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Cicadas leave bounty of nutrients for forests

Even in death, insects help ecosystem thrive

By Marsha Walton

The insects have bright red eyes and tiny hooks on their legs. They don't bite or sting.
The insects have bright red eyes and tiny hooks on their legs.
Cool Science
United States

(CNN) -- Every 17 years, billions of cicadas cause a loud stir in almost one third of the United States. Scientists now say the insects also leave a lasting and positive impact after they die.

"Even as dead bugs they are still influencing these forest ecosystems," said Louie Yang, whose research is published this week in the journal "Science."

Benefits of the cicadas-as-fertilizer include faster growing trees, and bigger seeds in some flowers for several years following the cicadas emergence.

Scientists call short, dramatic bursts of new resources, like the billions of cicada carcasses, "resource pulses." But unlike other pulses, such as those that occur randomly after El Nino year rainfalls, researchers can predict the appearance of cicadas almost to the day. That makes this phenomenon a lot easier to study.

"I think these are pretty charming and mellow insects," said Yang, a graduate student in ecology at the University of California at Davis. "They don't move very quickly, they don't sting or bite, and for the most part they are very easy to handle and to touch to observe in nature," he said.

Cicadas have a strange and fascinating life cycle. While flies and mosquitoes live for just a few weeks, cicadas, depending on species, spend either 13 or 17 years below ground in the nymph stage, feeding on plant roots.

During the few weeks they are above ground, there's a loud and frantic effort for the insects to mate, and for the females to deposit their eggs in trees.

"When you get thousands, or millions or billions of them together in one spot, it is a fantastic, deafening chorus," Yang said.

The most noted cicada population, known as Brood X, emerged last spring and summer in the eastern United States, from Georgia north to Pennsylvania, west through the Ohio River Valley. When the eggs hatch, the nymphs fall to the ground and burrow up to two feet below ground for their very long growth period.

While they are above ground, they become an unbelievably abundant food source for birds, lizards, snakes and fish. The massive number of cicadas means that predators can only eat about 15 percent of them.

As part of his research, Yang and others gathered tens of thousands of insect carcasses to see what impact they would have fertilizing bellflowers, a plant that's found in roughly the same region as the cicadas.

"What we found was that these plants are actually taking up nitrogen that comes from cicadas," said Yang. The seeds of the insect-fertilized plants were also 9 percent bigger than those in a control group.

Yang said recruiting field volunteers to gather insect carcasses can be a challenge. He's recruited family, friends, high school students, and strangers, who responded to a newspaper ad. And he says they soon become fans of the small creatures.

"These cicadas are everywhere, so everyone knows them. All the people on the street know your bug, and they often know the different species. The people I meet know a lot about the insects, and they often ask me really challenging questions about cicada biology," Yang said.

It's the safety in numbers that has kept the cicada population thriving since the last Ice Age. As individuals, Yang says they can be somewhat klutzy.

"They are not very agile fliers, and they don't seem to be very skilled at avoiding predators, " Yang said.

Especially with such an intriguing insect, Yang says it is important to study their entire life cycle, because they are fertilizing both below ground food webs as well as above ground populations.

"When things die a lot of people stop paying attention to them," he said.

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