What was the original relationship between megaherbivores and bambis in south-central African savannas, in terms of biomass?

It seems reasonable to expect that small-bodied organisms tend to be more numerous than large-bodied organisms.

Faunas and communities of hoofed mammals can include species of a remarkably wide range of body sizes, from bambis to megaherbivores.

At the collective level of biomass (https://en.wikipedia.org/wiki/Biomass_(ecology)), which weigh more in a given area under natural conditions: the relatively scarce, large-bodied hoofed mammals or the relatively numerous, small-bodied ones?

One might expect the small-bodied but numerous species collectively to outweigh the large-bodied, scarce species.

However, in African savannas this seems not to be the case among ungulates, under prehistoric conditions. It is the large-bodied species that seem naturally to have the overall competitive advantage. The trouble with large body-size is that it brings vulnerability to extermination by humans.

We have information on something approaching an original situation in what is now Zambia (https://en.wikipedia.org/wiki/Zambia). A survey of the fauna of hoofed mammals (http://www.rhinoresourcecenter.com/pdf_files/124/1245160404.pdf) was performed in 1931-1933 by the accomplished zoologist C R S Pitman (https://en.wikipedia.org/wiki/Charles_Pitman_(game_warden)).

This information has the disadvantage that it was just an educated guess, rather than a rigorous survey by modern standards. Furthermore, Pitman made no attempt to estimate populations for the abundant suids, Phacochoerus africanus and Potamochoerus larvatus.

However, the same information has the advantage that it was acquired before European influence much distorted the faunal communities by hunting and confinement to conservation reserves. For example, Pitman notes a rapid increase of the population of Syncerus caffer at the time as unwelcome; and Diceros bicornis was later completely exterminated from Zambia.

We can assess the ecological importance of body sizes by multiplying the population tally by the average body mass (of females, which are usually the more numerous and metabolically active sex).

Here are the estimates of Pitman (1933) for what is now Zambia, excluding Barotseland. Body mass refers to adult females. Suids are omitted. Species are listed in decreasing order of populations.

Kobus leche kafuensis 75 kg 250,000 and Kobus leche smithemani 150,000 https://en.wikipedia.org/wiki/Lechwe
Sylvicapra grimmia splendidula 15 kg more than 60,000 https://en.wikipedia.org/wiki/Common_duiker
Syncerus caffer 500 kg more than 60,000 https://en.wikipedia.org/wiki/African_buffalo
Hippotragus equinus 255 kg 60,000 https://en.wikipedia.org/wiki/Roan_antelope
Alcelaphus lichtensteini 135 kg 60,000 https://en.wikipedia.org/wiki/Lichtenstein%27s_hartebeest
Aepyceros melampus 45 kg 50,500 https://en.wikipedia.org/wiki/Impala
Taurotragus oryx livingstonei 400 kg 30,000 https://en.wikipedia.org/wiki/Common_eland
Connochaetes taurinus mattosi 200 kg 30,000 and Connochaetes taurinus cooksoni 2,000 https://en.wikipedia.org/wiki/Blue_wildebeest
Equus quagga boehmi and crawshayi 300 kg 30,000 https://en.wikipedia.org/wiki/Plains_zebra
Raphicerus sharpei 10 kg 25,000 https://en.wikipedia.org/wiki/Sharpe%27s_grysbok
Tragelaphus scriptus and sylvaticus 30 kg 25,000 https://en.wikipedia.org/wiki/Cape_bushbuck
Kobus vardoni 70 kg 20,000 https://en.wikipedia.org/wiki/Puku
Kobus defassa 15,000 and Kobus ellipsiprymnus 180 kg 12,000 https://en.wikipedia.org/wiki/Waterbuck
Redunca arundinum 48 kg 14,000 https://en.wikipedia.org/wiki/Southern_reedbuck
Ourebia ourebi 15 kg 12,000 https://en.wikipedia.org/wiki/Oribi
Loxodonta africana 3000 kg 12,000 https://en.wikipedia.org/wiki/African_bush_elephant
Strepsiceros strepsiceros 170 kg 10,000 https://en.wikipedia.org/wiki/Greater_kudu
Hippotragus niger 220 kg 10,000 https://en.wikipedia.org/wiki/Sable_antelope
Oreotragus oreotragus 10 kg 8,000 https://en.wikipedia.org/wiki/Klipspringer
Tragelaphus spekii 40 kg 8,000 https://en.wikipedia.org/wiki/Sitatunga
Damaliscus lunatus superstes 120 kg 6,000 https://en.wikipedia.org/wiki/Common_tsessebe
Hippopotamus amphibius 1500 kg 3,000 https://en.wikipedia.org/wiki/Hippopotamus
Philantomba monticola 5 kg perhaps 3,000 https://en.wikipedia.org/wiki/Blue_duiker
Diceros bicornis 950 kg 1,500 https://en.wikipedia.org/wiki/Black_rhinoceros
Cephalophus sylvicultor 70 kg approximately 1,500 https://en.wikipedia.org/wiki/Yellow-backed_duiker
Giraffa tippleskirchi thornicrofti 850 kg 300 kg https://en.wikipedia.org/wiki/Giraffe

