An interpretation of Ndumo Game Reserve, South Africa, part 1
Ndumo Game Reserve, with its striking stands of the fever tree (Vachellia xanthophloea, https://en.wikipedia.org/wiki/Vachellia_xanthophloea and https://www.forestryimages.org/browse/detail.cfm?imgnum=1589795 and https://www.alamy.com/fever-trees-and-sycamores-at-ndumu-south-africa-image4157380.html and http://www.mynewoldself.com/2015/09/13/time-in-nature-can-make-you-happy/fever-trees-shokwe-pan-ndumo/ and https://www.alamy.com/stock-photo-fever-trees-acacia-xanthophloea-on-the-edge-of-a-pan-in-ndumo-game-10487099.html), may seem mysteriously remote, tucked away in a wild, subtropical corner of South Africa on the border with Mozambique (https://en.wikipedia.org/wiki/Ndumo_Game_Reserve and https://offthehamsterwheel.co.za/2021/02/09/ndumo-game-reserve/).
And given the reputation of this small (about 10,000 hectares) reserve as the one of the most desirable destinations in South Africa for bird-watching, one might assume that this is simply a particularly lush remnant of the Pleistocene ecosystem of darkest Africa, happily preserved for perpetuity.
But neither image is realistic.
Ndumo is neither particularly natural nor is it located particularly far north in South Africa.
Instead of being a pristine remnant, Ndumo Game Reserve is interesting for the ways in which its original ecosystem has been distorted by humans.
This location is at the southern limit of flora generally accepted to be subtropical species, such as the leadwood (Combretum imberbe, https://en.wikipedia.org/wiki/Combretum_imberbe#:~:text=Combretum%20imberbe%20(leadwood%2C%20Afrikaans%3A,canopy%20of%20grey%2Dgreen%20leaves.) and the jackal-berry (Diospyros mespiliformis, https://en.wikipedia.org/wiki/Diospyros_mespiliformis).
In fact, the Makatini Clay Thicket or ‘Mahemane Bush’ (https://www.researchgate.net/figure/9-Habitats-of-the-Ndumo-Game-Reserve-northern-KwaZulu-Natal-South-Africa-8-Sand_fig4_267557018 and http://kenborland.com/tag/gorgeous-bush-shrike/) - which occupies much of the reserve - shares most of its plant genera with the Eastern Cape of South Africa, which lies in the temperate zone far to the south.
Albany thicket (https://en.wikipedia.org/wiki/Albany_thickets) or 'Addo Bush' in the Eastern Cape has among the densest natural populations of the African bush elephant (Loxodonta africana) and the hook-lipped rhinoceros (Diceros bicornis) known anywhere. This is contrary to expectations because it has relatively low, non-savanna vegetation with abundant succulents.
In this new interpretation of Ndumo Game Reserve - based on a visit in 2013 - my aim is not to spoil any nature-lover’s sense of escape to unadulterated wilderness, but to add a deeper level of interest for the scientifically curious. For this turns out to been the location of inadvertent experiments in ecological manipulation, in which important animals and plants have had their roles shifted on the ecological chessboard – and sometimes shifted off it altogether.
Ndumo Game Reserve provided natural habitat for the African bush elephant in much the same way as did the 'Addo Bush' of the Eastern Cape does (https://en.wikipedia.org/wiki/Addo_Elephant_National_Park). However, this megaherbivore was exterminated in the reserve as early as 1890. It continues to survive in the nearby – but significantly larger – Tembe Elephant Park (https://en.wikipedia.org/wiki/Tembe_Elephant_Park) to the east.
The elimination of the largest-bodied land mammal on Earth has disrupted the vegetation and faunal communities in unexamined ways. Although the extermination of the African bush elephant is unsurprising in such a small reserve, that of other species - for which this location should remain a paradise - is hard to understand.
Which other species are absent/scarce in Ndumo Game Reserve despite being indigenous?
The leopard (Panthera pardus, https://en.wikipedia.org/wiki/Leopard) is scarce here today, despite surviving southeast of Ndumo (https://www.inaturalist.org/observations/8747995).
The common eland (Taurotragus oryx), lion (Panthera leo), African wild dog (Lycaon pictus), chacma baboon (Papio ursinus), and yellow-billed oxpecker (Buphagus africanus, https://www.inaturalist.org/taxa/116831-Buphagus-africanus) have been locally exterminated.
The southern bushbuck (Tragelaphus sylvaticus) and Natal francolin (Pternistis natalensis, https://www.inaturalist.org/taxa/495975-Pternistis-natalensis) are scarce despite the habitat seeming suitable.
What seems obvious with the acknowledgement of these vacancies is the superabundance of species that seem to have compensated for absent species or been released from limitation in Ndumo Game Reserve.
Instead of the largest tragelaphin, the common eland, prevailing here, the nyala (Nyala angasii, https://www.inaturalist.org/taxa/42342-Tragelaphus-angasii), a medium-sized relative, is now plentiful.
