Tuatara: Volume 20, Issue 3, November 1973
The Floras of the South-Western Part of South Africa
The Floras of the South-Western Part of South Africa
A New Zealand point of view, and a suggested explanation of the high speciation
by G. B. Cone
The Extreme South-West of the Cape Province of South Africa has three different types of vegetation, each so distinct from the other that it constitutes an individual flora. The best known is the Cape Flora with its wealth of beautiful flowers and its exceptional number of distinctive species, genera and even families in a small area. The vegetation which it forms is a sclerophyll scrub (called ‘bush’ by South Africans) which covers the Cape Peninsula and a strip of the south-western mountains. (Adamson et al, 1929 and 1950) (Hutchinson, 1946).
The Karoo Flora which occupies the dry inland plateau of the Little Karoo forms a semi-desert vegetation of great interest though it lacks the distinctive families and genera which distinguish the Cape Flora from all other African Floras. Like the Cape Flora it is noted for large numbers of species; many are low-growing or shrubby succulents belonging to Mesembryanthemum and allied genera, and geophytes are abundant. Plants are often inconspicuous on the gravelly ground, the extreme example being the stone-mimicking species of Mesembryanthemum agg. Unpalatability, toxicity, and spinescence also protect the sparse vegetation from overgrazing by stock or game. On a hilltop near Worcester, amongst a scrub of Aloe microstigma and many other species, Dodonea thunbergiana without flower or fruit occurred, and would have passed for D. viscosa. The boundary, often a sharp line, between Karoo and Cape Floras is controlled by rainfall, the critical limit being 250mm., above which Cape Flora develops (Levyns, 1954).
The Forest Flora which survives on a small strip of range summit close to Knysna on the south coast where the annual rainfall exceeds 600–700mm. and in scattered gullies (‘kloofs’) throughout the region of the Cape Flora and beyond it, is considered to be the oldest vegetation in the south. Two features thought to indicate age are the lack of species dominance and the presence of many isolated outliers (Levyns, 1964). Species in the Forest Flora are not particularly numerous but are quite unrelated to those of either the Cape page 161 Flora or the Karoo Flora. The largest trees are Podocarpuss spp., Olea spp. are present, a common small tree is Cunonia capensis, very similar in appearance to the New Zealand Weinmannia racemosa (Cunoniaceae); tree-ferns, lianes, epiphytes, and a hygrophytic undergrowth make a general appearance and composition exceedingly like that of our podocarp-dicotylous forest, an apparently closed stable vegetation suited to a wet climate, though the South African forest is generally more sclerophyllous than our own. In an isolated kloof near Fransch Hoek Pass a small tree of Metrosideros angustifolius was seen to be very like a specimen of M. umbellata. In southern South Africa this forest vegetation appears to be lingering on at the minimal limit of rainfall.
On the Cape Peninsula within 200 square miles 2542 species of flowering plants of the distinctive Cape Flora are found, one-third of them monocots (Levyns, 1966). (Compare the totals for New Zealand and its outlying islands: within 104,250 square miles 1700 species of flowering plants are found, one-fifth of them monocots.) Families and genera rare in other African floras are typical: Restionaceae, Proteaceae, Penaeaceae, Bruniaceae, Geissolomaceae, Stilbaceae, Retziaceae, Grubbiaceae, and the tribe Ericoideae of Ericaceae; in addition the high proportion of Iridaceae, Orchidaceae, Ficoideae, and Geraniaceae, add to the unmistakable character of the Cape Flora. The shrub element of the vegetation includes many composites, many species of Protea, Leucadendron, Erica and other ericoid shrubs in diverse families, but with a lack of any species dominance or of ‘sociable’ plants. A few genera have many species, e.g. Erica 102, Mesembryanthemum agg. 61, Restio 32, Aspalanthus 51, Disa agg. 43, Senecio 44, Ficinia 33, Oxalis 33, Pelargonium 28, Crassula 30, Helichrysum 28, Gladiolus 22. The sparse ground cover of herbs and low shrubs includes many geophytes but there are few annuals. A strong link with Australia, especially West Australia, first noted by Hooker (1853–55) (1853) (1859) is shown at the level of families and orders but there are few genera common to both countries and no species. The relationship appears to be an old one with the characteristic and peculiar families of the two regions in parallel.
