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Plant story - Roridula dentata, endemic to South Africa

A tentacle leafed shrub, endemic to the Cape Province of South Africa.

Roridula dentata flower (Photo: A. Hitchcock)

The family Roridulaceae consists only of the genus Roridula, which has two species, Roridula gorgonias and R. dentata, both endemic to the Cape Province of South Africa. Roridula dentata is a tentacle-leafed shrub that has a unique symbiotic relationship with "assassin bugs" (Pameridea marlothii). Although commonly known as the flycatcher bush this plant does not consume the insects it traps but rather provides food for its insect guest and has been known to catch small birds.

Description

Shrubs of up to 2 m in height, the leaves are lanceolate having margins with teeth and tentacles. Leaves are covered with numerous sticky hairs, which are different lengths and covered with sticky glue. The flowers are pink in colour and are borne on the ends of hairy pedicles in the axils of upper leaves. Flowering time is in spring (September to October in the Southern Hemisphere). Plants do not live long and bush fires initiate seedling germination; these plants therefore suffer with too frequent and wrongly timed fires.

Ecology and distribution

Found at altitudes of 900-1200 m in the Cape Fold Mountains of Clanwilliam, Ceres and Tulbagh it prefers moist damp localities and is generally found in habitats such as bogs, marshes and on the banks of rivers. Although occurring in very moist habitats the biome type is arid mountain fynbos and annual temperature average 27°C, the dry summer winds and low rainfall make this an extremely harsh environment to survive in, yet Roridula dentata does.

Conservation

Roridula dentata is currently not listed on the South African Red Data list of 1996; however the Millennium Seed Bank team based at Kirstenbosch is working with the Threatened Species Program (TSP) currently revising the IUCN Red List Data. It has been noted that although this shrub had a wide distribution historically, many populations listed in herbaria are no longer in existence. This is due to too frequent fires, wrong season fires when seeds are germinating or habitat destruction by agriculture. With the data now at hand it is envisaged that Roridula dentata may be given a status of Vulnerable (VU) or even Endangered (EN). Although seed from this plant species is known to have an ample longevity in storage, the parent plant often does not produce large quantities of seed. Thus there is often insufficient seed for seed banking and also to regenerate the soil seed bank in situ.

To date only 75 seeds have been collected and banked, and plants are being propagated at Kirstenbosch Gardens' 'Wild Plant Orchard' for further seed production and also for use in re-habitation of disturbed habitats. The most devastating threat to R. dentata populations is agriculture, specifically the production of Rooibos tea, which is the only crop that can be grown in the mountains of the Cape, in exactly the same habitat as our species. Due to its unique relationship with a bug, it is not the only species that is rare or threatened - the bug Pameridea marlothii only occurs on Roridula dentata. This leggy insect may hold the key to the growth and pollination of this perennial shrub.

Plant - insect symbiosis

In spite of the fact that Roridula dentata has sticky insect trapping liquids on its leaves like the Drosera species in the Cape, this sap has no enzymes to digest this prey. In l996, a student of The University of Cape Town started to look more closely at this plant that captured an abundance of insect prey. Numerous small bugs (Pameridea - Miridae) were recorded living on the plants and after further examination it was seen that they had specially adapted feet which enabled them to run freely over the sticky sap. Pameridea then roam the plant for insects, on finding a trapped insect they probe the helpless victim with their proboscis, inject venom each time until the prey succumbs and dies.

"Within half an hour black bugs would have flown from close by and a massive seething scrum would have developed, vying for a place at the carcass, using their hind legs to lash out at rivals that get too close. The fat, red bug larvae creep unseen between the leggy forest, also keen for some of the bounty. Soon the prey is reduced to a dry husk and the bloated bugs slowly disperse" (Fernkloof web article)

They then defecate on the leaves of Roridula leaving nitrogen-rich faeces, which act as fertilizer for Roridula. The plants then absorb it through their leaves. The 'Assassin Bugs' occur nowhere else in the world and are thus also threatened. They need the plant to catch their food in order to mature and reproduce and the plant needs the juvenile bugs to pollinate the flowers whilst crawling around for food. It has also been noted that bees visit the flowers and allow pollination between different plants, without being caught. This symbiotic relationship allows for both the insect and plant to obtain food through the other's actions.

Story by Carly Cowell, SANBI, South Africa | More plant stories

Further reading

  • Leistner, O.A. (ed.). (2000) Seeds plants of southern Africa: families and genera. Strelitzia 10. National Botanical Institute, Pretoria.
  • Reiner, M. (2003) Cultivation and description of Roridula gorgonias and Roridula dentata. www.drosophyllum.com 
  • Smith, C.A. ( 1966) Common names of South African plants. Memoirs of the Botanical Survey of South Africa No. 35.
     

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