“Our point isn’t that temperature has no effect,” Willig says. “Our point is, because of the way data sets were analyzed and interpreted, we felt it was totally premature to say it’s a temperature effect and to avoid any disturbance discussion.”
The scientists laid out all of their issues in a detailed rebuttal letter, which PNAS published in late May alongside a response by Lister and Garcia. Reached by phone, Lister defended his analysis.
To deal with the discrepancies between the El Verde temperature records, Lister used a correction to merge the data sets, he said, and also looked at independent data from the nearby Bisley weather station. (The LTER researchers say Lister’s adjustments to the El Verde data were “erroneous” and that the Bisley data are inherently different because the sensor is affixed above the forest canopy rather than near the forest floor.) The cooling trend reflected in the latter half of the El Verde record? Not the result of canopy shading, but part of a global phenomenon in which climate warming stalled around the world.
Lister did agree, after learning that trees were not sampled randomly, that Schowalter’s data on canopy arthropods shouldn’t be extrapolated to the entire rain forest. He also agreed that the bird data would need to be revisited, as he had assumed that the researchers had conducted similar sampling efforts each year, based on the information he could find online. And there’s “no doubt,” he said, that hurricanes can influence population dynamics. But he sees them as shorter-duration influences “superimposed on these long-term and ongoing declines.”
Finally, Lister emphasized that the arthropod and lizard declines he saw in his own field data—from the 1970s to the 2010s—were very stark. “It’s a limited conclusion in a spatially complex forest,” he said. “We need more data.”
That’s a point on which insect researchers are likely to agree. Elsa Youngsteadt, an insect ecologist at North Carolina State University, thinks that the rebuttal raises serious questions, but that the original paper’s findings should not be dismissed outright, particularly given the authors’ field observations.
Read: Insects are in serious trouble
The entomologist Jessica Ware of Rutgers University voiced a similar sentiment. “I think it’s inevitable to have data reanalyzed—that’s what we do in science,” Ware says. To her, the most important takeaway from the hullabaloo is that “we’re missing a ton of information.” And even the best insect data are inherently noisy. “Insect populations bounce up and down like crazy,” says Christie Bahlai, an insect ecologist at Kent State University. “You can have orders-of-magnitude differences between years and that’s just normal population cycling.”
These limitations mean that a single study isn’t great evidence for a global insect apocalypse. “It’s almost impossible to unequivocally prove global insect decline everywhere,” says Manu Saunders, an insect ecologist at the University of New England in Australia. Demonstrating declines for just one ecosystem, she says, requires “continuous data for 10 to 15 years or more across multiple types of habitats and every insect in the system.” Many entomologists think we’ve described only a fraction of the insects that are out there.