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Last Updated: Saturday, 27 November, 2004, 12:50 GMT
Death deluge makes forest mulch
Cicada, Yang
The insects emerge from below ground and swarm over the landscape every 13 or 17 years
The nutrient boost given to North American forests when trillions of cicadas die en masse is explored in a new study in Science magazine.

The insects emerge from below ground and swarm over the landscape every 13 or 17 years to briefly sing and mate.

Their decomposing carcasses will increase bacteria, fungi and nitrogen in forest soils, Louie Yang shows.

This may explain why trees grow faster following cicada emergences, the University of California scientist says

"This observation doesn't necessarily mean the pattern comes from cicada 'fertilisation'; it's one possible hypothesis," Yang told BBC News.

"Another explanation says adult cicadas cause damage to trees that makes more light available in certain parts of the forest; but it's certainly a very interesting pattern."

Soil changes

The eastern states of the US have several broods of periodical cicadas (Magicicada spp).

1. Female lays eggs and dies soon after. Eggs hatch.
2. Bugs or 'nymphs' drop to the ground
3. Nymphs live underground feeding on tree roots
4. After 17 years, nymphs tunnel to surface, crawl up trees and shed skins to become adults
5. Adults mate during May and June of 17th year
The insects spend most of their lives as juveniles, feeding on the xylem in tree roots and growing slowly.

Then, timed to perfection, huge populations come out together to reproduce.

May and June of this year saw the emergence of the so-called Brood X, which swarmed over more than 10 US states, including Georgia, Indiana, Kentucky, New York and Ohio.

The cicada onslaught lasts only a few weeks - but is deafening. The males sing to their partners with high pitched trills, made by vibrating abdominal drums called timbales.

The mass emergence can be extraordinarily dense; up to 350 insects per square metre. It provides a feast for a host of other animals - including birds, racoons, foxes and skunks - but they cannot hope to eat all the cicadas and most will simply die and drop to the floor to rot.

Louie Yang set up experimental plots to examine the impact on the forest system of this death deluge.

His work demonstrates how the mass of carcases leads to a dramatic increase in fungal and bacterial decomposers in the soil. This in turn triples soil ammonium, and more than doubles soil nitrate concentration.

Seed size

"We can see a trend which shows that increasing the density of dead cicadas increases the amount of soil nitrogen in the ecosystem - it creates a pulse of nitrogen in the soil," Yang said.

Cicada, Yang
The cicada onslaught lasts only a few weeks - but is deafening
This rush of soil nutrients can be seen in the fillip it gives to a forest-floor plant, the American bellflower (Campanulastrum americanum), which grows across the range of the cicada emergences.

The plants in Yang's plots took up more nitrogen and increased the size of their seeds by 9%.

The cicadas are, of course, just returning nitrogen they "stole" when they were sucking on plant roots as nymphs. And the cycle will always show something of a deficit because of the substantial numbers of cicadas that are lost to the system through predation.

Nonetheless, it is an eye-catching observation that tree-ring growth data appears linked to the insects' cycle.

Cicadas, Yang
The mass emergence can be extraordinarily dense
"A couple of studies have found a robust result that there is a decrease in the growth of trees during cicada emergence years, but one of the results in more recent research suggests there may be increases in the growth rate of trees in some of the years following cicada emergence."

He added: "There's an awful lot we don't know about the ecology that happens in our backyard - and these are insects that truly do occur in our backyard; this year they were flying into our cars and bumping into us as we crossed the street.

"These are insects that are everywhere but there is still a great deal we don't know about them."

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