Monogastric herbivores taking weedy dicotyledonous pioneers rejected by ruminants - with notes on the grass Bothriochloa

(writing in progress)

Large herbivores are basically split into ruminants, with foregut fermentation of food, and monogastrics, with a simple stomach but fermentation in the large intestine.

Ruminants tend to be better than monogastrics at detoxifying plant matter, because bacteria break down the toxins in the rumen, before digestion takes place in the small intestine.

The diets of these categories overlap to a great extent, particularly w.r.t. grasses. However, various plants, usually uncommon in their habitats, are rejected by ruminants. Because such plants are chemically defended from the ruminants, they ‘heal’ disturbance by growing rapidly while being relatively free from herbivory.

These are available to monogastrics.

An example is Galenia africana (https://www.inaturalist.org/taxa/1038374-Aizoon-africanum). This weedy shrub tends to be a pioneer on bare, disturbed ground in the Karroo and nearby dry areas. It is ignored by ruminants. However, it is eaten in moderate quantities by the herbivorous rats Parotomys and Otomys (du Plessis, Kerley, and Winter, 1991, S Afr J Zool 27 (2), 45-49) and the donkey (Equus asinus, Jan Zimri, personal communication), all of which are monogastric.
 
Another example is the weedy indigenous daisy Pechuel-loeschea leubnitziae (https://www.inaturalist.org/taxa/634052-Pechuel-loeschea-leubnitziae).

Pechuel-Loeschea leubnitziae: http://www.zimbabweflora.co.zw/speciesdata/image-display.php?species_id=159170&image_id=1
 
This species - like Galenia africana - is restricted to southern Africa. It can be thought of as one of the few representatives, in the savanna flora, of the syndrome – so familiar in the fynbos (https://en.wikipedia.org/wiki/Fynbos) and karoo (https://en.wikipedia.org/wiki/Karoo) biomes – of asteraceous shrubs acting as pioneers on disturbed ground (see http://www.tandfonline.com/doi/abs/10.2989/10220119.2012.720280 ). In the Kruger National Park, this daisy occurs patchily in the north.
 
It is interesting that at least one species of daisy manages to maintain this niche in Kruger National Park, in a flora full of leguminous shrubs (particularly acacias in the loose sense), capable of response to disturbance.
 
The particular observation I report here is that of the plains zebra (Equus quagga crawshayi) eating this species, in Kruger National Park during drought in 2016.
 
What I saw was unlikely to be merely a case of the zebra resorting in desperation to an atypical food-plant. Instead, here we seem to have an intriguing aspect of the normal niche of monograstric grazers, which has been overlooked in the literature. Equids certainly are mainly grazers, but their foraging niches are complex enough to extend to certain dicotyledonous plants that have biochemical defences making them harmful to ruminants but acceptable to monogastric herbivores.
 
My observation was made on 8 Sept. 2016, at Pafuri in the far north of Kruger National Park. The habitat was a road verge in mopane (Colophospermum mooane, https://www.inaturalist.org/taxa/428749-Colophospermum-mopane) vegetation on basalt.

The daisy was bearing seed at the time, suggesting that part of the dietary attraction was the seeds. However, the zebra did not forage in a finnicky or careful way, as if to select the seed-heads. Instead, it ate the shrub wholesale, accepting leaves and stems as well as the seed-heads. My impression was that this daisy is a common food of the plains zebra in this area, with its mopane vegetation - generally unsuitable for equids.
 
I also noted that the plains zebra in this area ignored the abundant grass Bothriochloa radicans (https://www.inaturalist.org/taxa/347797-Bothriochloa-radicans), but accepted the shrub P-l. leubnitziae. This suggests to me an intriguing biochemical relationship. This grass - which has a distinctive odour to the human nose - is particularly adapted to be not worth eating to a specialised grazer depending on colonic and caecal fermentation. For its part, this shrub, correspondingly adapted to being not worth eating for foregut-fermenters (ruminants), can be exploited by the colonic-caecal fermenter.

It seems that the plains zebra can ‘fill in’ for the inability of ruminants to exploit this daisy, because its digestive strategy is not as vulnerable as that of ruminants to the particular biochemical formulation produced by this daisy.

The interesting point is that a grass, Bothriochloa, has proved capable of defeating the digestive system of the plains zebra despite zebras being bulk-and-roughage grazers able to take fibrous grasses unacceptable to many like-size ruminants, while at the same time the plains zebra can accept a dicotyledonous plant biochemically unacceptable to browsing ruminants such as the impala (Aepyceros melampus, https://www.inaturalist.org/taxa/42278-Aepyceros-melampus) or the greater kudu (Strepsiceros strepsiceros, https://www.inaturalist.org/taxa/42339-Tragelaphus-strepsiceros).

Perhaps the biggest surprise of said visit to Kruger National Park was the grass Bothriochloa, a genus closely related to Dichanthium. How could I have remained unaware of the existence of this whole genus of grasses, given that it dominates vast swathes of the Park, and is a problem in pasture management?
 
So I consulted the original magnum opus, edited by D Meredith and first published in 1955, “The Grasses and Pastures of South Africa”. This is a comprehensive book, running to 771 pages.
 
