Wrapping bridges for mayfly conservation?!

May 18, 2011

Guest author: BioFresh partner Szabolcs Lengyel (Assistant Professor of Ecology, University of Debrecen, Hungary) takes inspiration from the art world to suggest a novel solution to an unusual ecological problem for mayfly populations.

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What’s a mayfly to do when she meets a bridge?

One would assume that she flies over it. Or under it.

Still, according to a new study published in Journal of Insect Conservation, eighty-six percent of long-tailed mayflies (Palingenia longicauda) approaching a bridge never cross and turn back from it. To uncover the background of this peculiar behaviour, scientists from the University of Debrecen and Eötvös University from Budapest teamed up to study the flight of female mayflies on river Tisza in NE-Hungary.

The mayfly’s life is a fascinating one. Larvae develop in the riverbank for three years, then in one early summer afternoon they swim from their burrows to the water surface. Males come up first, moult on the water surface and fly to the riverbank for another, final moult. By the time they finish their second moult, female larvae emerge and undergo their own moult on the water surface. After mating with the males or escaping from them, female mayflies fly upstream. This flight presumably compensates for river flow and ensures that eggs deposited upstream reach the bottom where the egg-laying females had lived as larvae.

It is this compensatory flight which is interrupted by bridges. Because mayflies never actually touch the bridge, researchers focused on the optical properties of the bridge. It had been previously discovered that mayflies use a special signal of polarized light to identify water as such. Thus, the team performed measurements of polarized light at the bridge, enabling scientists to see with the eyes of the mayflies.

Ephemera vulgata © Cyril Bennett

The pictures clearly showed the water-identifying signal upstream and downstream from the bridge. But not at the bridge. The bridge disrupted the continuous band of polarized light on the water surface, easily fooling mayflies into believing they reached the edge of water and that it was time to turn back.

What happened to the mayflies that turned back from the bridge? After flying some distance downstream, they turned back and tried again at the bridge. Video recordings demonstrated that some mayflies attempt crossing the bridge several times in a few minutes. Energetic measurements showed that the repeated turning back may deplete the reserves of females, forcing them to lay their eggs just downstream from the bridge.

However, there is yet another mystery at the bridge. Counts of emerging larvae in three years suggested that there are roughly twice as many female than male larvae downstream from the bridge. Upstream from the bridge, however, there are more males than females. What is really happening at this bridge? Can this bias be related to the bridge’s barrier role?

Before 1941, when the bridge was built, all females could space out along the large bends of river Tisza. Ever since 1941, however, most females were restricted by the bridge to lay eggs only downstream from the bridge. This restriction could have been more serious to unmated females that did not participate in energy-consuming chasing by the males. These unfertilised females did not lose energy during mating and could have flown farther, had it not been for the bridge.

However, the bridge has been there for the last seventy years, forcing all females, mated and unmated, to lay eggs downstream from the bridge. If we assume an even number of male and female eggs for mated females and only female eggs for unmated females, a small surplus of female eggs and larvae will appear in every generation. With years, such small surpluses of female young downstream from the bridge may have thus led to the two-to-one ratio observed today.

Are bridges then evil and should we tear them down for mayflies? No. One solution may come from some plastics that are known to emit the same polarized light signal that aquatic insects use to identify water. Such materials may thus be used to cover some parts of the bridge so that there is a corridor over the bridge that mayflies can follow to fly over the bridge. After all, the Pont Neuf in Paris was covered in plastic once by Christo for art – why couldn’t this happen on river Tisza now for mayflies?