Thursday, 11 August 2011

Why is Africa so full of thorns?

Assorted browsers and browse lines, Selous GR, June 2010
I'm actually away this week, but left this to post itself whilst I'm off and keep people interested! One of the places I'm headed to is Mwiba Ranch, south of Serengeti and, as you might know, mwiba is swahili for 'thorn'. No doubt I'll have some specifics to talk about when I'm back, but one of my 10 things to talk about topics is thorns. Why? Well, many visitors from the north live in places where tere aren't lots of thorny trees, so going to a walking safari and discovering that just about every bush and tree is covered in massive needles is a bit of a shock, even if those of us lucky enough to live here barely notice them (until they get infected, of couuse...).

Giraffe (and shorter!) browse line, Arusha NP, June 2010
So, why should Africa, or at least African savannahs in particular, be so thorny? The answer, of course, is fairly simple - why do tourists visit the savannah? To see the animals, and what do the animals eat? Well, rather a lot of them like to eat bushes, which is no fun at all if you happen to be a bush. In fact, in many places the grazing pressure is so heavy, very distinct browse-lines form and the plants take on structures as if someone was pruning them into interesting sculptures. So, defending yourself against Africa's abundan browsing animal population is a very good idea, and I've always thought thorns must be pretty nasty things to eat. But, you say, if the thorns are a defense against browsing, how come I still see impala and giraffe and all the others happily choping on thorn trees? Obviously the defence doesn't work? That's a good point, and allows me to introduce one of the many ways in which nothing in biology makes sense except in the light of evolution. Imagine living in an Africa before there were any thorns - all the bushes are undefended, and equally appealing to a browser. Now imagine there's a little mutation in one of the offspring of those bushes, that means it grows small spines - all the other bushes are still nice and undefended. If you were an impala, which bush would you eat? I suspect you'd go for any of them, except the one with the small spines. Which, of course, means that the spiny bush is going to do very well and will produce lots of babies, also with spines. In time, all the bushes will have little spines and the poor impala, if he wants any lunch, just has to tuck into that thorny bush. But, of course, one bush might have a mutation making it's spines a little harder, a little longer, a little nastier - and you can see immediately what's going to happen now - bigger spines evolve.
Greater Kudu, carefully nibbling around thorns, Kruger NP, May 2011.

Of course there might also be heritable variation in the impala browsing technique or mouths - maybe for thicker skin, or a narrower nose that can squeeze between the thorns. Giraffe, of course, have evolved a huge long sticky tongue so they practically lick the leave out from between the thorns, rather than have to go to close. But both species certainly are aware of the thorns, even though they have no choice but to eat the prickly trees, of course - imagine what would happen to the poor thorn tree that, though some mutation, had no thorns. Ooops, poor thing! So clearly the thorns do have an impact - what's interesting to me to look for in places with very heavy browsing is evidence that the thorns do work, even though the plants get eaten.
Heavily browsed yellow-barked Acacia, Arusha NP, June 2010

To see this, you need to look at the shapes of trees and bushes. Here's a nice Vachellia xanthophloea (see, I'm trying to get you used to the new names!) that's been very hevily pruned - a favourite with the girafe. So how's it ever going to make the leap from heavily pruned bush to fully fledged tree? The secret is to grow wide, before growing tall. While the bush is still short and relatively narrow it doesn't stand a chance - any giraffe will bend down, and chew off the top bits. But if it can get wide enough the giraffe don't like the thorns on their skin, and they'll just nibble to top bits within easy reach - leaving a tuft in the centre to break away. And once that is done, those short, wide branches at the base of the tree are no longer important and soon die back, in favour of the taller tree. Not clear? She here's a little diagram showing how to escape giraffe broswing pressure...
Bushes escaping browsing. Honest!

Nearly there! This bush behing the giraffe has started to escpe from the centre, Lake Manyara NP, April 2010
Not the most artistic ever, I admit - but I'm an ecologist, not an artist... (It's a giraffe bending down on the left, not a funny, long-necked kangaroo). So on the left you have a tiny seedling been chewed by a giraffe. This makes the bush grow flat, like in the photo above. Then, when the plant is wide enough, the throns around the edge stop the animals from being able to reach the centre and in the third part of the diagram a new shoot is escaping browsing, and finally in the last picture a mature tree has grown, with just a little whisp of the original short, fat bush that had to escape browsing remaining as evidence.

Made it! Just a few whisps of short left, West Kili, April 2010
So, that all sounds very nice, but why, you ask, do some plants get away without thorns, even in the savannah? Both Terminalia and Combretrum are fairly thorn-free, but a typical savannah plants. So how do they do it? Well, the difference here is the nutrients. Remember that Acacias (or whatever we're going to start caling the group now) are legumes and are absolutely full of nutrients, whereas these other two species live on the nutrient poor ridges and represent pretty low-quality browse. So, if you're a tasty plant growing in nutrient rich areas, you're going to be browsed much more than those specialising in nutrient poor environments, and here you want to invest more in defence.

And that, for now, is that. Hope you're having as much fun as I will be having in Mwiba!

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