A reason for the diversity of the Cape Flora has been sought by some in the frequent burning which is known to have occurred over a long period, yet, although opportunity for many seedlings arises after fire, it has been concluded that burning would tend to destroy some species, especially annuals ad hard-wooded plants like those page 162 Ericas which do not sprout again from the caudex. It is reported that on mountain sides repeatedly fired plants with deep-seated rhizomes or bulbs prevail. Bolus and Wolley-Dod (1903–4) discuss this subject and conclude that burning appears to be more harmful to the Cape Flora than beneficial. These authors point out in connection with the problem of speciation that Hong Kong, an island near a continent, a similar geographical situation to Cape Peninsula, also has a very large number of species in a small area.
Levyns (1964), a lifelong student of the South African plants, suggests that the particular climate which covers both the Karoo and the Cape districts, i.e. a wet winter and a long, hot, dry summer, is the cause of the high speciation in both regions. Other countries with such a Mediterranean climate are also rich in species. Possibly the high insolation of the long, hot, dry summer encourages mutations but the plants of the Forest Flora, exposed to similar conditions, have not reacted in the same way.
The ‘bush’ vegetation of the Cape Flora is not a luxuriant one. The individual shrubs, often somewhat stunted, stand in stony ground among scattered low-growing flowering herbs and geophytes. Bare soil is always present, as it is also in the semi-desert Karoo, in contrast to conditions under the closed cover of the Forest Flora. The ‘bush’ or scrub appears in some places, e.g. in the Marloth Nature Reserve, as a sere in the development of the forest which today is but a tiny relict, the ‘scrub’ having become the main vegetation of the coastal parts of the south-west.
The same climatic change of reduced rainfall which led to the retreat of the forest must have acted also on the scrub, and possibly the change which has resulted in forest yielding territory to ‘bush’ has also led to ‘bush’ in other places being replaced by Karoo vegetation. Several members of the Cape Flora commence vegetative growth at the end of the summer, a growth rhythm ill-adapted to the present dry summer, and which suggests that the ancestors of these plants evolved in some place with a summer rainfall (Levyns, 1964). Many of the beautiful plants of the Cape Flora are cultivated abroad, especially under higher rainfall in New Zealand where they thrive far beyond the stature they reach in South Africa, suggesting that wetter conditions are what they are really suited to.
The present-day ‘bush’ vegetation of the Cape Flora appears to be out of step with its climate which has changed in recent millenia, even recent centuries, to a drier state. While the Forest Flora as a complete community retreated with little change into the restricted regions possible for it, the scrub vegetation of the Cape Flora appears to have spread out over formerly forested areas which today, though they allow it to survive, are not ideal for it. In the absence of any other vegetation able to invade this territory, the Cape Flora has remained established. Climate is known to have fluctuated since the last ice age (Lamb, 1969) and has left its mark, e.g. in comparatively page 163 recent changes in forest distribution in New Zealand where the podocarp-dicotylous forest suited to a wet climate has been replaced in some areas by Nothofagus forest which is adapted to drier conditions (Holloway, 1954). Other climatic changes with a longer time scale appear to have resulted in some New Zealand plants having distinct juvenile forms (Cockayne, 1899).
Comparable climatic changes in South Africa, it is suggested, have led to the Cape Flora becoming out of harmony with its environment which it is no longer able to fully exploit. Under these conditions, with much open unoccupied ground always available, and with plants which are relatively short-lived, many more variant seedlings may survive to maturity than would grow in a closed community of long-lived species such as comprise the Forest Flora. The special type of climate may allow many seedlings to grow in the rainy season but only a few to survive through the summer. Increasing variation of the vegetation might be expected to continue under these conditions till some new types arise better suited to the new aridity. The new species could then replace the old to form a more vigorous community with dominants capable of making a full exploitation of the environment and a close ground cover. In the Cape Flora this has not occurred. Many new species all so similar to the parents have arisen that they continue in the same type of open community to which they have given exceptional diversity.
