Diversity and endemism in the species-rich Ceropegieae (Apocynaceae) and Euphorbia in the highlands and escarpments of Angola and Namibia

Authors

  • PV Bruyns Bolus Herbarium, Department of Biological Sciences, University of Cape Town, Rondebosch, South Africa
  • P Hanácek Department of Plant Biology, Mendel University in Brno, Brno, Czech Republic
  • C Klak Bolus Herbarium, Department of Biological Sciences, University of Cape Town, Rondebosch, South Africa

Keywords:

Angola, Ceropegia, distribution, endemic lineages, Euphorbia, growth forms, highland, Namibia, species pairs, succulents

Abstract

We map the distribution of the species-rich groups Ceropegieae and Euphorbia in southwest tropical Africa (i.e., in Angola and Namibia). This shows that they are most diverse in the highland and escarpment areas of these countries, and in the coastal areas west of these highlands. Most of the endemic species occur here too. Several ‘hotspots’ within highlands and escarpments of Angola and Namibia (HEAN) are identified that are common to both groups. Diversity in both groups falls off rapidly in Angola north of Benguela. This diversity also tails off substantially on the Kalahari sands to the east of the escarpment (i.e., east of the HEAN), with some notable exceptions in Namibia in Ceropegia. In the Ceropegieae, many species occurring in these sandy regions are widespread in southern Africa. Areas covered by Kalahari sands are almost completely devoid of Euphorbia. Several lineages are identified in Ceropegia and Euphorbia that have diversified in and are endemic to the margins of the Namib Desert in Angola and Namibia (i.e., in and west of the HEAN). Apart from these endemic lineages, the species found in the HEAN (and more generally in Angola and Namibia) are mostly related to others occurring further east in southern Africa. Extreme examples of this are provided by closely related species pairs that occur on opposite sides of southern Africa. While the Asclepiadeae has significant numbers of geophytic herbs and few succulents, in the Ceropegieae and in Euphorbia succulents are particularly common. The proportion of succulents with greatly reduced leaves is also high, suggesting that this growth form is especially successful in the region of the HEAN and in the narrow coastal area west of it. These succulents are considered to have arisen within the last 8 million years.

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Published

2023-12-16