Diversity anomaly in Gladiolus

Researchers have investigated why there are more species of Gladiolus in the Cape of southern Africa than in the Mediterranean Basin.

View on map: South Africa,

19 Jul 2012

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Gladiolus alatus, one of 106 species of Gladiolus found in the Cape region of southern Africa (Image: J. Manning)

Biodiversity is not evenly distributed over the Earth. Some regions contain a much greater proportion of all known species than others. This disparity in species richness is all the more obvious when it occurs within one group of organisms. Why a single group should show greater diversity in one part of its range compared to another has puzzled biologists and it is usually assumed that a combination of historical, evolutionary and ecological processes have played a role.

Diversity disparity in Gladiolus

Gladiolus shows a marked geographical variation in species richness. Species of Gladiolus are found in two different Mediterranean climate biomes with similar climatic conditions: the Cape of southern Africa and the Mediterranean Basin. But, while 106 species are known from the Cape, only 7 species occur around the Mediterranean. Researchers from Real Jardín Botánico de Madrid, Imperial College London, the South African National Biodiversity Institute, Missouri Botanical Garden, and RBG Kew have attempted to find the causes of this spatial asymmetry in diversity.

Temporal vs evolutionary hypotheses

The team investigated whether the diversity disparities observed in the genus are better explained by recent colonization of species-poor areas (temporal hypothesis) or by differential rates of diversification (evolutionary hypothesis). Gladiolus has been present in the Cape region for much longer than in the Mediterranean, but the researchers found that the ‘diversity anomaly’ in Gladiolus cannot be explained solely by the extra time available for speciation in the Cape - locally reduced rates of diversification in the Mediterranean Basin were also a factor.

They conclude that the high overall diversity in southern Africa stems from an ancient origin in the Cape allied with high rates of diversification in the summer-rainfall region of the subcontinent.

Item from Kew Scientist, issue 41

Article reference:

Valente, L. M., Savolainen, V., Manning, J. C., Goldblatt, P. & Vargas, P. (2011). Explaining disparities in species richness between Mediterranean floristic regions: a case study in Gladiolus (Iridaceae). Global Ecology and Biogeography 20: 881-892.

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