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What the cicadas will leave behind

This story is part of Down to Earth, a Vox reporting initiative on the science, politics, and economics of the biodiversity crisis.

In the coming days, billions of cicadas will erupt from the earth across parts of the eastern US, crawl up trees, shed their skin, and begin a loud hunt for mates.

These particular insects are part of a group known as Brood X that emerges once every 17 years. And when they do, the lacy-winged critters hang around for two to four weeks before dying, assuming they don’t get nabbed by birds, pets, or a hungry zoo animal first.

But while their lives in the sun might be just a noisy blip, the members of Brood X will certainly leave a mark on forests from Tennessee to New York.

As researchers are learning, an eruption of periodical cicadas can send ripples throughout the ecosystem that alter the population dynamics of predators like birds and perhaps even fuel the growth of plants and trees. And, remarkably, some of those effects seem to last for years.

An adult periodical cicada.
Getty Images

Why cicadas flood the forest

For most of their lives — either 13 or 17 years, depending on the type — periodical cicadas are out of sight, residing underground and slurping up fluid from tree roots. Then, when the soil temperature hits about 64 degrees, they emerge by the billions, just like that.

This mass eruption, scientists believe, is strategic.

“They effectively satiate their predators,” said Louie Yang, an entomologist at the University of California Davis who has studied cicada ecology for years.

In other words, their defense strategy is to flood the forests so that predators, from blue jays to squirrels (and, during these eruptions, everything in between), become so full that they literally can’t stomach another bite. That leaves plenty of insects to mate and lay eggs that will become the next generation of 17-year cicadas.

This approach seems to work for cicadas, and it certainly satisfies birds.

A cicada surge likely drives an increase in birds

In the animal kingdom, a full stomach is a big deal. With more food, you often see more babies — even, apparently, if the supply only lasts a few weeks.

“Following [cicada] emergences, you do tend to get an increase in a lot of the apparent avian predator populations,” said Walt Koenig, an ornithologist at Cornell University and research zoologist emeritus at UC Berkeley.

What the cicadas will leave behind

Periodical cicadas emerge from their exoskeletons on May 16, 2021 in Takoma Park, Maryland.
Chip Somodevilla/Getty Images

One analysis, co-authored by Koenig and based on 37 years of population data for 24 predatory birds, found that cicada eruptions “significantly influenced” almost two-thirds of them. Eight species, such as the red-headed woodpecker and the common grackle, saw a population bump of 10 percent, on average, following the brood emergences — “presumably attributable to high survivorship or reproductive success.”

(The number of some other species actually fell, likely because those particular birds left the scene after filling up on cicadas, resulting in lower counts.)

Most interestingly, many of these knock-on effects lasted for years, Koenig said. The number of blue jays, for example, was significantly higher even three years after the cicada eruptions.

“These results indicate that, at least in some species, the effects of cicada emergences are detectable years after the event itself,” the authors wrote.

It wasn’t a huge surprise to Koenig that some birds benefit from cicada eruptions. But he did find something in the data that really caught his attention: Fewer birds overall tend to be around when — and where — cicadas emerge. “Bird populations are recorded as being relatively low during emergences,” he said.

That’s surprising, he added, because “you’d expect birds to really home in on these things.”

Koenig’s quest to explain this hasn’t yet turned up anything conclusive. The most intriguing theory, he says, is that cicadas have somehow influenced bird populations over centuries of evolution so there are fewer predators when they emerge. “It is possible,” he said.

Cicada carcasses fertilize the forest floor

The vast majority of cicadas are able to evade predators like birds and the occasional dog. They go on to mate, lay eggs, and eventually fall, as prickly carcasses, to the forest floor.

There they become a natural fertilizer, and it’s laid on thick. As Yang’s research suggests, a cicada burst can increase the biomass of microbes and the availability of nitrogen, an important nutrient for plants, in the soil.

Yang ran one experiment where he fertilized American bellflowers, a common understory plant, with dead cicadas, and found that the plants grew larger, produced bigger seeds, and had higher concentrations of nitrogen in their leaves. (He also found that they were more desirable to plant-eating mammals and insects.)

What the cicadas will leave behind

The empty skins of periodical cicada nymphs, following a 2007 emergence in Willow Springs, Illinois.
Scott Olson/Getty Images

It’s much harder to prove the impacts of cicadas on trees. One three-year experimental study Yang co-authored, however, found “that cicada fertilization strongly increased tree growth in the year of emergence,” and those changes persisted even two years later.

“The biggest effect to growth was in the first year, but the difference that [the fertilization] caused was maintained throughout three years,” Yang said.

Other observational studies that rely on correlation support the idea that dead cicadas fertilize the forest floor and might increase the size of trees years later as a result, though in general, they tend to show mixed results.

Does that mean that cicadas are good for trees?

In short, no. Some researchers argue that, if anything, periodical cicadas actually negatively impact trees.

It’s easy to forget that periodical cicadas are there, in the soil, all the time. For years, they’re parasitizing trees — sucking nutrients from their roots (specifically, something called xylem fluid), which those trees could otherwise use to grow. When they emerge, the cicadas lay eggs in twigs, causing the twigs to wilt or die. And there are loads of cicadas, mind you, perhaps as many as 580 per square meter in some places.

“They’re impacting their host trees almost certainly,” said Richard Karban, an entomologist at UC Davis who worked with Yang on the tree study. “They probably do have a fertilizing effect, and they probably also have quite a large negative effect.”

There’s some indication that female cicadas can harm young or weak trees when they deposit their eggs, so it could be worth holding off on new plantings.

What researchers still don’t know is how large those effects are, and for how long, exactly, they last. It’s possible, for example, that the changes in tree size could actually get more pronounced over time, Yang said.

And that’s just it: When it comes to these insects, there’s still plenty of mystery, partly because they live the majority of their lives underground and only emerge every so often. To put the time scale into perspective, the last time the 17-year Brood X cicadas sprang forth, in 2004, the original iPhone was still three years away from being unveiled.

“Cicadas are notoriously difficult to study,” Yang said. “There are things that we are just starting to understand.”

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