Plants can’t run away from herbivores who want to eat them. Lucky for them, they have some crafty friends…
Some of the fiercest battles in the natural world are fought between plants and their arch-enemies, the herbivores. Since plants are rooted to one place and cannot run away when herbivores approach, they have evolved clever ways to try and stay safe. Some bear nasty thorns and spines, while others produce toxic chemicals. Many herbivores prefer soft and juicy young leaves to tough mature ones. Therefore, some plants trick them by shifting the timing of producing young leaves so that it does not match with seasons when herbivores like caterpillars are plenty.
But herbivores are not so easily defeated. They too have found ways to counter the plants’ defences. Some herbivores are able to happily eat plants with thorns (have you seen a goat tackle a thorny shrub?), others have developed resistance to plant toxins (like certain caterpillars that can feed on toxic plants such as milkweed). So now plants need further defence strategies. And thus, even to this day, the arms-race between plants and herbivores continues.
In some tropical forests of the world, plants have evolved a most ingenious strategy to combat their herbivore enemies. They have struck up a friendship with one of the most ferocious and powerful group of animals, ants! Many ant species have sharp mandibles that can give a mean bite. Also, since they often work in groups, ants can overpower other animals that are much bigger than them. These ‘ant-plants’ therefore attract ants to stay on them by providing special shelters or food, or both. The ant-shelters they provide are called ‘domatia’ (pronounced do-may-shee-ah) which are usually a modification of a plant part. For instance in some Acacia plants, the thorns are huge, with hollow chambers where ants can nest. In the ant-plant that I know best, called Humboldtia brunonis, sections of a branch swell up and become hollow to form domatia, and a gate-like opening appears at each end.
The food that ant-plants provide for ants may vary: the leaves may either bear small nutritious globules, or the plants may produce a special sugary nectar on the leaves or other parts. The ants live in the domatia and eat the special foods, and dutifully patrol all over the plant. If they encounter a herbivores such as a caterpillar, the ants bite the caterpillar till it drops off or dies. Some ants that live on Acacia trees in Africa are so ferocious that they can even swarm and chase off huge animals such as giraffes and elephants who come to eat the leaves. Other ants are so meticulous in their job that they not only ward of herbivores, but also other plants that grow close to their home plant, or touch their home plant!
I don’t know about you, but I am pretty happy I have ants in my plants. Such a relationship between two organisms which benefits both of them is called mutualism. A nice way to be, right?!
A double advantage!
Humboldtia brunonis is a small tree that is a Western Ghats endemic – which means it is found nowhere else in the world! Joyshree's research on these trees in the forests of southern India has found that besides protection, the ants may also provide nutrition to the host plant. Ant carcasses and excreta, which accumulate in the domatia, are a source of nutrients that the host plant can absorb. So providing a home for ants gives the plant a double benefit! Non-ant interlopers sometimes also crawl into the domatia, and also provide extra nutrition to the plant. Some of these interlopers are earthworms. That's right – tree-climbing earthworms, isn't that wonderful?
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