Alleopathy and the revenge of the Sweet-thorn !

If you’ve spent any time at all in a garden you will know that plants are continuously communicating with the world around them. Using colour and scent plants send constant signals to insects, birds and other animal pollinators that they are ‘open for business’ and will be happy to have whoever is available to help with pollination and the survival of their species.

But what about plants that communicate with other plants ? If good communication is about sending out and receiving messages that are 100% understandable – then plants certainly do qualify! And, as with the an[LBH1]imals, plant motivation remains the same – the survival of the species.

As a classic example certain species of thorn-trees, in a process known as alleopathy, to protect themselves from being over-browsed by mammals, have evolved ways of communicating with each other to prevent wide-spread leaf predation !

I first learned first hand of alleopathic communication in 1974 when my wife and I ran environmental education courses at the Nyala Game Ranch ranch in the Nkwaleni valley just north of Eshowe. Nyala was quite a small ranch and, in addition to fabulous birds and interesting small mammals it was home to nyala, bushbuck, duiker, wildebeest and kudu – the latter two species being re-introduced at various times.

Now – kudu are best known as browsers in that they spend the day snacking on tree leaves, and when there is enough space to select their food they snack from as many as 150 species. However because Nyala was small ranch the choice of browsing trees was limited with the most common food item for kudu being the acacia trees – mainly Sweet Thorn (Acacia karroo) and the Umbrella Thorn (A. tortilis).

And this was when the trouble began for these newcomers to Nyala Ranch. For it wasn’t long that they began to die and when the owners of Nyala, understandably concerned by this turn of events, conducted autopsies they found each animal with a stomach full of undigested acacia leaves!

Why undigested ? Well it appears that when an acacia tree is ‘attacked’ by a browsing animal the tree registers the attack by pumping tannin into its leaves ! This makes the leaves unpalatable and so the browser moves on to another tree. The amazing things is that all the acacia trees downwind from the tree pick up the ‘scent’ of tannin, are thereby alerted to an imminent attack and so release tannin into their own leaves. Net result ? All the acacia trees downwind from the kudu browsing places became unpalatable to eat ! A classic case of alleopathic communication in action!

Because Nyala was so small the kudu were forced to eat the tannin-laced leaves, and because their stomachs could not digest this material they became malnourished and in some cases died !

What an amazing world we live in! The longer I live the more amazed I am at the diversity of survival techniques that our wild plants and plants display. Who ever said that Biology was boring !

Dr Lynn Hurry is a Pietermaritzburg based writer/ publisher in Sustainability Education. Contact at lynn@ecology.co.za