Sideroxylon

It is surprising that, within the genus Sideroxylon (https://www.inaturalist.org/observations?taxon_id=128749&view=species), some spp. are spinescent (https://www.inaturalist.org/posts/68481-diversification-of-spinescence-within-the-genus-rhamnus), whereas others are non-spinescent.

This is true both

• globally, and
• within the Africa-Arabian region.

It is even more surprising that, at a biogeographical scale, the spp. sympatric with megaherbivores are the non-spinescent ones, whereas the spp. long-disassociated from megaherbivores are the spinescent ones.

This, again, is true both

• globally, and
• within the African-Arabian region.

This apparent inversion of expectations can be shown by contrasting North Africa-Arabia with southern Africa, at compararable latitudes.

In North Africa-Arabia, two spp. in the genus in question remain extant on the mainland, viz.

• Sideroxylon spinosum of the Maghreb (https://www.inaturalist.org/taxa/1066286-Sideroxylon-spinosum), and
• Sideroxylon mascatense of the Horn of Africa and the nearby, southern, part of the Arabian Peninsula (https://www.inaturalist.org/taxa/537752-Sideroxylon-mascatense).

Both of these spp. are spinescent, facilitating the adoption of topiary-like shapes when the plants are repeatedly damaged by the domestic goat (Capra hircus) and herders who cut down foliage for their livestock.

Sideroxylon spinosum:
https://www.inaturalist.org/observations/135288571
https://www.inaturalist.org/observations/208984437
https://www.inaturalist.org/observations/199865259
https://www.inaturalist.org/observations/214754335
https://www.inaturalist.org/observations/107391644
https://www.inaturalist.org/observations/150077590
https://www.inaturalist.org/observations/209265384
https://www.inaturalist.org/observations/205986810
https://www.inaturalist.org/observations/202588842
https://www.inaturalist.org/observations/194832368

Sideroxylon mascatense:
https://www.inaturalist.org/observations/140137833
https://www.inaturalist.org/observations/191267582

Several non-spinescent spp. of Sideroxylon do occur near North Africa. However, these occur only on islands, viz.

• Sideroxylon marginatum (Cape Verde archipelago, https://www.inaturalist.org/observations/66383810),
• Sideroxylon mirmulans (Madeira archipelago, https://www.inaturalist.org/observations/192335915), and
• Sideroxylon canariense (Canary archipelago, https://www.inaturalist.org/observations/162981980).

Turning now to southern Africa:

There is only one sp. present, viz. Sideroxylon inerme (https://www.inaturalist.org/observations/246829244).

Sideroxylon inerme resembles the insular spp., above, in its non-spinescence. Furthermore, it is susceptible to overutilisation by the domestic goat, as evidenced by its recent extermination - together with most other woody spp. other than Pappea capensis (https://www.inaturalist.org/journal/milewski/100103-a-comparison-of-sapindaceae-in-the-mediterranean-and-adjacent-arid-climates-of-australia-and-southern-africa#) - over most of the thicket biome (https://pza.sanbi.org/vegetation/albany-thicket-biome#:~:text=Thickets%20in%20the%20Eastern%20Cape,%2C%20spekboom%20(Portulacaria%20afra).) of Eastern Cape province of South Africa.

It is unsurprising that the three spp. restricted to islands in the eastern Atlantic are non-spinescent. This is because their environments are naturally remote from the pressures of herbivory.

However, it is surprising that S. inerme conforms to its undefended congeners on islands. This is because this species occurs in a biome exceptionally rich in megaherbivores, e.g. in Addo Elephant National Park (https://en.wikipedia.org/wiki/Addo_Elephant_National_Park).

Posted on October 22, 2024 12:59 AM by milewski