COS 200-4 - Cascading effects of an ecological pulse: indirect effects of the 2021 Brood X cicada emergence on an eastern temperate forest

Background/Question/Methods

Perturbations, whether natural or man-made, can have cascading effects on complex multi-trophic food webs. In this study we documented the effects of a natural ecological resource pulse: the synchronous emergence of billions of 17-year periodical cicadas (Magicicada spp.). Because this phenomenon occurs only once in a generation, its ecological effects have been poorly documented. We quantified both the magnitude and the duration of the ecological effects resulting from this insect biomass pulse on the trophic cascade involving insectivorous birds, oak-feeding caterpillars, and white oak trees. We quantified changes in bird predation using sentinel plasticine caterpillars surveyed weekly on understory oak saplings throughout the season one year prior to and again the year of the 2021 brood X emergence. We also quantified the density of insect herbivores on understory oak trees before, during, and after the cicada emergence in the spring and summer of 2021. Finally, we estimated resulting foliar herbivory levels on the trees in both years.

Results/Conclusions

Compared to the pre-emergence year, we found that bird predation drastically decreased during the periodical cicada emergence and this reduction was most pronounced during the spring and early summer of the emergence year. Compared with a historic dataset of insect herbivores on white oaks, we observed a dramatic increase in herbivore density just after the cicada emergence, which resulted in a commensurate increase in herbivory on these trees compared to the previous, non-emergence year. By studying predictable, natural perturbations like periodical cicadas, it may be possible to understand how less predictable anthropogenic perturbations will affect forest food webs. Developing our understanding of these perturbations is thus critical to preserving long-term ecosystem integrity.