Why no gazelle at the southwestern tip of Africa?

Mediterranean-type climates, with dry summers and rainy winters, occur both in South Africa and along the Mediterranean coasts of North Africa and the Levant. And gazelles have been evolving and shifting their ranges across Africa and Asia for millions of years. However, a puzzling faunistic difference is that four species of gazelles (Gazella cuvieri, Gazella dorcas massaesyla, Gazella gazella gazella, and Eudorcas rufina) are indigenous to the northern area whereas no species of gazelle is indigenous to the southern area.

In South Africa there is one species of gazelle, namely the springbok (Antidorcas marsupialis, see https://www.inaturalist.org/observations/35692467). However, this has not naturally penetrated the mediterranean-type climate of Western Cape Province (the many observations in iNaturalist being of introduced populations). Furthermore, even within its distribution in South Africa the springbok avoids stony slopes, in contrast to Cuvier's gazelle in the Atlas ranges of Morocco-Algeria (see https://www.inaturalist.org/observations/39161994) and the mountain gazelle, which is named after its association with stony slopes in e.g. Israel (see https://www.inaturalist.org/observations/5305686).

Why are there these disparities? One partial explanation involves competing members of the ruminant fauna.

A crucial difference is that the grey rhebok (Pelea capreolus, see series of photos in https://animalia.bio/grey-rhebok), mountain reedbuck (Redunca fulvorufula), common eland (Taurotragus oryx, see http://shutterstock.puzzlepix.hu/kep/1614041191), steenbok (Raphicerus campestris, see https://www.zoochat.com/community/media/steenbok-raphicerus-campestris.488693/) and Cape grysbok (R. melanotis) of South Africa have no counterparts in North Africa or the Levant. These species were, until recently, common in the relevant environments near the southwestern tip of Africa. Among them, they arguably usurped the niche of gazelles.

The grey rhebok and mountain reedbuck prefer stony slopes, and the distribution of the former included most of the South African area of mediterranean-type climate. Both are similar in body size and partly similar in diet to the springbok. The common eland is extremely large (adult female about 500 kg) and the two species of Raphicerus smaller than gazelles, but all have diets which overlap those of gazelles enough for them to be potential competitors.

To 'rewild' the climatically similar stony slopes near Casablanca in the north and Cape Agulhas in the south of the same continent, we would reintroduce to the northern area only Cuvier's gazelle, but to the southern area at least three species: grey rhebok, common eland and steenbok/Cape grysbok. One would be naive to think that the same ecological function could be restored by simply 'reintroducing' the springbok in Agulhas National Park (see https://www.inaturalist.org/observations/3975537), even though it is by far the most similar antelope to Cuvier's gazelle that is available in the southern African fauna.

Publicado por milewski milewski, 04 de mayo de 2021

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Are there no fossil gazelles in the Agulhas megafauna? Wiped out by modern humans in the last 50,000 years?

Venter, J.A et al., Large mammals of the Palaeo-Agulhas Plain showed resilience to extreme climate change but vulnerability to modern human impacts, Quaternary Science Reviews, https://doi.org/10.1016/j.quascirev.2019.106050

For instance, were was Antidorcas australis found?

Publicado por tonyrebelo hace más de 1 año (Marca)

Hi tonyrebelo, thanks for your comment. Not only is it possible that Gazella once occurred on the Agulhas Plain, but Antidorcas is well-known formerly to have occurred in East Africa. Both genera, namely Gazella and Antidorcas, have at various times lived in both South and East Africa. This means that the Recent pattern of distribution, in which Antidorcas has completely replaced Gazella in southern Africa, is presumably owing to the adaptive superiority of Antidorcas to Recent environmental conditions in southern Africa. The particular puzzle is why Antidorcas, so abundant in semi-arid southern Africa, failed to occur (or once occurred but failed to survive in the last few centuries) on the mesic coastal forelands (the Swartland and probably also the Ruensveld) of the southwestern Cape. It is hard to explain any of this according to human predation because this has been a constant throughout subSaharan Africa throughout the waxings and wanings of both genera of gazelles (Gazella and Antidorcas) in southern Africa including the southwestern Cape, including the area of mediterranean-type climate in Western Cape province, and including the Agulhas Plain.

Publicado por milewski hace más de 1 año (Marca)

Could it not be a areal issue. WIth the demise of the huge Agulhas plains, the Med mammals were squeezed to critical population sizes that could not cope with hunting and sheep and cattle. The problem with mountain gazelles is the extremely low carrying capacity of the Cape Med mountains so mountain specialists probably did not occur, but plains specialists would have.
We have large buffalo, large hartebeest and large springbok, that succumbed to human predation and rising sea levels - problems that were not an issue in tropical Africa because of its huge areas and relatively extensive populations (the s Cape Med region is less than 1% of the area)..

