The endemic vascular plants of the Cape Verde Islands, W Africa

Abstract

Abstract The archipelago of Cape Verde consists of nine main islands of oceanic origin situated 500 km west of the African mainland. The present study aims at increasing our knowledge of the endemic vascular flora and its origin and evolution, and at providing a scientific basis for its protection. One endemic genus (Tornabenea) and 82 endemic species and subspecies were accepted, including one new species in Tornabenea (T. humilis) and three new combinations in Campylanthus and Kickxia. Morphological descriptions, illustrations, typifications, chromosome numbers, phyto- and ecogeographic information, and conservation status, based on field-, herbarium-, garden-, and literary studies, are provided. The degree of endemism was 10.5% at the species level. The endemic flora had a mixture of holarctic (65%, including I 5% in a NW-Moroccan element) and tropic (35%) affinities. Thirty-three Capeverdean endemics ( 40%) were most closely related to taxa with a Canaro-Madeiran distribution. The endemic flora was divided into five distributional elements (northern, western, southern, eastern, and ubiquitous; with 26, 21, I 5, 3, and 17 taxa. respectively) and three ecological elements (hygrophytic, mesophytic, and xerophytic; with 31, 34, and I 7 taxa, respectively). Although distributional patterns corresponded closely to geographic positions of island groups, the patterns could be sufficiently explained by ecological factors. Endemic species richness was strongly correlated with humidity and maximum island altitude. The tropic proportion decreased with increasing altitude and humidity. More than half of the endemic flora is presently threatened (Red List taxa), and 16 single-island extinctions were recorded. The proportion of polyploids was 27% based on 63% of the endemics, which is similar to the proportion in the endemic Canarian flora. Most taxa (90%) were schizoendemic diploids or mesopolyploids, typically differentiated ecogeographically among islands and along humidity gradients. This pattern has frequently been complicated by parallel inter-island evolution along similar gradients, and many ecogeographically ubiquitous mesophytes showed large and complex interpopulational variation. In such cases, differentiation along continuous, steep, and tree-less coast-mountain gradients of humidity has resulted in eco-morphological clines of populations rather than classic adaptive radiation into distinct taxa. Contrary to previous beliefs, we suggest that the present Capeverdean flora is very young, possibly only a few hundred thousand years old. It contains no palaeoendemics in the strict sense, only a single endemic genus, and most endemic taxa are only slightly differentiated morphologically from their sister groups. This conclusion is supported by recent geological, palaeontological, and molecular data. A possible scenario with origins and extinctions of successive Capeverdean floras is outlined, following the dramatic fluctuations between wet and dry climates in northern Africa during the Quaternary. The present flora was probably founded by two main waves of immigrants, both via dispersal from (north-)westem Africa, but in different climatic periods.