Night of the strangling figs: a biological horror story

Imagine the scene. You are standing somewhere deep in the middle of a tropical rainforest. You’ve been there a while, you know the territory, and you feel safe. But no-one is truly safe in these most dynamic of habitats, and a tiny seed lands on your shoulder.

Slowly, it germinates and a tiny little root pierces into your flesh. The little plant is sucking the vital fluids from your body and using them to grow. Its stem winds downwards, coiling around your body, and soon it reaches the ground and takes root. Before long it has wrapped itself around your body and begun to squeeze. You cannot breathe! Everything goes dark.

This actually happens, in tropical forests all over the world. Except the victims are not people, but trees! Strangling figs begin their lives as semi-parasites, starting much like the British mistletoe, when a seed lands on a branch of a suitable host tree. Like the mistletoe, it then attaches itself firmly to the host plant, with organs called haustoria, which plug themselves into the vascular tissue of the host and suck out water, nutrients and sugars to feed the parasite. However, whereas the mistletoe is content to remain relatively small as it sits on the host tree, the strangling fig continues to grow.

In a British wood you can often see ivy plants snaking upwards along the trunks of trees, like oak. Ivy is not technically a parasite, it doesn’t draw food from trees. It simply uses them to gain elevation, although ultimately the weight of the ivy can sometimes help bring the tree down.

The host eventually gives up the ghost

The strangling fig also hugs the trunks of its host trees, but in this case, it starts near the top and works downwards, and hence it is roots rather than stems that snake downwards towards the ground. Once they reach the ground they take root there, and from this point on the strangler is no longer dependent on the host. It can take water and nutrients from the soil, and energy from its own leaves.

The host tree is now dispensable! The strangler’s roots encircle the stem of the host, fusing and merging into a lattice and eventually a rock-hard tube. Some species then choke the life out of the host by constricting its trunk. In other cases, the host may limp on for a while but still suffers from the strangler shading it from above and competing with it for water down below. Eventually it gives up the ghost. Either way, the dead host then rots away and on the spot where it stood now stands a monstrous strangling fig tree.

A killer stalks the streets of Kathmandu

Incredibly, this murderous strategy is employed by most of the numerous species of fig (genus Ficus) found throughout the tropics of the world. Our familiar fig (Ficus carica) is an exception to the rule. In Kathmandu many or most of the street trees are strangling figs, while others are in the process of being killed and replaced.

More remarkably still, this lethal strategy has evolved independently in two other unrelated families of tropical plants. Biologically speaking, it is similar to the strategy employed by parasitoid animals, which begin their lives as parasites on an unfortunate host before ultimately destroying that host and continuing without it. Ichneumon wasps are a good example, as is the titular Alien from the Ridley Scott films.

So, while the horrors we may imagine roaming the night during Halloween are entirely fictional, spare a thought for the poor forest trees that have much to fear from this very real botanical monster.

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