Lessons on succession and megaherbivory in African savannas, from Malelane Mountain Bushveld as viewed in 2012

In June 2012, I spent about 6 days hiking in the Malelane Mountain Bushveld (http://www.thekruger.com/gertenbach/gertenbach2.htm) in Kruger National Park. I studied the fauna carefully.

I write this Post in the present tense, but it refers to a time when the populations of megaherbivores were at their historical peak. I felt as if I had been transported back to the Pleistocene, revealing the full expression of the relationship between large animals and the vegetation. Since then, the square-lipped rhino has been depleted by illegal hunting.

The Malelane Mountain Bushveld is broken topography ('koppie country') on granite/gneiss, with savanna generally dominated by Combretaceae. Acacias are strictly interstitial, and Vachellia tortilis (https://www.inaturalist.org/taxa/489563-Vachellia-tortilis) is diagnostically absent.

The grass is generally 'sour', and burnt deliberately every couple of years. In the drainage lines are belts of woodland of Spirostachys africana (https://www.inaturalist.org/taxa/340253-Spirostachys-africana).

Various species of herbivores are, in June 2012, common in the Malelane Mountain Bushveld, and others are scarcer than expected. The general pattern is that the currently common species are typically found in dense populations attracting intense predation, whereas the currently scarce species are poorly adapted to cope with the collateral predation.

The currently common herbivores in the Malelane Mountain Bushveld are as follows, in order of decreasing abundance relative to expectations: impala, buffalo, square-lipped rhino, kudu, common duiker, steenbok, elephant, scrub-hare, giraffe, zebra, hook-lipped rhino, mountain tortoise, blue wildebeest. Klipspringer (in its stereotyped habitat) is also present in what appear to be 'full' populations.

Anybody with experience in African savannas would hardly expect greater populations of these species, given the topography, water availability, and vegetation of the Malelane Mountain Bushveld.

The currently scarce herbivores in the Malelane Mountain Bushveld are as follows, in order of decreasing scarcity relative to expectations:

• rock hyrax (https://www.inaturalist.org/taxa/43086-Procavia-capensis),
• oribi (https://www.inaturalist.org/taxa/42380-Ourebia-ourebi),
• bushpig (https://www.inaturalist.org/taxa/42138-Potamochoerus-larvatus),
• mountain reedbuck (https://www.inaturalist.org/taxa/42306-Redunca-fulvorufula),
• common eland (https://www.inaturalist.org/taxa/75192-Tragelaphus-oryx),
• sable antelope (https://www.inaturalist.org/taxa/42334-Hippotragus-niger),
• tsessebe (https://www.inaturalist.org/taxa/42276-Damaliscus-lunatus),
• roan antelope (https://www.inaturalist.org/taxa/42332-Hippotragus-equinus),
• Lichtenstein's hartebeest (Alcelaphus lichtensteinii),
• chacma baboon (https://www.inaturalist.org/taxa/57556-Papio-ursinus),
• southern reedbuck (https://www.inaturalist.org/taxa/42305-Redunca-arundinum),
• common warthog (https://www.inaturalist.org/taxa/42122-Phacochoerus-africanus),
• bushbuck (Tragelaphus sylvaticus), and
• waterbuck (Kobus ellipsiprymnus).

Another species that might possibly be added to this list is Pronolagus saundersiae (https://www.inaturalist.org/taxa/74957-Pronolagus-saundersiae).

The hippopotamus (https://www.inaturalist.org/taxa/42149-Hippopotamus-amphibius) can be discounted since there is naturally insufficient perennial water in the Malelane Mountain Bushveld for this species.

Anybody with an appreciation of how widespread in Africa, or favoured by rocky slopes, the above species are should wonder what it is that sets the current limit to their occurrence in the Malelane Mountain Bushveld. Their scarcity/absence seems anomalous.

