Notes on a visit to Silhouette Hiking Trail, on a semi-arid part of the Great Escarpment in South Africa

@tonyrebelo @ludwig_muller @jeremygilmore @yvettevanwijk1941 @botaneek @karoopixie @chris_whitehouse @frankgaude @sethmusker

In early August 2001, I visited Silhouette/Ezeljacht farm, the site of the Silhouette Hiking Trail (https://www.safarinow.com/destinations/sutherland/hikingtrail/silhouette-hiking-trail.aspx), near Sutherland (https://en.wikipedia.org/wiki/Sutherland,_South_Africa) in the Northern Cape province of South Africa.

This location is on a semi-arid part of the Great Escarpment of southern Africa (https://en.wikipedia.org/wiki/Great_Escarpment,_Southern_Africa), immediately adjacent to the region of winter rainfall (scroll to first map in https://theconversation.com/scientists-split-on-south-africas-winter-and-summer-rainfall-zones-109513).

I discussed the natural history of the area with the landowner and pastoralist, Rudolph Steenkamp.

The substrate is dolerite (https://en.wikipedia.org/wiki/Diabase).

The general vegetation was a dense form of Nama Karoo (https://en.wikipedia.org/wiki/Nama_Karoo), marginal to mountain renosterveld (https://en.wikipedia.org/wiki/Renosterveld).

The whole area was naturally exempt from wildfire. Furthermore, fire was not used to manage the vegetation, even in dense stands of Dicerothamnus rhinocerotis (https://www.inaturalist.org/taxa/1200667-Dicerothamnus-rhinocerotis).

ANIMALS

The main species of livestock was Ovis aries (https://www.inaturalist.org/taxa/121578-Ovis-aries), which grazed all over the landscape, including the boulder-outcrops. Capra hircus (https://www.inaturalist.org/taxa/123070-Capra-hircus) was absent.

Procavia capensis capensis (https://www.inaturalist.org/taxa/43086-Procavia-capensis) was common here.

I assume that Sylvicapra grimmia grimmia (https://www.inaturalist.org/observations?taxon_id=698822) was present.

The distribution map for Oreotragus oreotragus (https://www.inaturalist.org/taxa/42384-Oreotragus-oreotragus) shows this area as marginal. However, the habitat is suitable.

Pelea capreolus (https://www.inaturalist.org/taxa/42336-Pelea-capreolus) survives in this general landscape. However, I did not inquire about its presence on the property.

The place-name 'Ezeljacht' hints at the former occurrence of Equus zebra (https://www.inaturalist.org/observations?taxon_id=132668).

Rudolph Steenkamp told me that he had been losing about ten individuals of O. aries each year to Caracal caracal (https://www.inaturalist.org/taxa/42042-Caracal-caracal). However, the more predaceous local carnivore, for livestock, was Lupulella mesomelas mesomelas (https://www.inaturalist.org/taxa/1210966-Lupulella-mesomelas).

Aquila verreauxii (https://www.inaturalist.org/taxa/5077-Aquila-verreauxii) was present, with a population of two pairs on his property. Its specialised diet was Procavia capensis.

PLANTS

The predominant dwarf shrubs were mainly asteraceous, e.g.

• Euryops (https://www.inaturalist.org/taxa/585725-Euryops-lateriflorus and https://www.inaturalist.org/taxa/585720-Euryops-imbricatus),
• Felicia (https://www.inaturalist.org/taxa/528743-Felicia-filifolia),
• Eriocephalus (https://www.inaturalist.org/taxa/467680-Eriocephalus-ericoides and https://www.inaturalist.org/taxa/430981-Eriocephalus-africanus), and
• Oedera (https://www.inaturalist.org/taxa/524508-Oedera-genistifolia).

A noteworthy aspect of the vegetation was that there were no foliar-succulent dicotyledonous plants whatsoever - despite this escarpment bordering the Tankwa Karoo (https://en.wikipedia.org/wiki/Tankwa_Karoo_National_Park), a prime area of Succulent Karoo (https://en.wikipedia.org/wiki/Succulent_Karoo).

Small-scale stem-spinescence, and the micro-hedging of individual plants, were noticeable to me.

For example, Asparagus capensis (https://www.inaturalist.org/taxa/580633-Asparagus-capensis) was common here, as a dwarf shrub even lower (<20 cm) than the asteraceous shrubs. It was puzzlingly exempt from herbivory, despite having

• a similar in growth-form to karoid dwarf shrubs, and
• spines (https://www.inaturalist.org/observations/10950255), which indicate palatability to herbivores, in plants generally.

