Ladders to Heaven – the Secret History of Fig Trees

It’s not often I read two books which make me want to proselytise about plants in quite such quick succession but Mike Shanahan’s quick read on fig trees is a no brainer.  I guess lots of people know part of the story about figs being pollinated by wasps but who knew that there are more than 750 species of Ficus, many pollinated by just a single, different, species of fig-wasp with a remarkable level of co-dependence? Or that figs form part of the diet of at least 1274 species of birds and mammals?

Shanahan makes an enthusiastic case for figs as keystone species in tropical regions throughout the world.  Their vigorous growth and multiple annual fruit crops allow them to support many birds and herbivorous animals.  These, in turn, disperse the seeds of other plants they eat onto the forest floor beneath the figs and support thriving and diverse ecosystems.

One of the reasons for their widespread success is that fig trees come in all shapes and sizes – the smallest are excellent colonisers of bare ground, producing strong, rapidly growing roots from tiny, easily-transported seeds. 

Fig tree bearing fruit on stony ground near Ghar il-Kbir caves, Malta, April 2015

Figs were amongst the earliest plants to colonise both Long Island off Papua New Guinea and Krakatoa after devastating volcanic eruptions and still play a key part in the flora of both islands today. ‘Strangler’ figs are opportunists of a different sort – they hijack mature rainforest trees to reach the precious sunlight they need.  Seeds deposited by birds in damp hollows high up on a mature tree germinate quickly, producing leaves to harvest the light and rootlets to access water and minerals held in the host tree’s bark.  Very soon the seedling starts to produce much longer aerial roots, which travel downwards until they reach the ground where they thicken and lengthen, taking up water and minerals directly from the soil and eventually encasing the host tree, limiting its ability to grow.  Even when the host tree eventually dies the strangler fig can thrive using the dead trunk as scaffolding to keep its leafy crown high up in the canopy.

 So, what is it that puts such very different species into the same biological genus?  Perhaps the most obvious common characteristic is the way figs are pollinated which, in turn, is the result of and reason for their distinctive, urn-shaped ‘fruits’. Because fig flowers are so well hidden inside this ‘urn’, it wasn’t until the 19th Century that botanists realised that the trees do reproduce by flowers rather than spores of some sort. The process of pollination is a complex one, with nearly all fig species pollinated by a single species of tiny fig-wasp.  In this case of the Sycamore fig, Ficus sycomorous, which grows wild in much of tropical Africa, the partner wasp is Ceratosolen arabicus.  

Male C. arabicus are born and spend nearly all their lives inside fig ‘fruits’ and even females, which must leave one fig tree and travel to another to carry out pollination, also spend all but around 48 hours of their life inside a fig.  Each tiny female wasp, carrying fertilised eggs and pollen, emerges from her natal fig and flies until she finds another Sycamore fig tree with younger figs, still in need of pollination – she is attracted by a specific mix of volatile organic compounds secreted by the hard, green figs.  She has a wedge-shaped head and jaws which function like a rachet to allow her to force her way into a tiny hole at the tip of an unfertilised fig and then into a narrow tunnel. This leads to a central chamber lined by a carpet of flowers.  As the wasp walks over the flowers she sheds pollen picked up from her birth fig, allowing each flower to develop into a miniscule fruit containing an even smaller fig seed – what we think of as the edible fig fruit is actually an enlarged, fleshy stem (the syconium) carrying many fruits.  


Left: Fig syconium showing the hole where wasps enter and leave.  Right: Mature syconium – the ‘pips’ are the true fruits

Not all flowers will develop into seeds, though – in some the wasp lays her eggs, and these will develop into galls which nurture the larva until they metamorphose into adults.  The male offspring emerge first, chewing their way out of their gall after a few weeks then digging through the flowers to find galls with female wasps inside.  They gnaw a hole in the gall and fertilise the female whilst she is still trapped within – who said romance is dead!  In some fig species the female collects pollen passively on her way out of the fig but, in Sycamore figs, males and females wasps work together to harvest and collect pollen into pockets beneath the female’s thorax.  The male wasps then cooperate in a unique act of altruism, chewing a hole in the fig wall through which they fall to their deaths so that the females can escape and fly to a new tree anything up to a remarkable 160 km away, starting the cycle all over again.

Of course, it doesn’t always go quite so smoothly – there are often ants waiting to catch female wasps leaving a fig and other wasp species can enter figs and lay eggs without bringing pollen. Some even inject their eggs into the figs from outside and, when their larvae hatch, they feed on the pollinator-wasp larvae.  Ironically, though you’d think it would have a negative impact on fig dispersal, this complex web of invertebrate interactions is one reason why figs are so important in supporting entire tropical ecosystems.  The other reason is that many species produce figs all year round rather than just at a specific time of year. This is essential because of the short life span of fig-wasps, which need to have immature figs to lay eggs their eggs in within a short time of leaving a mature fig.  The benefit to other birds and animals is obvious, making figs often the most important fruiting plants in rainforest.  Having a range of different figs only enhances the reliability of the food source.  

Deforestation and all that comes with it are hardly new problems but, because of their role as a keystone resource, fig trees can be used very effectively to repair some of the damage.  Since the 1970s, Wangari Maathai has been planting native trees such as the Sycamore fig to re-green damaged land in Kenya and, more recently, what is known as the ‘framework species approach’ has been very effective in restoring degraded forests both in north Queensland and in Thailand’s Doi Suthep-Pui national park.  Ensuring that at least one in five seedlings planted when restoring land is a species of fig is key.  Figs grow rapidly, shading out weeds, and produce fleshy fruits to attract seed-dispersing animals from nearby patches of intact forest.  These animals bring with them the seeds of other plants, which can then germinate and thrive in the weed-free shade provided by the fig trees allowing the ‘natural’ species composition to regenerate. 

Any biologist will see this as excellent news but, in this time of climate emergency, locking away carbon in rapidly-growing, restored forests must be a priority for all of us.  In an attempt to make this feasible on a large scale, Thailand’s Forest Restoration Research Unit (FORRU) are developing ways of dropping fig seeds in hydrating gel into remote sites using unmanned drones and also hope, in the future, to be able to use drones to collect wild seed from deep inside inaccessible forest.  It sounds like a more appropriate use of drones than to somehow replace the bees we are killing with insecticides!

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