To elaborate in the case of the currently common herbivores:

• the impala is not generally associated with 'sourveld', and yet it is the most abundant ungulate in the Malelane Mountain Bushveld;
• the square-lipped rhino is not generally associated with rocky substrates, and yet it extends from plains on to such substrates in the southern part of Kruger National Park, being the second most-frequently spotted ungulate in the Malelane Mountain Bushveld in my experience.

The African bush elephant occurs right to the tops of the boulder outcrops, foraging side by side with the klipspringer. The effect of the proboscidean are, indeed, so great that one common species of tree, Pterocarpus rotundifolius, is completely suppressed to clonal coppice lower than 1.5m high, throughout the Malelane Mountain Bushveld.

To elaborate in the case of the currently rare herbivores:
Rock hyrax, mountain reedbuck, and chacma baboon seem ideally suited to the topography in the Malelane Mountain Bushveld. Yet, they are extirpated, reduced to a few individuals, and restricted to a few troops, respectively.

Sable antelope and common eland are both tolerant of rocky slopes and 'sourveld', yet are only occasionally sighted in the Malelane Mountain Bushveld. It is unknown which species, in the case of sable/roan and tsessebe/Lichtenstein's hartebeest, is more suited to the Malelane Mountain Bushveld. However, it seems certain that one species of each pair would potentially find the Malelane Mountain Bushveld well within its range of potential habitats.

The current abundance of the square-lipped rhino and the African savanna buffalo, both mainly grazers, is likely to have reduced the food base available to smaller grazers such as roan, sable, tsessebe, Lichtenstein's hartebeest, and waterbuck. In the case of the southern reedbuck, the necessary cover of tall grass is also reduced. The current abundance of giraffe and kudu is likely to have reduced the food base available to other browsers, such as common eland and bushbuck.

The currently abundant African bush elephant is likely to compete with the chacma baboon, even though it may facilitate other grazer/browsers such as the impala.

The current abundance of the impala has possibly resulted in competitive exclusion of the mountain reedbuck.

A pattern emerging here is that, other things being equal, the abundance of a larger species seems to result in the scarcity of a smaller species in the same guild (grazer, browser, or herbivorous omnivore).

Besides competition for food, there is a potential effect of collateral predation.

The currently abundant species, particularly the impala, have probably supported an increased population of predators, including lion, leopard, spotted hyena, and hunting dog.

This relative abundance of predators is possibly prohibitive for slow-reproducing species such as the rock hyrax, or relatively slow-moving species such as the mountain reedbuck. Even elephant and rhinos, which are essentially beyond predation, can boost the population of the spotted hyena through carcases being scavenged.

The African bush elephant is (as at June 2012) so common in the Malelane Mountain Bushveld that - as for the square-lipped rhino - it is difficult to imagine it more abundant here. I noticed the following examples of how this megaherbivore reduces the height of the tallest plants, suggesting that the vegetation might be taller in the absence of elephants.

A dominant tree in the Malelane Mountain Bushveld is Combretum apiculatum. Although this species is abundant in Kruger National Park, it has two noteworthy features.

Firstly, it has extremely dense wood, being one of the few species of tree in Kruger National Park that possess wood which, when dry, sinks in water.

Secondly, many or most of the individual trees of this species, at least in the Malelane Mountain Bushveld, have been pushed over by elephants. They have survived this, but have 'grown upwards from their sides'. This has created a distinct (induced) life form of a tree with a horizontal trunk just above ground level, giving rise to vertical branches that reach about 5 m.

Although the density of the populations of trees of C. apiculatum may not have been reduced by the proboscidean, it seems likely that the maximum or average height of the trees has declined, simply because the trees generally grow not upright, but on their sides instead.

An interstitial tree in the Malelane Mountain Bushveld is Senegalia nigrescens (https://www.inaturalist.org/taxa/594427-Senegalia-nigrescens).

This species exists here in two forms.

The first is 'saplings' <4m high, which have dense crowns obviously hammered by elephant as well as being browsed by giraffe. The branchwork has been mangled with a hedged result in the crown of each individual 'sapling'. It seems obvious that these 'saplings' are being held at a height within the browsing reach of the megaherbivores, perhaps for decades, before finally managing to break out of this constraint and to grow into mature, reproducing trees.

