16 Aug 2023
The most plant species in the smallest space can be found where it rains a lot. Like in the Amazon rainforest. The plant genus Petalidium, however, seems to turn this rule of thumb on its head. Because 37 of its 39 species, which are known worldwide so far, have developed in an area that is considered to be extremely dry: in the northern Namib.
This astonishing fact was pointed out by the US botanist Kyle Dexter in a talk at the National Botanical Research Institute (NBRI) in Windhoek. Two of these 37 Petalidium species (P. aromaticum and P. oblongifolium) also exist in Limpopo, one species is not yet considered certain. Only one species (P. barleriolides) occurs in India – in much wetter distribution areas.
According to Dexter, the Indian species can be traced back 850,000 years. Its sisters in northwestern Namibia are much younger, with a maximum of 510,000 years. What is astonishing about this is the relatively short period of time in which the diversity of the 37 (or 38) species has developed.
In a research project of botanist Erin Manzitto-Tripp, Dexter is also investigating the question of why and how this species diversity came about in the dry area on the eastern edge of the northern Namib. The researchers assume that there were wet phases during which new species formed in batches.
Petalidium species can also be distinguished by the shape, colour, size and smell of their flowers. With these, they each attract specific pollinators, such as bees or flies. This raises questions of when and how some of the species have changed their pollinator in the course of evolution.
The shrub genus Petalidium belongs to the acanthus family (Acanthaceae). Kyle Dexter is Associate Professor at the University of Edinburgh and Research Associate at the Royal Botanic Garden Edinburgh.
His lecture was arranged by the Botanical Society of Namibia (BOTSOC). BOTSOC also organises regular guided tours of the Botanic Garden on the grounds of the NBRI in the centre of Windhoek.
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