The Elephant and the Acacia
(a short excerpt from the excellent book The Private Life of Plants)
[Discussing the effects of fire on the flora of the African plains]
You may also find the young seedlings of acacia thorn bush. If they are only a foot high or less, their leaves and regenerative buds will have been in the hottest part of the flames and little will be left of them on the charred stem. Such seedlings cannot survive. As long as fires regularly burn here, the plains will remain the dominion of the grass.
That suits the herds of game that graze upon the plains and depend upon grass for their food. One might suppose that the animals that crop leaves with such assiduity would be the enemies of the grass, but in the long term, they are its allies. Like the fire, they do not damage the horizontal stems. The grasses even have structures that make it easier for animals to remove the leaves, for there are special points of weakness at the base of each blade. That gives a grazer an easier mouthful, but it also ensures that the all-important horizontal stem is not ripped out by its roots. That would certainly be the end of the grass plant. The acacia seedlings, however, are as easily destroyed by nibbling teeth as they are by fire. So long as game feed here, the dominance of the grass seems assured.
How is it, then, that over many areas there are wide areas of acacia scrub? How did they get their start? In good years, when the rains are abundant and the grass grows strongly, there will be a big increase in the numbers of game on the plain. As the dry season drags on however, so many mouths trim the grass with such severity that there is virtually nothing to fuel a fire. The animals themselves, having exhausted their food supply, begin to move away to look for better grazing elsewhere.
In the absence of both flames and biting teeth, the year's crop of young acacia seedlings have a chance. If they escape damage for four or five seasons, the probability of their survival soars. If they can gain a few feet in height before the next fire, their buds may be above the most damaging of the swiftly moving flames. Their small leaves, by now, are guarded by sharp thorns and they are of little temptation to habitual grass-feeders like antelope or zebra. Their branches start to spread, taking more and more of the light and eventually the grass cannot grow beneath them. So a tract of acacia bush appears on the plains, sometimes in a small patch, sometimes extending for many miles, with all its member trees of about the same size and age and standing on bare earth.
But the victory of such thorn bushes is not permanent. As they increase in size and produce more leaves, so the begin to attract the attention of those animals that habitually browse brush, such as giraffe and gerenuk. When they get bigger still, elephants start to rip off whole branches, chewing leaves, thorns, twigs, even the wood in the branch itself. Commiphora, a shrubby thorny tree belonging to the balsam family which produces a rubbery resin known as myrrh, also grows on these plains if it gets the chance. Such trees are not to the elephants' tastes. That, however, does not save them. The elephants deliberately push them over and then leave them uneaten. The groves of thorn trees are destroyed and light floods back onto the ground. Grass once more spreads over the site. So the whole cycle, that may have taken a man's lifetime to run its course, repeats itself.
You could claim that the elephants, by destroying the acacias and uprooting the commiphora, are in effect gardening, creating conditions in which one of their favoured foods, grass, will continue to flourish and provide them with a good annual crop. But equally you could argue that the grass has recruited the elephants to help extend its empire and by growing more leaves than it needs for its survival, and developing a structure which allows it to be cropped without lethal damage, it is simply paying its employees a decent wage that it can well afford.