I have multiplied population tally by body mass, to produce values for biomass. Species are listed in decreasing order of biomasses.

Loxodonta africana 36,000,000 kg
Kobus leche 30,000,000 kg
Syncerus caffer 30,000,000 kg
Hippotragus equinus 15,300,000 kg
Taurotragus oryx livingstonei 12,000,000 kg
Equus quagga 9,000,000 kg
Alcelaphus lichtensteini 8,100,000 kg
Connochaetes taurinus 6,400,000 kg
Kobus defassa/ellipsiprymnus 4,860,000 kg
Hippopotamus amphibius 4,500,000 kg
Aepyceros melampus 2,272,500 kg
Hippotragus niger 2,200,000 kg
Strepsiceros strepsiceros 1,700,000 kg
Diceros bicornis 1,425,000 kg
Kobus vardoni 1,400,000 kg
Sylvicapra grimmia 900,000 kg
Tragelaphus sylvaticus/scriptus 750,000 kg
Damaliscus lunatus superstes 720,000 kg
Redunca arundinum 672,000 kg
Tragelaphus spekii 320,000 kg
Giraffa tippelskirchi thornicrofti 255,000 kg
Raphicerus sharpei 250,000 kg
Ourebia ourebi 180,000 kg
Cephalophus sylvicultor 105,000 kg
Oreotragus oreotragus 80,000 kg
Philantomba monticola 15,000 kg

Here are the species-tallies categorised by body size, with biomasses expressed in tonnes:

  • Megaherbivores (more than 850 kg) 36,000 + 4,500 + 1425 + 255 = 42,180 tonnes
  • Largest-bodied ruminants (300-850 kg) 30,000 + 12,000 = 42,000 tonnes
  • Ungulates 75-300 kg 15,300, + 9,000 + 8,100 + 6,400 + 4,860 + 2,200 + 1,700 + 720 = 48,280 tonnes
  • Ruminants 17-75 kg 30,000 + 2,272.5 + 1,400 + 750 + 672 + 320 + 105 = 35,519.5 tonnes (which would possibly have approached 40, 000 tonnes had the two species of suids been included)
  • Bambis (ruminants less than 17 kg) 900 + 250 + 180 + 80 + 15 = 1,425 tonnes

My commentary:

Each of of the five categories of body mass other than bambis approximates 40,000 tonnes of biomass. Megaherbivores consisted mainly of the savanna elephant, the largest-bodied ruminants consisted mainly of the savanna buffalo, ungulates 75-300 kg consisted mainly of the roan antelope and the plains zebra, and ruminants 17-75 kg consisted mainly of lechwes.

By contrast, the category of bambis provided minor biomass. Even if Pitman undercounted bambis - which are diminutive enough to hide easily - by five-fold, their biomass would be only one-fifth of those for any of the other categories.

The savanna elephant and the bush duiker are both widespread in African savannas, with generalised diets; and both were originally widespread in what is now Zambia. However, the estimated biomass of the former was 36,000 tonnes, whereas that of the latter was only 900 tonnes.

This is a difference of 40-fold, suggesting that even if small-bodied species compensate greatly by increasing their populations after the extermination of large-bodied species, it is unlikely that they can emulate the latter ecologically.

Indeed, I would argue that the very existence of niches for bambis in the first place in African savannas is by virtue of the continual rejuvenation of the vegetation by megaherbivores. Once all megaherbivores are exterminated, I would expect bambis to disappear as well owing to their thermodynamic inefficiency. In reality, humans tend to emulate the megaherbivores they have exterminated, by breaking woody plants and disturbing the ground, thus ensuring that the smallest ungulates continue to survive, and possibly with population-densities exceeding those in the original, natural regime.

Publicado el abril 11, 2022 11:29 MAÑANA por milewski milewski


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