‘Hyperalarming’ study shows massive insect loss

Insects around the world are in a crisis, according to a small but growing number of long-term studies showing dramatic declines in invertebrate populations. A new report suggests that the problem is more widespread than scientists realized. Huge numbers of bugs have been lost in a pristine national forest in Puerto Rico, the study found, and the forest’s insect-eating animals have gone missing, too.

In 2014, an international team of biologists estimated that, in the past 35 years, the abundance of invertebrates such as beetles and bees had decreased by 45 percent. In places where long-term insect data are available, mainly in Europe, insect numbers are plummeting. A study last year showed a 76 percent decrease in flying insects in the past few decades in German nature preserves.

The latest report, published Monday in the Proceedings of the National Academy of Sciences, shows that this startling loss of insect abundance extends to the Americas. The study’s authors implicate climate change in the loss of tropical invertebrates.

“This study in PNAS is a real wake-up call — a clarion call — that the phenomenon could be much, much bigger, and across many more ecosystems,” said David Wagner, an expert in invertebrate conservation at the University of Connecticut who was not involved with this research. He added: “This is one of the most disturbing articles I have ever read.”

“We went down in ’76, ’77 expressly to measure the resources: the insects and the insectivores in the rain forest, the birds, the frogs, the lizards,” Lister said.

He came back nearly 40 years later, with his colleague Andrés García, an ecologist at the National Autonomous University of Mexico. What the scientists did not see on their return troubled them. “Boy, it was immediately obvious when we went into that forest,” Lister said. Fewer birds flitted overhead. The butterflies, once abundant, had all but vanished.

García and Lister once again measured the forest’s insects and other invertebrates, a group called arthropods that includes spiders and centipedes. The researchers trapped arthropods on the ground in plates covered in a sticky glue, and raised several more plates about three feet into the canopy. The researchers also swept nets over the brush hundreds of times, collecting the critters that crawled through the vegetation.

Each technique revealed the biomass (the dry weight of all the captured invertebrates) had significantly decreased from 1976 to the present day. The sweep sample biomass decreased to a fourth or an eighth of what it had been. Between January 1977 and January 2013, the catch rate in the sticky ground traps fell 60-fold.

“Everything is dropping,” Lister said. The most common invertebrates in the rain forest — the moths, the butterflies, the grasshoppers, the spiders and others — are all far less abundant.

Lister and Garcia attribute this crash to climate. In the same 40-year period as the arthropod crash, the average high temperature in the rain forest increased by 4 degrees Fahrenheit. The temperatures in the tropics stick to a narrow band. The invertebrates that live there, likewise, are adapted to these temperatures and fare poorly outside them; bugs cannot regulate their internal heat.
…
Lister pointed out that, since 1969, pesticide use has fallen more than 80 percent in Puerto Rico. He does not know what else could be to blame. The study authors used a recent analytic method, invented by a professor of economics at Fordham University, to assess the role of heat. “It allows you to place a likelihood on variable X causing variable Y,” Lister said. “So we did that and then five out of our six populations we got the strongest possible support for heat causing those decreases in abundance of frogs and insects.”

The authors sorted out the effects of weather like hurricanes and still saw a consistent trend, Schowalter said, which makes a convincing case for climate.

“If anything, I think their results and caveats are understated. The gravity of their findings and ramifications for other animals, especially vertebrates, is hyperalarming,” Wagner said. But he is not convinced that climate change is the global driver of insect loss. “The decline of insects in northern Europe precedes that of climate change there,” he said. “Likewise, in New England, some tangible declines began in the 1950s.”