However, in Australia where arid conditions have prevailed for much longer, species have evolved much better adapted to the dearth of water in the present Cape region than any of the indigenes, e.g. Acacia cyanophylla Lindl., Port Jackson willow, two other Acacia spp., Hakea acicularis and two other Hakea spp. (Levyns, 1966). Where these plants have been introduced by man they rapidly invade the natural Cape Flora and oust it entirely, making an abrupt change from an open community of great variety and interest with a large number of species to a new closed community with a single or very few dominants. Leptospermum laevagatum is another Australian species which behaves as an invasive weed in the ‘bush’ while one or two species of Pinus grow as volunteers in the Cape Flora and are seen to be spreading across the summit of Table Mountain. Only vigorous preventive action by interested people can save the wonderful Cape Flora from extinction by the invaders.
Although this situation has been brought about by the deliberate action of man, it might have happened over a longer period of time without human intervention. Had the geographic positions of the two countries been reversed the strong prevailing westerlies could have carried seed of these unwelcome colonists with birds* or ocean drift page 164 before now, and the wonderful Cape Flora with all its flowers would have disappeared naturally after its fruitless burst of speciation.
It may be suggested in general terms that whenever a community, following a fairly rapid climatic change to less favourable conditions, becomes out of harmony with its environment, it will fail to fully exploit its territory. Increased opportunity may then be given for juveniles differing from the parents, thus allowing a burst of speciation to continue till some new species better adapted to the new conditions arrive on the scene. Although these might arise within the old community, in the event of a rapid climatic change where a considerable alteration in the character of the community is necessary for renewed success, they are more likely to come as invaders from without. If a suitable alternative type of community is present in the same region, the invasion or replacement could occur more or less contemporaneonsly with the climatic change, but if no suitable indigenous stock exists speciation may continue for a long time till one arrives. Only then would the old multi-speciate community disappear suddenly with the emergence of a completely new cover dominated by the new invaders.
Adamson, R. S., et al, 1929: The Botanical Features of the South Western Cape Province. Specialty Press of South Africa, Cape Town.
——, and Salter, T. M., 1950: Flora of the Cape Peninsula, Juta and Co., Cape Town and Johannesburg.
Bolus, H., and Major Wolley-Dod, 1903–4: A List of the Flowering Plants and Ferns of the Cape Peninsula. Trans. S.A. Phil. Soc. 14: 208–373.
Cockayne, L., 1899: An Inquiry into the Seedling Forms of New Zealand Phanerogams, and their Development. Trans. N.Z. Inst. 31: 419–424.
Holloway, J. T., 1954: Forests and Climates in the South Island of New Zealand. Trans. Roy. Soc. N.Z. 82: 329–410.
Hooker, J. D., 1853–55: Flora Novae-Zelandiae. London.
——, 1853: Introductory Essay to the Flora of New Zealand. London.
——, 1859: Introductory Essay to the Flora of Tasmania. London.
Hutchinson, John, 1946: A Botanist in Southern Africa. Gawthorn, London.
Lamb, H. H., 1969: Climatic Fluctuations. World Survey of Climatology 2: 173–242. Elsevier, Amsterdam, London and New York.
Levyns, M. R., 1964: Migrations and Origin of the Cape Flora. Trans. Roy. Soc S.A. 37: 85–107.
——, 1966: A Guide to the Flora of the Cape Peninsula. Juta, Cape Town.
* It is not suggested that such seed dispersal is due to any unnatural constipation in the birds but the feathers of even such small species as the white-eye which is common in South Africa as well as in Australia could easily carry dust containing small seeds like those of Hakea, Leptospermum spp., and others.