To my disappointment, the only coverage of Bothriochloa is a perfunctory account of the South African species, as part of a complete treatment of the whole flora of grasses in South Africa. In this whole book, there is nothing even to hint at the importance of Bothriochloa in Kruger National Park.
 
The only part of “The Grasses and Pastures of South Africa” devoted to Bothriochloa is pp. 482-484, where the genus and its various species are described, with negligible interpretation or commentary. There is no mention of secondary chemicals or fire-proneness.
 
Among the few facts that I learned:
 
The distributions of the various spp. of Bothriochloa are given as follows:
 
B. radicans: from Graaff Reinet across the Highveld to the northern and eastern Transvaal, plus Hereroland in Namibia, growing usually on rocky ridges. (the range sounds right for Kruger National Park but the habitat seems too restricted)
https://www.inaturalist.org/taxa/347797-Bothriochloa-radicans
 
B. glabra: from the eastern Cape through Natal and Swaziland to the Transvaal, plus Grootfontein district of Namibia, growing in wet places, usually on river banks or in vleis. (again, the distribution makes this species possible in Kruger National Park, but the habitat seems wrong)
 
B. insculpta: from the southeastern Cape through Natal and Swaziland to the Transvaal, Botswana, and the Grootfontein district of Namibia, growing in undisturbed or disturbed grassland, often on dry hillsides. (This sounds most like what I observed in Kruger National Park.)
https://www.inaturalist.org/taxa/279566-Bothriochloa-insculpta
 
B. insculpta var. vegetior: described from North Africa as ‘rather striking on account of its reed-like rough leaves’ (the leaf-blades up to 30 cm long and 1.2 cm wide).
 
Disappointed by the authoritative book, I turned to Wikipedia.
 
Now, I have Posted previously about the biogeographic and phylogenetic oddity of grasses in general: how the genera mean little in terms of consistency of ecological type, and how the taxa are puzzlingly widespread across the continents. Familiar though I am with these quirks of the Poaceae, Bothriochloa still managed to surprise me by illustrating the confusion and futility of traditional botanical and biogeographical approaches to the grasses.
 
The reality is that there is nothing special about Bothriochloa as a genus. Like so many other grass genera, it is cosmopolitan; and indeed even B. insculpta (the species likely to be common in Kruger National Park) occurs as far afield as India and southern Europe, apparently indigenous there.

Several species are palatable grasses, noteworthy in farming. However, one species is even a lawn-former, known inter alia to provide the main grazing for the rusa deer (https://www.inaturalist.org/taxa/75052-Rusa-timorensis) in New Caledonia. This species, native to China inter alia, sounds almost domesticated.
 
With respect to chemical defence:
‘Turpentine grasses’ or ‘lemon grasses’ in the form of Cymbopogon (https://www.inaturalist.org/observations?place_id=any&taxon_id=54494&view=species), a genus that seems fairly consistent in this chemical defence. So it was easy to assume that Bothriochloa would be similarly consistent. But far from it: not only do most spp. in this genus seem to lack any particular odour, but the only mention of these volatile substances, even for the species occurring in Kruger National Park, is the name 'smelly grass'.
 
What all of this adds up to is that a botanical phenomenon familiar to ecologists in Kruger National Park is absent from the literature – from the original expert treatment in the ‘fifties to the modern Wikipedia. It is as if we are dealing with two different worlds.
 
For ecologists familiar with Kruger National Park, the world is one in which this grass, typically unpalatable and flammable, is particularly adapted to deter grazers such as the plains zebra. This has converted the potentially palatable mopane-dominated vegetation in particular into something of a desert for grazers. And this applies to extensive areas of Kruger National Park.

However, for the rest of us, Bothriochloa may as well be just one more of the many grass genera worldwide (including Australia) that does the same old thing, repeated for genus after genus of Poaceae: the genus occurs widely in the Old World and South America, even its species are widespread with scant regard for biogeography, and the congeners vary radically and apparently randomly in growth-form, successional role and palatability/fire-proneness.
 
In the end, the genus Bothriochloa may as well not exist as far as ecology and biogeography are concerned: all we have is the species found in the Kruger National Park and its particulars in that situation.
 
Why, in the voluminous literature on grasses, have I never seen discussion of this pattern of grass phylogeny being relatively meaningless in ecological terms? One could read everything written on grasses, and never get a major point about them: they make taxonomy and biogeography relatively meaningless.
 
In summary: there is no doubt that Bothriochloa (probably insculpta) is an extremely important grass in Kruger National Park, and that it poses problems both theoretically and practically in the understanding and management of this reserve. However, the nature and occurrence of its genus sheds no light on these problems. The fact that B. insculpta, that problematically unpalatable grass on nutritious soil, is a Bothriochloa, as opposed to any other genus of grass, seems to be virtually irrelevant.
 
https://en.wikipedia.org/wiki/Bothriochloa

https://en.wikipedia.org/wiki/Bothriochloa_saccharoides

https://en.wikipedia.org/wiki/Bothriochloa_pertusa

https://en.wikipedia.org/wiki/Bothriochloa_ischaemum

https://en.wikipedia.org/wiki/Bothriochloa_bladhii

https://en.wikipedia.org/wiki/Bothriochloa_barbinodis

(writing in progress)

Posted on July 6, 2022 08:47 AM by milewski