Acacias don’t just stand there being eaten.

Migunga haisimami kukubali kuliwa tu.

Just because trees can’t move to escape from predation doesn’t mean they’re just standing there letting themselves be attacked. In fact, because plants have been around for so long, they’ve had a lot of time to evolve some very complex defense strategies.

One of the iconic species of the savannah is the Acacia tortillis (Vachelia tortillis) or Umbrella thorn. They obviously use thorns as their defense. Their thorns come in little paired hooks, or long straight pairs, and sometimes a combination of both. These thorn combinations are not distributed evenly on the whole tree. Look at a young tree with branches that are low enough to be browsed by impala and giraffe and you’ll notice that there are a lot more long, straight thorns than little hooks. Tall trees that have grown too tall to be browsed by giraffe have much fewer straight thorns on the branches. If you look carefully, you’ll also notice that there are more long straight thorns at the ends of the branches where the leaves are easiest to get to. In fact, if you protect an Acacia from being browsed on, it will stop investing energy in long thorns. Producing thorns is an energetic cost to the trees, so these Acacia’s are actually responding to the threat.

A male giraffe feeding on the Whistling thorn (Vachelia drepanolobium). This as another interesting story for another blog post, but look at those long white thorns.

Now of course, there are still a lot of animals that eat Acacia’s despite the thorns, but you’ll also notice that these browsers don’t spend a lot of time at any one tree, and keep moving. Why is this? This is a product of two things. Animals need to fulfil their nutritional needs as quickly as possible to reduce competition and reduce the amount of time they are distracted from looking from predators, and so they get to a bush and eat what is easiest to get to. When it starts to get harder to get a good mouthful, they move on. The second thing is that the plants respond to being browsed on by concentrating chemicals in their leaves. In a study of giraffe’s feeding on Acacia nigrescens (Senegalia nigrescens), the tannin content of leaves on a tree went up within 2 minutes of being “attacked” by the giraffe.  

One of the things I find very interesting is that there is evidence that thorns actually evolved in African plants to protect them from antelopes that invaded Africa from Europe and Asia about 15 million years ago. It appears that the ancestors of the hyrax and elephant that lived in Africa before that were not deterred by thorns. You can read more about that here.If you’ve ever wondered why Acacia thorns tend to be an obvious white color, there’s also evidence that this deliberately to warn herbivores.

As an end note, what is also fascinating is that Acacia tortilis trees that have escaped mammal herbivores actually produce more chemicals in their leaves which may be a response to not being able to escape from insect herbivores.

Swahili version

Miti hayawezi kukimbia ili kukwepa adui, lakini haimanishi kwamba miti yanasimama tu na kuruhusu kushambuliwa na adui. Kwa kweli, mimea mbalimbali imekuwepo muda mrefu sana na imepata muda mrefu wa kubadilika na kutunga ulinzi wa kujitetea.

Spishi mmoja maarufu kwenye savanna (mbuga za wanyama) ni Acacia tortillis (Vachelia tortillis) au Mgunga. Migunga huwa inatumia miiba kama ulinzi. Miiba yao inatokea kwa mipangilio mitatu- ndoana mbili, ndoana ikiwa imekaa pamoja na mwiba ulionyoka, na miiba miwili ilionyoka. Mipangilio hii haijatandazwa sawa kwenye mti mzima. Ukiuangalia mgunga mdogo wenye urefu sawa wa kuliwa na swala na twiga utakuta kuna miiba mirefu zaidi kuliko miiba ya nduano. Miti mirefu kuliko uwezo wa twiga kula haina miiba mirefu. Ukichunguza vizuri, utakuta kuna miiba mirefu zaidi mwisho wa tawi kwenye sehemu rahisi kufikiwa na mlaji wa majani yake. Ukizuia mgunga ili majani yake yasiliwe na mammalia, itapunguza matumizi ya nishati kutengeneza miiba mirefu. Kutengeneza miiba ni hasara ya nguvu kwa mti na inamaanisha kwamba migunga inaitikia matisho.

Lakini, kuna wanyama wengi wanakula majani ya migunga bila kujali miiba, lakini utakuta kwamba hawa walamimea hawabaki kwenye mti mmoja tu, na huwa wanazungukia migunga mingi. Ni sababu gani? Hii ni mchanganyiko wa sababu mbili. Wanyama wanahitaji kutosheleza mahitaji yao ya malisho haraka iwezekanavyo ilikupunguza ushindani na kuwa makini na adui kwa kupunguza muda wanapobanwa kwa kula, na kwa hivyo wakifika kwenye kichaka wanakula kilichorahisi zaidi kula. Sababu ya pili ni kwa sababu mimea inaitika kwa kulinda majani yao yasiliwe kwa kuongeza kemikali kwenye majani yao. Kwenye utafiti uliochunguza twiga wakila spishi nyingine linaloitwa Acacia nigrescens (Senegalia nigrescens), kiasi cha ‘tannin’ kwenye majani ya miti iliongezeka ndani ya dakika mbili tangu kushambuliwa na twiga.

Kitu kinachonivutia pia ni kwamba kuna ushahidi ya kwamba miiba ilitokea kwenye mimea ya kiafrika kwa ajili kuilinda mimea kutokana na paa (antelope) waliovamia Afrika kutoka Ulaya na Asia karibia miaka milioni 15 iliopita. Inaonekana kwamba miiba haikuwazuia wanyama wakazi wa kale kama pimbi na tembo walioishi Afrika kabla ya kipindi kile. Unaweza kusoma ziadi hapa. Kama uliwahi kufikiri kwa nini migunga ina miiba miyeupe sana, kuna ushahidi pia ya kwamba inasaidia kutahadharisha wanyama wacheuzi.

Kwa kumalizia, kitu kingine cha kushangaza ni kwamba miti ya Acacia tortillis (Migunga) iliyoponyoka kuliwa na walamimea mamalia huwa inatengeneza kemikali ziaid kwenye majani yao, bila shaka ni kuitikia mashambulizi ya walamimea wadudu.

Sources:

Furstenburg, D & Van Hoven, W. 1994. Condensed tanning as anti-defoliate agent agains browsing by giraffe (Giraffa camelopardalis) in the Kruger National Park. Comp. Biochem. Physol. Vol 107A.

Ward. D, Kirkman, K. 2007. Inducible defences in Acacia sieberiana in response to giraffe browsing. African Journal of Range and Forage Science.

University of the Witwatersrand. “Browsing antelope turned ancient African forests into grassy savanna ecosystems: Study of thorn trees in African savanna reveals astonishing comparisons to the evolution of thorns and the arrival of antelope in Africa.” ScienceDaily.

Midgley, J. 2015. Further evidence that in African acacia, white is a warning colour to herbivores: the white pseudo galls of Vachellia seyal.

4 replies

  1. Thanks for the deeply explanation of the thorns in acacia, as a guide and orator, Iam always looking forward to learn from professional so that I can deliver good information to my guests, thanks again and good luck.

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s