What environmental impact did 17-Year cicadas leave across northern Illinois?

After the deluge and deafening roar of billions of 17-year cicadas across northern Illinois, the question arises, what happens when they all fall silent?

The answer may be found not in the tall trees above, but in the entangled ecosystems of interdependent microbiomes thriving underfoot.

At Morton Arboretum’s Root Biology Lab in Lisle, research program manager Marvin Lo is leading a team crushing and pulverizing hundreds of of deceased Brood XIII cicadas down to their basic elements.

“How much are the cicadas really contributing to our forests?” Lo asked. “Do their dead bodies create such huge pools of nutrients that without them our forests would be drastically changed? Would the complex soil and forest ecosystems be altered if the cicadas disappeared?”

The lab’s research is part of a hypothesis that the excess amount of nitrogen and carbon contained in billions of dying cicadas creates a nutrient pulse, providing super-charged fertilizer to the microbiome ecosystems of forest soils and the thirsty tree roots slurping up all the nutrients they can get.

“Cicadas, like most insect shells and bodies, are made up of a lot of chitin, which contains a lot of nitrogen and carbon,” Lo said. Nitrogen and carbon are crucial nutrients to promote growth in trees and plants. They help enrich the soil and feed the bacteria in the soil. The whole ecosystem functions together.”

To explore their hypothesis, the team created rectangular plots throughout different forest patches on the Arboretum’s grounds to collect decaying cicadas and their shells. The team also is examining soil nitrogen samples, measuring tree stem and root growth and measuring tree fruit/seed production.

“Cicadas, like most insect shells and bodies, are made up of a lot of chitin, which contains a lot of nitrogen and carbon. Nitrogen and carbon are crucial nutrients to promote growth in trees and plants. They help enrich the soil and feed the bacteria in the soil. The whole ecosystem functions together.”

—  Marvin Lo, research program manager at Morton Arboretum

The first step is measuring the amount of biomass the dead cicadas and their shells actually provide to each plotted quadrant. This helps calculate the size of their potential impact on the soil. Hundreds of deceased cicadas are carefully collected and weighed back in the lab.

Lo said that the second step is the fun step. The team pulverizes their collected samples down into tiny folded boats for use in a beastly machine called “The Elementor.” The machine is a carbon and nitrogen analyzer, burning at special wavelengths to absorb the carbon and nitrogen, revealing their ratio content number.

Once the average biomass and carbon and nitrogen content per plot is calculated, the researchers can begin to examine the soil to see if the added nutrients are indeed making a statistical difference in tree growth and soil health.

The team has been monitoring their forest plots for five years before the emergence of the 17-year cicadas to establish a baseline pattern in their data. They factor in the measurement for different weather patterns like droughts, which result in diminished tree stem and root growth. All is set to see if the cicadas cause any statistical downstream effects later this year, next year, and even five to 10 years down the line.

Lo said there are intricacies to forest ecosystems making them more challenging to account for all the variables, but even more fascinating to try to understand.

“There’s fallen trees with growing fungi and everything is in a constant state of growth and decaying,” Lo said. “[The lab] studies fallen leaf inputs to see how they affect the soil and tree growth. The ecosystem is so complex with insects and animals above ground. Underground there’s insects feeding on microbes, microbes feeding on other microbes, fungi feeding and spreading; we still don’t understand how it all fully works.”

Lo said it’s an interesting concept to think of cicadas as potential stewards of their ecosystem. Because Brood XIII cicadas are dependent on a single tree for the vast majority of their 17-year life span, mostly as nymphs feeding off the tree roots’ nutrients underground, adults tend to lay their eggs on mature trees they know will continue to be healthy over the next two decades.

Brood XIII cicadas were noticeably fewer in neighborhoods with recent construction, vast areas of concrete, and disturbed areas with few mature trees.

“It is possible if we see a decline in cicadas it may be an indication of poor soil health or poor plant and tree health in that area,” Lo said. “We need to look out for signs that our environment may be changing in a way that is negative for nature and for us; tree health is so integral to human health.”

Lo said we can gain a greater appreciation for the living world around us by seeking to understand just how interconnected all the complex systems designing our shared existence truly are.

“Even though it’s hard to see these connections, from people in suburbs, to people in cities, everybody is a part of nature,” Lo said. “Cicadas affect us because we’re a part of the environment. Some people might see cicadas as a nuisance, but they may be vital to both us and the health of our forest ecosystems.”