African Milk-barrel

Host to a parasitic mistletoe

Euphorbia polygona var. horrida has a stem with many spines and outwardly resembles a cactus. The plant is named after its 4 cm long spines (horrida). As an extra deterrent the plant contains a toxic milky sap. The species is a textbook example of evolutionary adaptation to a dry, barren environment with herbivores. Whereas in America cacti have evolved to live in these habitats, in Africa and Europe they support the succulent spurges.

Euphorbia polygona var. horrida is the host for parasitic mistletoe, Viscum minimum, which becomes obvious because its produces orange berries visible on the stem of the Euphorbia. This mistletoe only grows on Euphorbia polygona var. horrida and its close relative Euphorbia polygona. Given the mistletoe’s preference, it is perhaps a better botanist than the authors who originally described E. horrida because the latter species has now been lumped together with E. polygona.

Ecology and habitat

This plant grows in the hills of the South Africa’s Eastern Cape. It is a region with a mosaic of subtropical thicket vegetation (spekboomveld) and Succulent Karoo. Both vegetation types are extremely species-rich. The spekboomveld habitat, named after the elephant bush (Portulacaria afra, or spekboom in Africaans), comprises more than 8000 species, of which 23% are endemic, and the Succulent Karoo tallies 10,000 species. This strip is very dry (20-290 mm rain per year) and the temperature is often in excess of 40°C.

Threat

The species is under threat predominantly from habitat destruction and grazing sheep. The habitat of Euphorbia polygona var. horrida is shrinking as a result of expanding common ground and associated overgrazing. Moreover more land is being taken up by mining. Less that 3% of the Succulent Karoo is a protected biotope. Illegal collecting probably plays a small role in the decline of this species.