Publicado por tonyrebelo hace más de 1 año (Marca)

I agree that there is nothing puzzling about the lack of gazelles in the sandstone/quartzite mountains of the southwestern Cape. I know of no similarly nutrient-poor substrate, anywhere in Africa or Eurasia, inhabited by any of the eight genera of gazelles. However, what remains puzzling is that Antidorcas did not include in its habitat the nutrient-rich stony slopes, freshly weathering from dolerite, in the Karoo. From a South African perspective it might seem that Antidorcas is morphologically unsuited to negotiating obstacles in the form of rocks; but it is hard to sustain this view once one sees the competence of Gazella cuvieri and Gazella gazella on rocky slopes. Turning to the main question you just posed: if I understand it correctly, what you are suggesting is that the relatively nutrient-rich coastal forelands in the southwestern Cape occupied such a small area, after sea levels rose, that Antidorcas could not survive human predation within this area. My answer is as follows. In ruminants, reproductive rates are generally correlated with body size, the smaller species reproducing more rapidly than the larger species. This helps to explain why, by about 1900, the only species of wild ruminants surviving in either the Karoo or the coastal forelands of the southwestern Cape were those of body mass less than 30 kg (a size-range which includes the local form of Antidorcas). Applying this principle to the coastal forelands before European arrival: we would predict that, if anthropogenic extermination were to occur owing to the restricted area available to ruminants, it would be the larger species which disappeared first. More particularly, we would predict that the eland (Taurotragus) and the buffalo (Syncerus) would have disappeared before Antidorcas disappeared. This prediction fails, because in reality the eland (500kg), the buffalo (500 kg), the hartebeest (150 kg), and the bontebok (70 kg) - but not the springbok (Antidorcas, 30 kg) - remained at the time when Europeans arrived at what is now Cape Town. Broadening the scope of ungulates, three species of megaherbivores remained common at this time near what is today Cape Town, despite their reproductive rates being far more limited again than those of eland and buffalo. How, then, would anthropogenic factors explain why the local vegetation type in question, namely the renosterveld, was named after the hook-lipped rhino, and not named 'bokkeveld' after the springbok? Surely a habitat too restricted for the springbok would also be too restricted for the hook-lipped rhino/elephant/hippo/eland/buffalo/hartebeest?

Publicado por milewski hace más de 1 año (Marca)

I was thinking of the bigger megaherbivores (above Buffalo=Rhino size). No large gazelles? Not even on Agulhas?

Publicado por tonyrebelo hace más de 1 año (Marca)

Bergmann's rule (https://en.wikipedia.org/wiki/Bergmann%27s_rule) would predict that gazelles at high latitudes would be relatively large-bodied. The largest species of gazelle, including extinct forms, has/had average body mass for adult females of about 50 kg, and was/were not associated with high latitudes. Gazelles survive today as far north as the Gobi Desert, but the body mass of Gazella subgutturosa there remains less than 35 kg in adult females. There is another reason to doubt that gazelles have ever been particularly large in southern Africa. The pattern with bontebok/blesbok (Damaliscus pygargus), black wildebeest (Connochaetes gnou) and bloubok (Hippotragus leucophaeus) is to the contrary, because these are all - despite Bergmann's rule - remarkably small compared to their congeners in the tropics/subtropics.

Publicado por milewski hace más de 1 año (Marca)

Please see http://www.scielo.org.za/pdf/sajs/v103n1-2/12.PDF, which indicates that Antidorcas recki, the extinct southern African ancestor of modern Antidorcas marsupialis, was the smaller-bodied of the two.

Publicado por milewski hace más de 1 año (Marca)

If you find the link http://www.scielo.org.za/pdf/sajs/v103n1-2/12.PDF defective, please Google Temporal variation in Plio-Pleistocene Antidorcas...South African Journal of Science 103, January/February 2007.

Publicado por milewski hace más de 1 año (Marca)

Please see https://www.researchgate.net/publication/283993201_The_feeding_niche_of_an_extinct_springbok_Antidorcas_bondi_Antelopini_Bovidae_and_its_paleoenvironmental_meaning and https://www.sciencedirect.com/science/article/abs/pii/S1040618217315896. These refer to Antidorcas bondi. The ancestral Antidorcas recki was replaced in the Pleistocene by both marsupialis and bondi, the latter being a relatively specialised grazer comparable to Eudorcas thomsoni. It is possible that bondi was the main species on the Agulhas Plain (although the fossil distribution shows no evidence of it so far south). And if it occurred here it is possible that it shared the Agulhas Plain with Syncerus antiquus, Megalotragus priscus (which were larger than any modern relatives), Damaliscus niro, Equus capensis (similar to modern grevyi) and Ceratotherium simum (the still-extant square-lipped rhino, which did formerly range to the southern tip of Africa). It is even possible that all besides Antidorcas bondi were exterminated by human predation on the coastal forelands of the southwestern Cape. However, I doubt that human predation could have exterminated Antidorcas bondi here without ecological changes adverse to bondi, because bondi seems to have been a similarly small, rapidly reproducing species to marsupialis.

Publicado por milewski hace más de 1 año (Marca)
Publicado por milewski hace más de 1 año (Marca)

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