In being widespread among continents, many genera of grasses defy biogeography

(writing in progress)

Everyone knows that, in general, the various continents have different biotas. In plants as well as in animals, it is unusual - because of the tyranny of distance - for any given genus to occur naturally on several continents.

There are many exceptions to this rule, partly because Asia and North America have been connected for most if the last two million years by a Broad land-Bridge across Beringia. However, plant genera with cosmopolitan distributions beyond the Holarctic tend to have diaspores easily transported over great distances. These tend to be either:

  • windblown, as in e.g. daisies (Asteraceae), or
  • dispersed by birds which eat fleshy fruits and then pass seeds in viable condition.

Because of this pattern of continental distribution, any globally-experienced botanist, familiar with many genera, can generally infer the continent concerned from any local floristic checklist.

Grasses (Poaceae) are, however, anomalous. Their general pattern is that many or most genera occur naturally on several continents.

I first noticed this many years ago while trying to identify which genera of grasses are particularly associated with the fauna of grazers of Africa, which is so rich in mammals including ungulates. I expected that the extreme favourability of Africa for grazers, from termites through rodents and lagomorphs to the hippopotamus and square-lipped rhino, would be based at least partly on a distinctive flora of African grasses at the level of genus.

This turned out to be not the case: almost all of the palatable grasses of Africa belong to genera shared with at least one other continent, and in many cases all three southern continents.

Typical examples of this pattern include Urochloa, Panicum, Cynodon, Pennisetum, Setaria, Cenchrus, Digitaria, Eragrostis, Sporobolus, Brachiaria, etc. Many of these genera contain species particularly favoured by grazers, to the point of near-mutualism in the sense of adaptation to form lawns when repeatedly grazed. Yet at the level of genus they span the continents.

The case of Urochloa mosambicensis (https://www.inaturalist.org/taxa/170075-Urochloa-mosambicensis and https://www.tandfonline.com/doi/abs/10.2989/10220110209485784 and https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.572.3781&rep=rep1&type=pdf and https://www.classicafrica.com/images/customer-files//HensmanMasters.pdf) is as intriguing as any. This species forms stoloniferous lawns associated with the square-lipped rhino in Zululand. In the past, when this species of rhino had been nearly exterminated, much of thus area was dominated by the tussock-grass Themeda triandra. As the population of the square-lipped rhino increased, lawn-forming grasses including U. mosambicensis took over.

Despite this association, Urochloa is not restricted to Africa, occurring naturally also in Madagascar, China, India, and Mexico (https://en.wikipedia.org/wiki/Urochloa).

The cosmopolitan tendency of grass genera, including those most associated with large-bodied grazers, is puzzling because these plants have no known mechanism of dispersal across sea barriers.

(writing in progress)

Posted on Απρίλιος 12, 2022 0840 ΠΜ by milewski milewski

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Αναρτήθηκε από milewski περίπου 2 χρόνια πριν
Αναρτήθηκε από milewski περίπου 2 χρόνια πριν

Here is another example of how the genera of grasses are widespread, with little regard to any ecological integrity or biogeographical distinctiveness.
 
The genus Chrysopogon (https://en.wikipedia.org/wiki/Chrysopogon) is widespread in Australasia, Asia, Africa and even southeastern North America. In the semi-arid lands of East Africa (please see below), it is an important grass for grazing ungulates, in the form of the species C. plumulosus.
 
Despite its association with the ‘big game country’ of East Africa, the genus Chrysopogon is also perfectly at home in Australia. For example, C. fallax occurs in eucalypt woodland in Kakadu National Park in the Northern Territory. Here it had few indigenous grazers, but the same genus crops up regardless.
 
This is a puzzle from two points of view. How did all these grass genera find their way to Australia in the first place? And how come they are common in Australia as in Africa, despite the great difference in the incidences of large grazers on the different continents?
 
http://www.fao.org/docrep/008/y8344e/y8344e06.htm

Αναρτήθηκε από milewski σχεδόν 2 χρόνια πριν

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