The population of the nyala has expanded in its natural habitat, spreading into savannas that it probably did not inhabit a century ago. And in its superabundance the nyala seems in turn to have usurped the niche of the southern bushbuck - as has happened in other conservation areas in Zululand where the glamorous nyala has been favoured at the expense of the smallest tragelaphin.
While an unusual prominence of the nyala should be obvious to any naturalist visiting the reserve, the superabundance of the southern bushpig (Potamochoerus larvatus koiropotamus, https://www.inaturalist.org/taxa/42138-Potamochoerus-larvatus) – a far more secretive species than the nyala – may go unnoticed.
This checkerboard of vacancies and substitutions makes for an interesting challenge of discerning biological cause and effect. Do we understand ecosystems well enough to make sense of the interactions among the species in Ndumo Game Reserve?
Odd elements of the vegetation drop clues. A prime example is the many-stemmed false-thorn (Albizia petersiana subsp. evansii, https://www.inaturalist.org/taxa/342703-Albizia-petersiana and https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:473353-1 and https://plants.jstor.org/compilation/albizia.petersiana), which is both strange in its life-form and remarkably common in Ndumo Game Reserve. This is a ‘signature plant’ of this reserve, even though the distribution of the species ranges as far north as Kenya.
The many-stemmed false-thorn is an unusual (sub)species in two main ways:
- it differs from other false-thorns (Albizia spp., https://en.wikipedia.org/wiki/Albizia) in combining a multi-stemmed growth-form with both deciduousness and a lack of spines, and
- it dominates patches of low forest in Ndumo Game Reserve despite competing with more conventional nitrogen-fixing members of the same family (Mimosaceae), such as horned thorn (Vachellia grandicornuta, https://www.inaturalist.org/taxa/595949-Vachellia-grandicornuta and https://www.inaturalist.org/observations/116100934 and http://www.acacia-world.net/index.php/africa-me/south-africa/vachellia-grandicornuta/ and http://redlist.sanbi.org/species.php?species=15470-9).
My explanation for the local success of the many-stemmed false-thorn is linked to the absence of the African bush elephant. I suspect that this plant was originally limited by this megaherbivore, to which its growth-form seems specifically adapted as a defence.
The African bush elephant is known to eat Albizia spp. It tends to break down tall shrubs and low trees, which defend themselves in various ways such as the spines on the trunk of the knobthorn (Senegalia nigrescens, https://www.inaturalist.org/taxa/594427-Senegalia-nigrescens).
With this destructive pressure released by the local extermination of the megaherbivore, the many-stemmed false-thorn increased to patchy dominance in Ndumo Game Reserve. Indeed, it should now replace the fever tree in the mind of the educated visitor as the symbolic plant of this location. This seems ironic because
- Albizia is a cosmopolitan rather than an African genus, and
- the growth-form of this subspecies (https://www.inaturalist.org/observations/71153621) resembles that of mallee eucalypts (https://en.wikipedia.org/wiki/Mallee_(habit)) in Australia, rather than the typical flat-topped leguminous trees of Africa.
Indeed, even the most comprehensive book on the vegetation of South Africa (Mucina L and Rutherford M C 2006 The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19: 540-567. South African National Biodiversity Institute, Pretoria, South Africa, https://www.researchgate.net/publication/287992846_The_vegetation_of_South_Africa_Lesotho_and_Swaziland) misses a point about Ndumo Game Reserve.
It is true that the local ‘Makatini Clay Thicket’ is dominated by tall shrubs and short trees (e.g. Pappea, Schotia, Sideroxylon, Euclea, Diospyros, Carissa, Euphorbia, Azima, Ehretia) akin to that other 'megaherbivore thicket' much farther south, the 'Addo Bush' of the Eastern Cape. However, much of surface in Ndumo Game Reserve is actually sand rather than clay, and the many-stemmed false-thorn – which is associated with sand more than clay and belongs to a tropical genus – is incongruous relative to the typical plants of Makatini Clay Thicket. Patches of vegetation dominated by the many-stemmed false-thorn are poorly described by either ‘clay’ or ‘thicket’, because this species forms a low forest, open at ground level, and grows typically on sandy surfaces.
Another plant apparently released from herbivory by the absence of the African Bush elephant is the elephant's pudding (Cissus rotundifolia, https://www.inaturalist.org/observations/116957085 and https://www.inaturalist.org/observations/71153605 and https://www.inaturalist.org/observations/9293441 and https://www.inaturalist.org/observations/108359608 and https://www.inaturalist.org/observations/71153496).
This plant is noteworthy for two reasons:
- it has large succulent leaves, and is possibly the only example worldwide of a high-growing liane with leaves of this texture, and
- its current superabundance in Ndumo links the vegetation with the Addo Bush, in which the common succulent food of the African bush elephant is elephant bush (Portulacaria afra, https://www.inaturalist.org/taxa/326086-Portulacaria-afra). This plant is relatively scarce in Ndumo Game Reserve.
...to be continued in https://www.inaturalist.org/journal/milewski/66634-an-interpretation-of-ndumo-game-reserve-south-africa-part-2#
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