Pacific Views

Figs and Wasps

Humans are omnivores, we will and can eat just about anything given sufficient need. We can live in nearly any habitat, and even change our environment to suit us better. As we lose touch with where our food comes from, and the need to put up with other creatures, we can begin to think that we don't need anything that we didn't plant or raise ourselves.

Yet human actions can have indirect consequences. Many plants and animals don't have the ability to adapt to new climates and food sources. Some are so specialized in diet and habitat that the loss of only one other organism would mean the end of them.

And we do depend on them for our well being, even though it may not seem obvious. The tropical forests have been described as the lungs of the world, and a good number of those forests depend on figs and wasps to stay healthy. And by healthy, I mean self-sustaining.

Fig trees are a central food producing species for many animals. And the fig, as you may have guessed, is dependent on a singular relationship with tiny, stingless wasps that live the majority of their lives inside clusters of fig flowers.

First, Some Background

For those of us that live in northern latitudes, the idea of winter dormancy is a cornerstone of our conceptions of how nature works. But in tropical regions, both animals and plants are active all year round. Even the idea that you can always tell the age of a tree by it's rings falls short in the tropics.

In climates with a harsh winter, trees only grow significantly in spring and summer, and little or not at all during fall and winter. In spring, new cells are laid down rapidly and tend to be larger. In summer, the process slows and the cells are narrower. The intersection of dark, end of summer wood, with brighter spring wood gives the appearance of rings.

Tropical trees, by contrast, add girth steadily all year. They may get the odd ring during times of exceptional drought or wetness, but these could lead someone counting them to come to the conclusion that a tree you can't put your arms around is maybe 10 years old.

Because the food sources don't go dormant, neither do the animals. So the survival possibilities of an animal with no seasonal metabolic dormancy and whose diet is highly dependent on fruit, goes up. Fruit, of course, doesn't store well even if an animal were to try and hide it for later. And most fruit trees "mast", that is, they produce all of their fruit at a particular time of year.

It would be very bad for fruit trees in general if the animals that disperse their seeds were to starve. The whole point (to the plant) of producing fruit for animals is seed dispersal. That's their payoff for spending a lot of energy producing food that's intended for another organism. But in temperate climates, the only animals that have survived have had to adjust their rhythms to cope with the cold and lack of food. In the tropics, animals have figs.

The Lives of Wasps

This family of fruit trees flower and fruit all year in tropical climates. A fig tree is a central attraction for birds and mammals, with animal diets in some forests composed of up to 70% figs. But the fig flowers won't set fruit until they've been visited by a wasp species suited to that type of fig.

Figs produce a bundle of flowers in an inward facing pouch called a syconium, which is approximately the same general shape and size as the mature fig. The tiny hole at the bottom allows the correct female wasp driven by the scent to enter and lay her eggs. In the course of what will be her last act, she pollinates the fig flowers.

The male wasps hatch first. They fertilize the as yet unhatched females, and die, never leaving the syconium. When the females hatch and leave, the male flowers dust them with pollen on their way out. The fig then begins to ripen into a tasty fruit which will be spread far and wide by many animals.

The female's short life is spent looking for another suitable cluster of fig flowers. She may never even eat before laying her eggs and dying.

Delicate Balance

Such a total dependency between two species for reproduction and continuance poses interesting questions about the effects of human actions.

...Deforestation destroys figs, many of which take years to re-establish themselves. Insecticides kill off the pollinating wasps. A threat to figs also threaten the wide range of forest life that depends on them. Studies show that about 100-300 trees of one particular fig species is required to sustain a pollinator wasp population for 4 years. Because figs grow far apart, like all other rainforest plants, the area required for this many figs is about 800 acres. ...

A sufficiently thorough application of insecticide would be enough to ensure that there were no more figs in a given area. A sharp reduction in habitat size could do the same, gradually dropping the local wasp population below a sustainable threshold. The die-off of animals that eat the figs and spread the seeds would eventually result in a 'naturally' much reduced habitat over the long term, with the seeds of figs that fell in the shade of their parent going to waste.

As you can see, the loss of either figs or wasps would be enough to shatter a local food chain. Not only the animals dependent on the figs, and the predators dependent on those animals, but also other fruit trees that depend on these animals for seed dispersal would be impacted.

These are the kinds of natural systems and relationships that can't be replaced by replanting a few seedlings and hoping they do alright. These ecosystems are dynamic, complex, have a cast of thousands. And like many great dramas, they are much reduced by the loss of key actors.