Testimony to this is the scattered incidence of tall trees of S. nigrescens in the Malelane Mountain Bushveld. However, even these mature individuals continue to be hammered by elephants, this time by way of debarking. Most of the mature individuals of S. nigrescens here are partially debarked, and some are ring-barked and doomed. Thus elephants reduce the height of the vegetation w.r.t. S. nigrescens both by suppressing most individuals to an extended sapling stage and by killing mature individuals and thus gradually thinning them out.

An abundant species in Malelane Mountain Bushveld is Pterocarpus rotundifolius (https://www.inaturalist.org/taxa/416647-Pterocarpus-rotundifolius).

This is potentially a valuable timber tree, but not one tree of this species was seen in the Malelane Mountain Bushveld. Instead, all individuals are suppressed chronically to patches of unbranched stems emerging separately (unbranched) from the ground, presumably representing an underground clonal system of rhizome-like stems. This is a tree species held as a suffrutex.

Each of these repeatedly regenerating stems, which is about 1 cm thick and up to 1.5 m high, is predictably killed each year by elephants, which pull out the whole stem complete with its spray of leaves, work the stem through the mouth to remove the bark (the cambium portion of which is eaten), and discarding the dead, peeled stem complete with its terminal spray of leaves.

The African bush elephant, presumably in combination with fire, thus presumably reduces the entire, abundant population of P. rotundifolius in any given area of the Malelane Mountain Bushveld to a height of zero every year or every second year. The species is common enough in the Malelane Mountain Bushveld that, if every one of these clonal patches were allowed to mature into a tree, the resulting canopy cover would be considerable.

Instead, although the African bush elephant does not exterminate P. rotundifolius, it holds it semi-permanently in suppression to the degree that the plant hardly even qualifies as a shrub, let alone a tree.

A final species apparently suppressed height-wise by the African bush elephant in the Malelane Mountain Bushveld is Philenoptera violacea (ex Lonchocarpus capassa, https://www.inaturalist.org/taxa/340211-Philenoptera-violacea). This species is abundant (although subordinate to the dominant Combretaceae) in the Malelane Mountain Bushveld. However, it seldom attains a mature form.

Instead, most individuals have been grossly broken at a juvenile stage by elephants so that the remaining woody stem (trunk) is only about 2-4 m high, with a mangled condition in which it continues to send out branches but does not resemble a mature tree. The outright snapping of the trunks of P. violacea (which has light, pine-like wood), as opposed to the only partial snapping of the trunks of C. apiculatum (which has dense wood), means that one seldom sees trees of P. violacea that have been felled but continue to grow by means of side-branches growing vertically from a horizontal trunk.

Nonetheless, it is clear that, were all individuals of P. violacea in the Malelane Mountain Bushveld allowed to grow to their potential height, the overall height of the vegetation in this ecosystem would be increased.

The overall result of suppression of the height of the vegetation by the African bush elephant in the Malelane Mountain Bushveld is that

• the dominant Combretum apiculatum, potentially a tree of perhaps 10 m, is reduced to say 5 m, by the conversion of vertical trunks to horizontal trunks;
• Senegalia nigrescens, potentially a tree of 15 m, is usually reduced to an upright but 'miniature' tree of <4m;
• Pterocarpus rotundifolius, potentially a tree of 10 m, is suppressed to a height of only <2m, with complete loss of the arborescent growth form; and
• Philenoptera violacea, potentially a tree of 10 m, is truncated to a third of this height.

What remains in the presence of abundant megaherbivores is still a savanna. However, this seems shorter than it might be in the absence of the proboscidean. The trees reach 10 m high near drainage lines, owing to the fact that Spirostachys africana, the dominant species on alluvia in the Malelane Mountain Bushveld, is hardly touched by the African bush elephant.

Posted on July 30, 2022 11:23 PM by milewski