Surprising floristic links between the Galapagos and Africa, with particular reference to spinescence

@ludwig_muller @jeremygilmore @tonyrebelo @troos @botaneek @graham_g @richardgill @magdastlucia

The Galapagos archipelago (https://en.wikipedia.org/wiki/Gal%C3%A1pagos_Islands) is remote, which explains why its flora is poor. All plants growing there had to reach the islands over the ocean.
 
However, what surprises me about this flora is how many genera would be familiar to, for example, a South African naturalist.

The two regions are in different hemispheres, and one is equatorial whereas the other is mainly at temperate latitudes. Despite such separation, I can offer many examples of shared genera.
 
A preliminary list (in no particular order) of genera indigenous to both the Galapagos and South Africa is as follows:

Here are notes on a few of these, with some surprising details, and a first attempt at explanation.
 
The spp. of Vachellia (https://en.wikipedia.org/wiki/Vachellia) in the Galapagos are similar to those typical of Africa (bipinnate-leafed and stipule-spinescent (https://www.darwinfoundation.org/en/datazone/checklist?species=488). They seem adapted to damage by large animals, despite the absence of mammalian herbivores on this archipelago.

This is not to imply that the main trees and shrubs in the Galapagos belong to this genus. However, for some reason Vachellia is present, in Africa-like form, on these remote islands.
 
Maytenus (https://en.wikipedia.org/wiki/Maytenus) in the Galapagos and the coastal mainland of Ecuador (https://www.inaturalist.org/observations/118724576) is similar to that in the Fynbos Biome of South Africa (https://www.inaturalist.org/observations/22705918).

These congeners share the property of having extremely hard wood. This is true despite the fact that forests in the Galapagos, composed of the daisy genus Scalesia (https://en.wikipedia.org/wiki/Scalesia), have such flimsy stems that they seem, in evolutionary terms, like 'instant trees' created in-situ from herbaceous ancestors, to fill available niches.
 
Sida (https://www.inaturalist.org/taxa/765800-Sida-poeppigiana) is a major food of the giant tortoises (https://en.wikipedia.org/wiki/Gal%C3%A1pagos_tortoise) of the Galapagos. It is also eaten by a large-bodied species of tortoise in Africa (https://africantortoise.com/AfJEcoPardalis2001-1.pdf and https://www.inaturalist.org/observations/70082659), albeit as part of a broad diet.
 
Scutia in the Galapagos (https://www.inaturalist.org/observations/32622619) is spinier than that in South Africa (https://www.inaturalist.org/observations/114695840). This seems paradoxical in terms of the regimes of herbivory.
 
There are many plants with burrs or spiny seeds in the Galapagos, that achieve dispersal by having the diaspores adhere in various ways to animals. It surprises me that a spine-diaspore as gross as Tribulus (https://en.wikipedia.org/wiki/Tribulus) has been successful on islands with no large mammals until the recent introduction of livestock.
 
The presence of Polygala in the Cape Flora (https://en.wikipedia.org/wiki/Cape_Floristic_Region), as well as that of the Galapagos, surprises me (https://www.inaturalist.org/taxa/327827-Polygala-galapageia and https://www.inaturalist.org/taxa/474386-Polygala-fruticosa).
 
At first, I was even more surprised by the sharing of the genus Plumbago (https://en.wikipedia.org/wiki/Plumbago) between the Galapagos and the Eastern Cape (https://en.wikipedia.org/wiki/Eastern_Cape).

In my experience, the only situation in which Plumbago is abundant in a wild state is in in evergreen thicket characterised by Portulacaria afra (https://en.wikipedia.org/wiki/Portulacaria_afra). This vegetation is unusual, even in South Africa.

Here is a genus that contains few species, one of which is typical of the habitat of megaherbivores (https://en.wikipedia.org/wiki/Addo_Elephant_National_Park). So, I asked myself, what is it doing, ostensibly wild, on islands in the Pacific Ocean? My surprise was reduced when I realised that Plumbago is remarkably widespread on Earth (including USA, western South America, Mediterranean Basin, Madagascar, southern Asia, and northern Australia).
 
Zanthoxylum is a specialised genus, with peculiar leaf-form, spinescence, and chemical defence against herbivores (https://www.inaturalist.org/observations?place_id=6986&taxon_id=54837&view=species).

Not only is this genus present in the Galapagos (https://www.inaturalist.org/taxa/170401-Zanthoxylum-fagara). It is dominant on slopes above the Scalesia ‘forest’, where the vegetation is noticeably ‘brown’ owing to the epiphytes associated with mist (https://www.inaturalist.org/observations/111435613). Zanthoxylum is spinescent enough in the Galapagos to be called ‘Wait-a-Minute Bush’ here.

Why have spines, in several families and genera, been retained on islands with such an odd and rudimentary fauna of herbivores? Alternatively, why have the Galapagos ‘recruited’ genera that are spinescent, in the first place?

Such recruitment implies pre-adaptation. But how could a genus as specialised as Zanthoxylum, in terms of anti-herbivory, be pre-adapted to such remote islands?

A parallel puzzle applies to cyanide as an anti-herbivore defence, in the same flora (https://www.jstor.org/stable/3545363).
 
My interpretation:
The soils on these volcanic islands, although generally shallow, tend to be rich in phosphorus, zinc, and other nutrients. This makes plants particularly attractive to whichever herbivores occur.

This nutritional factor may be as important as isolation in having shaped which plants, having reached the islands, have succeeded there. Although the only indigenous large animals on land are tortoises and lizards (https://en.wikipedia.org/wiki/Galapagos_land_iguana#Diet_and_longevity), these seem effectively to have exerted enough pressure of herbivory on plants to account for several floristic parallels with mainlands, including South Africa.
 
In other words, what I suggest is that the nutrient-richness of the soils in the Galapagos is such an important ecological factor that it has

  • outweighed, in some cases, the great isolation, and
  • ensured that, of the various candidate lineages that have reached the islands by long-distance dispersal, those that have prevailed have tended to represent certain familiar lineages associated with ‘big-game habitats’ on mainlands.

Which herbivores do I invoke, as climbing shrubs and trees, and eating leaves and shoots? The genus Conolophus (https://en.wikipedia.org/wiki/Conolophus), and in particular

  • juveniles of the largest-bodied spp., which are more agile than the mainly terrestrial adults while being large-mouthed enough to be affected by spines, and
  • recentlly-extinct spp. (body mass about one kilogram) in the same genus, specialised for climbing even as adults, but not yet detected in the fossil record.
Publicado el julio 9, 2022 01:36 MAÑANA por milewski milewski

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https://www.jstor.org/stable/3892078

Publicado por milewski hace casi 2 años

Very interesting Antoni.

Publicado por graham_g hace casi 2 años

Interesting!

Publicado por richardgill hace casi 2 años

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