Nitrogen-fixing species, generally scarce among the dwarf shrubs of the Karoo, were present here, in the form of certain Faboideae (https://en.wikipedia.org/wiki/Faboideae), particularly Melolobium candicans (https://www.inaturalist.org/taxa/589770-Melolobium-candicans).

In the case of Indigofera (https://www.inaturalist.org/taxa/587840-Indigofera-meyeriana and https://www.inaturalist.org/observations/61408909), the plant was shaped by browsing, to the point of obvious suppression.

There were no Fynbos elements (https://en.wikipedia.org/wiki/Fynbos) in this vegetation. However, a karoid Agathosma (Rutaceae), shaped by herbivory (https://www.inaturalist.org/observations/10909376), was fairly common on the property.

Euclea undulata (https://www.inaturalist.org/taxa/533457-Euclea-undulata) was absent. However, a species of dwarf ebony was present. Diospyros austro-africana (https://www.inaturalist.org/taxa/583880-Diospyros-austro-africana), not eaten by O. aries, reached up to 2m high on this property.

Rudolph Steenkamp told me that

• Procavia capensis ate the same plant species eaten by O. aries, viz. most of the karoid shrubs in the local flora,
• Asparagus capensis, common here, was categorically not eaten by either O. aries or P. capensis, and
• the tubers of Moraea, a genus toxic to herbivores, were excavated and eaten by Hystrix africaeaustralis (https://www.inaturalist.org/taxa/44173-Hystrix-africaeaustralis).

I observed the following additional species:

• Melianthus major (https://www.inaturalist.org/taxa/181791-Melianthus-major),
• Stachys rugosa (https://www.inaturalist.org/taxa/594734-Stachys-rugosa),
• Cyphia digitata (https://www.inaturalist.org/taxa/567751-Cyphia-digitata),
• Muraltia spinosa (https://www.inaturalist.org/taxa/526214-Muraltia-spinosa), uncommon and obviously shaped by herbivory,
• Lycium horridum (https://www.inaturalist.org/observations/164389050),
• Massonia depressa (https://www.inaturalist.org/taxa/467685-Massonia-depressa), and
• Moraea miniata (https://www.inaturalist.org/observations/63903346) and possibly Moraea marlothii (https://www.inaturalist.org/observations/64779864), conspicuously exempt from herbivory in the farmyard.

ECOLOGY OF DICEROTHAMNUS RHINOCEROTIS

Dicerothamnus rhinocerotis was present, despite the absence of fire. Its percent foliage cover was about 2%.

Rudolph Steenkamp told me that D. rhinocerotis was

• most vigorous on the richest soils on his property,
• greyish in the dry season, and greenish in the wet season,
• uneaten by either P. capensis or O. aries (although the black-headed dorper breed, https://en.wikipedia.org/wiki/Dorper, will reputedly accept small quantities when the plants are desiccated during droughts),
• flammable, with both the foliage and the wood used to fuel domestic fires in this region,
• particularly abundant in one of the ravines on this property, where it grew in dense stands, up to 1.5 m high, and
• controlled by artificial cutting, in lieu of any natural agent of consumption.

After my overnight stay, I noticed that much ash remained after the wood of D. rhinocerotis was burned in the domestic hearth.

DISCUSSION

The abrupt incidence of Nama Karoo on the escarpment above an extensive plain of Succulent Karoo is possibly because succulents cannot tolerate frost.

The vegetation here differed from typical karoid vegetation, in containing

• more nitrogen-fixing plants than expected, and
• Dicerothamnus rhinocerotis and Muraltia spinosa, both associated with mesic rather than semi-arid climates.

I infer that, in the original ecosystem, it was Diceros bicornis bicornis (https://www.inaturalist.org/observations?taxon_id=147644) that controlled D. rhinocerotis, a plant unpalatable to all the current herbivores.

Given the absence of both this megaherbivore and wildfire, and the choice not to use anthropogenic fires to burn the stands, the local pastoralists had the practice of cutting the shrubs of D. rhinocerotis mechanically, to control this plant. This had the benefit of providing fuel for the domestic hearth in the cold winters, in an ecosystem otherwise devoid of firewood.

Although the exemption from herbivory today is plausibly explained by the extermination of D. b. bicornis, a puzzle remains in the case of the monocotyledonous dwarf shrub, Asparagus capensis, which poorly fits the profile of a food-plant for this rhino.

Posted on June 23, 2023 08:54 AM by milewski