Convert the dispersal number into an annual rate and the pessimism becomes arithmetic. About 46 metres per emergence, spread across a 17-year cycle, is roughly 2.7 metres a year [1]. Nine decades on from that clearance, a surviving brood at that rate has had about five emergences and advanced something like 230 metres [2]. A kilometre of replanted woodland sits about 370 years away for a 17-year brood, and about 280 years away for one of the four species on a 13-year cycle [3]. Those figures come from a single measured step distance, which is what the movement model was built on [12].
The mechanism is odd because the animal is. Nymphs feed on tree roots for the whole underground stretch, and the study treats the roots of old-growth forest as their sole essential food source [2]. Cut the trees and the cohort beneath them cannot walk out mid-cycle. As Andrew Forbes, the corresponding author, puts it, the animals die out and it may take hundreds of years for them to get back [5], long after new trees have filled the gap [4].
Two different kinds of evidence sit under that conclusion. The historical half is observational: a sequence assembled from newspapers, aerial photographs and other records, with the timing lining up, but no unlogged tract in the county reported as a comparison [9]. Correlation of that shape is still worth something, because the direction is not ambiguous. Forests came down; broods that Forbes went looking for near Iowa City were not there [17].
The relocation is closer to an experiment, and its failure answers a different question than the one about food. Periodical cicadas have no defence against predators other than abundance, in emergences that can reach hundreds of thousands of insects per acre [7][8]. The number released into the new site was at most about a tenth of that peak density [4]. So the test established that a sub-threshold cohort does not survive predation, which Forbes concedes directly: "I thought if we got big enough numbers, it would work. I was wrong" [15]. Whether a brood seeded at natural density could establish in restored forest remains untested.
The thing this does not tell you is when, or whether all seven periodical species share the trajectory [6]. The reconstruction covers one Iowa county [9]; broods embedded in large, continuous forest elsewhere are a different sum. Nor does the account of the study weigh warming against land use [18]. The argument runs on clearance and a dispersal rate and needs no climate term to reach its conclusion [2][3], which makes it cleaner than a multi-driver story and also narrower.
An insect lineage that traces to a common ancestor nearly four million years old [6] now carries a range map largely written by when the trees came down, read back at 2.7 metres a year [1].
