Warming wasps pollinate poorly

Researchers from Uppsala University and elsewhere have been studying the effect of rising temperatures on the lifespan of pollinating fig wasps. Their findings, published in Ecology and Evolution, show that the wasps lived much shorter lives at high temperatures, which would make it difficult for them to travel the long distances between the trees they pollinate.

Lisette van Kolfschoten and colleagues found that all fig wasp species showed a dramatic decrease in lifespan with increasing temperatures. This brevity is going to be a problem as the planet warms. Temperatures may rise by as much as 4°C by the end of the 21st century. The authors write: “A 4°C increase in temperature from the daytime mean 26–30°C reduced pollinator wasp median lifespan to 67% for some species. However, temperatures fluctuate during the day, and a 4°C increase could lead to daytime temperatures frequently reaching 33.8°C… This would severely impact fig pollinator lifespan, reducing the median lifespan to merely 20%–46% of the current lifespan.”

Wasp lifespans are critical for figs and so for forests in the tropics. Fig trees are vital components of tropical forests worldwide, as they ensure a food source for forest birds and mammals, even during periods when other plants do not bear fruit. Fig trees produce fruit all year round, but only if the fig is first visited by its pollinating fig wasp. Each species of fig tree is pollinated by its own species of fig wasp. Fig wasps are only two to three millimetres long and live an average of two to three days, but often fly 10 kilometres or more to get from a ripe tree to a flowering tree.

The researchers used experiments in which the fig wasps were allowed to spend their entire adult lives in carefully controlled temperatures. All five pollinator fig wasp species studied lived shorter lives at higher temperatures. When the temperature rose to 36 degrees, the wasps only lived an average of two to ten hours.

In addition to a warmer future shortening the lifespan of the wasps, deforestation of tropical forests is increasing the distance between fig trees, making pollination more difficult. Fig wasps might be able to avoid higher temperatures by adapting their behaviour, such as flying at night when it is cooler. If the fig wasps are no longer able to pollinate, fig trees will not be able to produce fruit. This could have significant consequences for all animals in tropical forests that depend on figs for food.

During the study, the scientists collected ripe figs from five different species of fig trees found in Panama: Ficus obtusifolia, Ficus citrifolia, Ficus popenoei, Ficus insipida and Ficus maxima. The figs were opened in the lab, and the wasps that had developed inside the figs were kept in climate chambers at different temperatures. The number of wasps still alive was counted about every four hours. In total, the researchers tested over 40 000 wasps from about 400 figs.

van Kolfschoten and colleagues conclude: “The projected local temperature increases in Panama could seriously decrease the lifespan of fig tree pollinators. By reducing the lifespan of fig wasps and therefore the chances of successful pollination of fig trees, increasing temperatures add an additional threat to this keystone resource of tropical forests. Anthropogenic ecosystem changes in the form of habitat destruction, fragmentation, and temperature increases, require species to cope with new situations. Particular attention should be paid to species in tight mutualistic relationships as they are vulnerable also to effects on their mutualistic partners.”

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van Kolfschoten, L., Dück, L., Lind, M.I. and Jandér, K.C. (2022) “Rising temperatures threaten pollinators of fig trees—Keystone resources of tropical forests,” Ecology and Evolution, 12(9). https://doi.org/10.1002/ece3.9311