Winds of change blow for plant survival

The windscreen of my father’s Ford prefect was peppered with dead insects. Even short journeys, especially nocturnal ones, added to the carnage.

The distances we travelled in the 1950s were shorter, and vehicles much slower, than those of today, but insects no longer foul windscreens and radiator grills.

The decline in their populations has gone largely unnoticed and unlamented. Some people deem the loss a good riddance but, for insect-eating creatures, it’s a tragedy.

Nor is it only fish, frogs, and birds that suffer; spare a thought for plants which need insects to pollinate them. How will they cope in a world increasingly devoid of bees, wasps, and butterflies?

Not all flowering plants are equally vulnerable. According to a paper just published, some species have a second string to their bow. In the absence of insect carriers, they use an alternative, more ancient, means of distribution.

From early in their evolution, plants have relied on the wind to disperse their pollen.

‘Cast thy bread upon the waters; for thou shalt find it after many days’ was the approach. Random wind dispersal worked, but there was a downside. While some pollen ‘fell on good ground and sprang up, and bore fruit a hundredfold’, most of it ‘fell by the wayside’. Wind dispersal is a hit-and-miss affair, wasteful and expensive.

Over a hundred million years ago, some plants hit on a more efficient, and focused, way to get their genetic material out. Doing a deal with flying insects, they established a new courier service.

Offerings of juicy nectar induced the little aviators to visit them. When they did so, pollen became attached to their bodies and was carried to other plants.

Nectar bribes became increasingly focussed, persuading the visitors to concentrate on particular host species, ensuring that pollen went to desired destinations rather than not-known-at-this-address ones.

The loyalty card, beloved of supermarket proprietors, had arrived. Flowers were the first helicopter pads, and their powerful fragrances the original air traffic control systems. They would, in time, become the most diverse structures in nature.

But insect pollination seems to have disappointed some plant lineages; at any rate they reverted back to dispersal by wind. According to David Timerman and Spencer Barrett of Toronto University, this has happened on at least 65 occasions. About 10% of flowering plant species use wind for fertilisation.

Given the growing ‘pollination crisis’, this tendency may become more widespread in the future, so Timerman and Barrett are examining the plant mechanisms involved. Their results appear in a paper just published.

Stamens are the ‘male’ pollen-bearing organs of flowers.

Those of wind-dependent plants are long and flexible; they vibrate vigorously when air currents are strong.

The Toronto researchers focused on a typical flowering species, tall meadow-rue, native to North America.

They used an electronic shaking device to measure vibration in the stamens and placed specimens in a wind tunnel to simulate conditions in the field. Meadow-rue stamens, they found, vibrate readily and have a particular resonance frequency.

Wind, the researchers conclude, is a ‘more consistent agent’ for pollen dispersal than insects whose numbers tend to fluctuate erratically. With the decline in insect populations worldwide, fall-back mechanisms, such as the one used by meadow rue, are likely to become increasingly important for plant survival.

David Timerman and Spencer Barrett. Divergent selection on the biomechanical properties of stamens under wind and insect pollination. Proceedings of the Royal Society B: Biological Sciences. 2018.