Patterns of plant diversity and endemism in the Horn of Africa

Abstract

Data on the distribution of 5518 indigenous taxa of vascular plants in 25 Flora regions on the Horn of Africa have been analysed. The sample is derived from families that are covered in the published parts of all of the following three works, the Flora of Ethiopia and Eritrea, the Flora of Somalia and Les plantes Vasculaires de la Republique de Djibouti, or from families or smaller groups covered for all parts of the Horn in other works or in unpublished accounts. The sample contains 4247 taxa that occur in Ethiopia, 2520 in Somalia, 1619 in Eritrea and 554 in Djibouti, and it is assumed to cover ca. 80% of the total flora on the Horn. According to the sample, the highest diversity (number of taxa per Flora region) is found in southern, eastern and parts of western and central Ethiopia and western Eritrea. The number of Horn of Africa endemics plus near-endemics in the sample is 1935 (35%), with 1330 endemics (24%) and 605 near-endemics (11%). The endemics make up 29% of the taxa that occur in Somalia, 17% in Ethiopia, 7% in Eritrea and 3% in Djibouti. The near-endemics make up 16% of the taxa that occur in Somalia, 10% in Ethiopia and Djibouti and 8% in Eritrea. The highest number of Horn endemics is found in north-eastern Somalia. The highest number of near-endemics is found in southern Ethiopia, while near-endemics make up the highest proportion of the flora in inland Flora regions of southern Somalia. The highest number of single-region endemics (Horn endemics restricted to one Flora region only) is also found in north-eastern Somalia. On the tip of the Horn there is a high concentration of endemics with their total range restricted to one Flora region or two, while the endemics in other regions of Somalia and even more so in the Ethiopian highlands tend to occur in several Flora regions. A number of ecogeographical parameters (e.g. position, distance to the tip of the Horn, altitudinal range, area, perimeter and amount of coastline of the Flora regions) have been analysed and show varying degree of correlation with diversity and endemism. The higher diversity in Ethiopia than in Somalia is correlated with the higher altitudinal range in the former country and agrees also well with the higher climatological diversity in Ethiopia, while the higher endemism in Somalia than in Ethiopia agrees with the observation that the Flora regions in Somalia have higher proportion of their perimeter as coastline and shorter distance to the tip of the Horn, possibly a “peninsular effect.” The high number of near-endemics in southern Ethiopia and the high proportion of near-endemics in southern Somalia agree with the observation that many taxa with restricted range that are shared between the Horn and northern Kenya.