The ecological biogeography of Atriplex

(writing in progress)

The genus Atriplex (https://en.wikipedia.org/wiki/Atriplex and https://www.inaturalist.org/observations?taxon_id=58112), with as many as 300 species, is remarkably widespread and common in semi-arid climates. Saltbushes occur on all vegetated continents and even on oceanic islands.

And in Australia, Eurasia, the Americas, and North Africa, Atriplex dominates large areas of vegetation on sodic soils. Because the shrubs tend to look silvery rather than green, this dominance is easy enough to see.

Based on this wide-ranging success, it would have surprised nobody to find a similar diversity and abundance of Atriplex in the dry parts of southern Africa. But the genus is oddly scarce in the Karoo. Why this lapse?

The lapse involved is both biogeographical and botanical. For although the scacity of Atriplex is one of the salient features of South African vegetation, scientists – including the small army of South African botanists – tend to overlook it.

The few species of this genus, which are naturally restricted to southern Africa, such as Atriplex vestita, are easily overlooked because sodic soils under dry climates in that subcontinent are covered instead by another halophytic genus of amaranths.

Salsola is so successful in the Karoo of southern Africa that it has virtually usurped the niche of Atriplex. Salsola, unlike Atriplex, has extremely small leaves despite the succulence of these dark, dull green leaves. Although Australia has counterparts for Salsola in the form of Maireana pyramidata and M. aphylla, and these species of Maireana do dominate patchily on sodic soils in semi-arid Australia, the difference between continents is that Atriplex dominates even larger areas than does Maireana in Australia whereas it dominates not even one hectare of the natural vegetation in southern Africa.

The few biologists who appreciate this biogeographical anomaly have suggested that for some reason Atriplex may have arrived in southern Africa only relatively recently. But such an explanation is implausible geographically and floristically, and seems tautological.

My explanation is an ecological, not historical one, and it involves the herbivorous faunas.

All forms of Atriplex are potentially vulnerable to herbivory, because the leaves of this genus are large enough to be picked off relatively easily and because the stems are extremely brittle.

Atriplex relies on the salt excreted on the leaf surface - rather than toxins or spines – for defence.

There are rodents that specialise on the leaves of Atriplex. Species eating these as a staple include

• Muridae: Murinae: Leporillus conditor in Australia,
• Muridae: Gerbillinae: Psammomys obesus in North Africa,
• Octodontidae: Tympanoctomys barrerae in South America, and
• Heteromyidae: Dipodomys microps in North America.

In most semi-arid parts of the world, the salt deters large herbivores. This includes the red kangaroo (Megaleia rufa), which relies on grass and other herbaceous plants.

However, something that sets southern Africa apart is that it has a ruminant fauna extremely well-adapted to drought. The gazelle-like bovids epitomising this, namely the springboks (Antidorcas marsupialis), have no counterparts in Australia or the Americas. In North Africa and Asia, gazelles occur but there are no counterparts for the browsing ruminants of an extreme range of body sizes, common in semi-arid South Africa. The common eland (Taurotragus oryx), steenbok (Raphicerus campestris), grey rhebok (Pelea capreolus), and common duiker (Sylvicapra grimmia) lacked counterparts in North Africa.

Ostriches have diets similar to those of gazelle-like bovids and echo their biogeography, remaining common in semi-arid southern Africa although they have become extinct in Asia [TASK: look up ostrich diet in captivity, e.g. in Little Karoo to verify that this species is sometimes fed Atriplex].

Atriplex is also unprofitable food for ruminants except as a minor part of the diet. Experiments with the domestic sheep (Ovis aries) have shown that ruminant physiology wastes much of the protein in this plant, and that the carbohydrate content is insufficient.

Herbivores with hindgut fermentation may be somewhat better suited to utilising Atriplex than ruminants are, but the hares naturally occurring in the habitat of Atriplex eat this genus only as a small part of their diets even in the case of species which rely mainly on shrubs as opposed to grass.

It is only among small rodents that we find species specialised for a diet of halophytic amaranths, particularly Atriplex [more needed here].

A fundamental pattern of Nature is that, in ecosystems dominated by herbivory as opposed to fire or decomposition of plant matter, the most successful plants tend to be those mutualistic with the large animals. This is best exemplified by natural grazing lawns, but it applies also to the dwarf shrublands characteristic of the Karoo. The dominant plants in such ecosystems tend to be those not only tolerant of, but capable of benefitting from, the damage inflicted by herbivores. (Explain further.)

Because Atriplex is fundamentally unsuited to intense herbivory, it has been relegated to no more than an interstitial status – easily overlooked by even the most ardent of Karoo botanists - in the vegetation types to which it would be suited in climatic and edaphic terms. Under the naturally intense herbivory in southern Africa, the amaranth that has instead prevailed, heavily browsed and happily so, is Salsola.

A broad biogeographical pattern emerges:
Although Atriplex is essentially cosmopolitan under dry climates and on sodic soils, its largest body sizes occur in Australia, the continent of minimal herbivory, and the least incidence occurs in southern Africa, the subcontinent of maximal herbivory.

Atriplex nummularia, common in south eastern Australia as a large shrub (up to 3m high and 4m wide) and naturally dominant over considerable areas, is testimony to the particular difference between two southern continents. Places like Patagonia and the south-western semi-deserts of North America fall somewhere in between.

(writing in progress)

Posted on June 13, 2022 08:54 AM by milewski