Warmer wasps could mean fewer figs

By Tim Radford

High temperatures could have devastating effects on some of the most important trees in tropical ecosystems.

A global average temperature rise of 3°C might not do lasting harm to a tropical fig variety – but if it killed the little creature than pollinates it, there could be problems.

There are more than 750 known varieties of the genus Ficus, or fig.

Each is home to dozens, perhaps hundreds, of creatures, and figs provide year-round food for a huge range of birds and animals, including orang-utans, fruit bats, capuchin monkeys, at least one species of parrot, pigeons, hornbills and the caterpillar of the giant swallowtail butterfly.

But each fig is dependent on one species of fig wasp to pollinate it. The wasp lays her eggs in the syconium – the botanist’s name for the fig fruit, which is not strictly a fruit, but an inflorescence, a complex enclosure of flowers and seeds – and scrambles out, taking pollen with her to the next syconium.

The little wasp doesn’t have long to work. Some adult females live less than 24 hours. So in extreme conditions the relationship could be a precarious one.

Researchers believe fig trees could be extremely vulnerable to anthropogenic global warning (Pic: RuTemple)

Nanthinee Jevanandam of the National University of Singapore, and colleagues, asked themselves what would happen to this reciprocally obligate mutualism – their phrase for the intimate evolutionary relationship between fig and pollinator – if global temperatures started to soar.

Singapore is only about 85 miles from the Equator, and has a stable climate, ranging on average from 27°C to 31°C, and occasionally soaring to 36°C. There are no seasons, as people in temperate zones understand the term.

They report in Biology Letters, the journal of the Royal Society, the UK’s national academy of science, that they took more than 4,000 wasps of four species, and 240 syconia, and exposed the wasps to increasing temperatures in laboratory growth chambers.

They had expected the conditions in Singapore would tend to select for a narrow range of heat tolerance. It looks as though their hunch was right. They toasted the wasps tenderly at a range of temperatures between 25°C and 38°C and in a variety of humidities.

Chain reaction

They recorded wasp survival at two hour intervals, but above 36°C three of the pollinator species died within two hours. The lifespan duration for all species fell steadily with increasing temperature.

The message is that, with average temperatures increasing, and with ever higher day time maximums, fig wasps could be in for a hard time. Some of them fly ten kilometres or more in search of receptive host figs: once the heat is on, they would have less time to find a host syconium to pollinate and lay eggs.

Fewer fig wasps would mean lower pollination levels, which would mean reduced fig populations, which would mean less food available for forest fruit-eaters, which in turn would mean secondary population declines.

This was a laboratory experiment: it provides a guide to things that could happen in the real world, but only a guide. The authors concede that individual wasps may vary in their ability to adapt to temperature variation; and they concede that even on the same tree, different syconia would represent different microclimates.

But they argue that three of the wasp species they tested represented three very different evolutionary groups, and they think their results could offer a general guide to other fig wasps.

“This implies that fig populations, not just in Singapore but in the entire aseasonal tropics, could be extremely vulnerable to anthropogenic global warning”, they warn.

“Because of their ecological importance, any loss of fig species or reduction in their abundance would be of major conservation concern.”

This article was produced by the Climate News Network

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