges of three equid species native to north-east Africa are analysed with respect to annual rainfall, several temperature par ameters and a satellite-derived multispectral index of primary product ivity. Equus africanus Fitzinger, Equus grevyi Oustalet and Equus burc helli Gray used to largely replace each other, geographically, with na rrow zones of range overlap occurring between E. africanus and E. grev yi in the Awash valley, and between E. grevyi and E. burchelli in sout hern Ethiopia and northern Kenya. The three species are shown to succe ed each other along an environmental gradient. The position of each sp ecies on this gradient and the resulting location and extent of its ra nge are discussed. Competitive exclusion, specific adaptations and his toric events are likely determinants of equid distribution. In the are a of sympatry between E. grevyi and E. burchelli, mixed habitat charac ters as well as environmental fluctuations seem to prevent either spec ies from excluding the other. Different social organizations of E. gre vyi and E. burchelli and the resulting migratory patterns may be adapt ations to the environment in their allopatric ranges; in their sympatr ic range they could alleviate competition.