In many areas, the Brood X cicadas that are emerging across a 15-state region are spottier in their prevalence than they were 17 years ago. Some neighborhoods have been inundated with the insects, while others are seeing only a few.

Why it matters: The 17-year periodical cicadas are one of nature's wonders, but the patchiness of the emergence, particularly in suburbia, points to one of the key threats to these unique insects: urbanization.

The big picture: America's cities have expanded at a rapid rate during the past few decades, with sprawl spreading out far beyond city centers. In Washington, D.C., a 2016 study using satellite data found a rapid expansion in impervious ground cover, such as pavement and buildings, from the mid-1980s through 2010.

• That trend has likely continued through the present day, says study co-author Xia-Peng Song of Texas Tech University.
• Much of the growth in the Washington region has occurred along and beyond the Beltway that rings the city, particularly to the west, in Tysons Corner and Springfield, as well as College Park and New Carrollton.
• Knocking down trees to construct new homes, businesses and parking lots can kill the insects or block their emergence, since immature cicadas feed upon their roots while underground, University of Maryland entomologist Paula M. Shrewsbury tells Axios.

What's happening: The two biggest threats to periodical cicadas, which emerge every 13 or 17 years, are urbanization and climate change, Shrewsbury says.

• "Even if they're close to the end of the [17-year] cycle, they can't get out from underneath the pavement. It's not like they can travel, you know 100 feet or 200 feet and find another way out, that's not happening," Shrewsbury says.
• There are already two periodical cicada broods that have gone extinct, one in Connecticut and another in Florida, she says, and scientists are keeping close watch on a group in Long Island. Urban development there may be wiping out that population, scientists fear.

Climate change also poses a threat to cicadas because warming temperatures could alter the timing of their emergence, or encourage interbreeding between 13-year and 17-year populations. That would reduce the number of cicadas emerging in any given year.

• And that's a big problem (for the cicadas), since the insect's main defense mechanism against predators is "predator satiation" — meaning there are so many cicadas emerging at once that predators can eat as many as they want, yet many of the insects will still survive long enough to lay their eggs.
• Cicadas help feed species ranging fromvarious types of small birds to turkeys and squirrels, with their decomposing exoskeletons helping to add nutrients into soils to boost tree growth.
• Perhaps more importantly, they've been successfully following this unique life cycle for generations, so any trouble now is a warning sign for humans.

Our thought bubble: The periodical cicadas are a reminder that we now live in an era known as the "Anthropocene," as human forces are re-shaping the planet. This is harming biodiversity, driving climate change, and having a host of other effects worldwide.

What's next: You can track cicadas in your area through the Cicada Safari citizen science app.