On 13- or 17-year cycles, billions of cicadas emerge from the ground to reproduce in eastern North American deciduous forests. One of the largest emergence events of these insects happened in 2021 when the Brood X cicadas emerged. Researchers who studied that once-in-a-generation event are now unveiling the impact of this occurrence on forest ecosystems, specifically on birds, caterpillars and trees.
In a new study published October 19, 2023, on the cover of the journal Science, researchers at the University of Maryland, George Washington University and Georgetown University quantified the widespread changes the 2021 periodical cicada emergence had on the feeding patterns of birds and its downstream effects. The research team said this is one of only a few studies that has explored the indirect effects of periodical cicada emergence events on forest food webs.
The study found that more than 80 species of birds opportunistically switched their diets to cicadas during their emergence. That feeding frenzy on cicadas relieved the predation pressure on the birds’ typical prey, caterpillars, which consequently increased in abundance, resulting in more leaf damage on their host oak trees. Researchers collected their data at two study sites in the Mid-Atlantic portion of the Brood X range in the United States.
"This study shows that massive resource subsidies can have far-reaching ripple effects through forest food webs," said study co-author Daniel Gruner, a professor of entomology at UMD. "Many bird species switched to feed on cicadas, which doubled caterpillar populations and their damage to trees. Because the Brood X periodical cicadas emerged in the billions across 15 states, their indirect ramifications were geographically extensive."
Periodical cicada emergence events only last for about five to seven weeks. The research team collected data before, during and after the cicada emergence, deploying decoy caterpillars to track bird feeding patterns. They also recruited the help of the broader birding community to crowdsource additional observational data about which birds were feeding on cicadas. In addition to the clay decoy caterpillars, the researchers counted the number of caterpillars on oak trees during the emergence and measured the subsequent damage they caused to the leaves of their host trees.
As an undergraduate at UMD, Grace Soltis (B.S. ’22, biological sciences) designed and organized the bird foraging part of the project.
"I was out watching bird foraging behavior and documenting the shifts almost every day in summer 2021," Soltis said. "As part of my undergraduate honor's thesis, I analyzed the bird data and wrote part of the paper’s results. This work on cicadas inspired me to continue studying insect food webs for my Ph.D. at Florida State."
Not only do these massive emergence events help scientists better understand how a sudden influx of a particular food source can rewire complex food chains and disrupt forest ecosystems, the study authors said their findings have larger implications for bird conservation.
"Birds are a really important regulator of insect herbivores and the amount of damage that plants receive. Over the last several decades, there’s been a significant decline in bird populations," said study co-author John Lill, a professor of biology at GW. "Our study provides a glimpse of what a world with fewer birds might look like. That would include increased damage to both forestry and agricultural crops, which has consequences for productivity and the economy."
The research team suggests that their findings related to the ecological consequences of the cicada emergence may represent a more general, though little-studied, pattern.
"Whenever a big resource pulse, like the annual bonanza of salmon eggs in inland streams, or horseshoe crab eggs in the Delaware Bay, alters the diets of generalist predators in a region, ecologists should take a look at the changes in the food web that occur when the typical prey temporarily escapes predation," said study co-author Martha Weiss, a professor of biology at Georgetown.
Scientists are anticipating a historical convergence of two broods of periodical cicadas, Brood XIII and Brood XIX, that are scheduled to emerge at the same time in the Midwest in the spring of 2024, an event that last occurred in 1803. The research team plans to continue exploring the indirect impacts of cicadas on other parts of the forest food web.
"I believe we need to get a handle on how land use and environmental change may impact populations of periodical cicadas, which require 13 or 17 years of ecosystem stability to emerge and reproduce successfully,” Gruner said. "We must conserve this extraordinary natural phenomenon, not just for the insects but for the entirety of the food webs they support."
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This article was adapted from content provided by George Washington University and originally posted by the UMD College of Computer, Mathematical, and Natural Sciences
The paper, “Periodical cicadas disrupt trophic dynamics through community-level shifts in avian foraging,” was published in Science on October 19, 2023.