Even in death, the 17-year cicadas made their mark.
Their decaying carcasses gave a super-size boost in nutrients to forest soil and stimulated seed and nitrogen production in a plant important to the forest ecosystem, researchers reported in Friday’s issue of the journal Science.
The findings might explain why tree growth increases for several years after a major cicada emergence, experts said.
Bard College professor Felicia Keesing likened it to someone pouring a pound of fertilizer per square yard over the forest floor. She co-wrote an article accompanying the research paper on the impact of cicada carcasses on soil and plants.
Louie H. Yang, a graduate student at the University of California, Davis, conducted the studies in 2002, 2003 and 2004 in cicada-infested areas in the eastern United States.
Last summer’s infestation was the largest, with 17-year cicadas known as Brood X inundating the mid-Atlantic region and filling forests from Georgia north to Pennsylvania and west through the Ohio River Valley to the Mississippi.
The beady-eyed insects spawned a public fascination — along with a multitude of cicada recipes and worries about the insects’ impact on trees, plants, pets and even driving visibility.
While most worries weren’t borne out, scientists have long been concerned about the impact of such outbreaks in which there is a sudden hyperabundance of organisms with potentially vast consequences for ecosystems.
Andrew Sugden, Science’s international managing editor, said in an interview from Cambridge, England, that Yang’s research was significant because “it quantifies for the first time what is likely to happen when you get large outbreaks of insects all at once,” like the swarms of locusts that recently hit portions of Africa.
Following the nitrogen
In his field studies, Yang experimented with naturally occurring densities of cicada carcasses of as much as 300 bugs per square yard. For each density he measured the soil’s nitrogen and bacterial and fungal growth over varying periods of time after carcasses were applied.
Soil content of a form of nitrogen used by plants was many times higher — 199 percent to 412 percent — in ground littered with cicada carcasses. Bacterial and fungal growth also increased.
Yang also measured the nitrogen content in the leaves of bellflower plants, with and without cicada carcasses applied to their soil, as well as bellflower seed production. The bellflower is a forest floor plant common to the geographic range in which cicadas occur.
He found bellflowers supplemented with cicada carcasses produced seeds 9 percent larger and foliage with a nitrogen content 12 percent higher than bellflowers without cicadas.
“The nitrogen itself has an unusual isotopic signature that suggests it’s actually from the cicadas themselves,” Yang said in an interview.
Explaining other data
Yang’s findings that decaying cicada carcasses apparently stimulate a rush of soil nutrients might explain why other analyses have shown that tree-ring growth among oaks in areas infested with 13-year and 17-year cicadas increased for the first four years after cicada emergence, Keesing said.
Cicadas spend most of their time underground as nymphs, sucking on tree roots and diverting some of the nitrogen that would otherwise go to the plant. In their last few months they emerge from the ground, crawl up trees and shed their hard skins. Over the next few weeks they sing to attract a mate.
The sheer number of such cyclical cicadas overwhelms potential predators, and only about 15 percent of the insects are eaten. The rest fall to the ground, leaving massive numbers of carcasses left to decay.
“The death of the adult cicada is returning some of the nitrogen to the soil the nymphs took up,” Yang said.