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Insects around the world are in crisis, according to a small but growing body of long-term studies showing dramatic declines in invertebrate populations. A new report suggests the problem is more widespread than scientists thought. The study found that large numbers of beetles have been lost in a pristine state forest in Puerto Rico, and the forest’s insectivorous animals have also disappeared.
In 2014, an international team of biologists estimated that over the past 35 years, the population of invertebrates such as beetles and bees has declined by 45 percent. Where long-term insect data are available, particularly in Europe, insect numbers are declining. A study from last year showed a 76 percent decline in flying insects in German nature reserves over the past few decades.
The latest report, published Monday in the Proceedings of the National Academy of Sciences, shows this astounding loss in insect abundance extends to the Americas. The authors of the study link climate change to the loss of tropical invertebrates.
“This study in PNAS is a real wake-up call — a wake-up call — that the phenomenon could be much, much larger and in many more ecosystems,” said David Wagner, an expert in invertebrate conservation at the University of Connecticut, who wasn’t involved in this investigation. He added, “This is one of the most disturbing articles I’ve ever read.”
Fewer bugs banging on your windshield these days? It could be because flying insects are disappearing. (Video: Anna Rothschild/Washington Post)
Bradford Lister, a biologist at Rensselaer Polytechnic Institute in New York, has been studying rainforest insects in Puerto Rico since the 1970s. If Puerto Rico is the island of enchantment – “la isla del encanto” – then its rainforest is “the enchanted forest on the enchanted island,” he said. Birds and coqui frogs chirp beneath a 50-foot emerald canopy. The forest called El Yunque is well protected. Spanish King Alfonso XII claimed the jungle as a royal reserve in the 19th century. Decades later, Theodore Roosevelt made it a national reserve, and El Yunque remains the only tropical rainforest in the national forest system.
“We went down in 1976, 1977 to specifically measure the resources: the insects and insectivores in the rainforest, the birds, the frogs, the lizards,” Lister said.
He came back nearly 40 years later, along with colleague Andrés García, an ecologist at the National Autonomous University of Mexico. What the scientists didn’t see when they returned worried them. “Boy, it was immediately obvious when we walked into that forest,” Lister said. Fewer birds flew overhead. The butterflies, once plentiful, were all but gone.
García and Lister again measured the forest’s insects and other invertebrates, a group called arthropods, which also includes spiders and centipedes. The researchers trapped arthropods on the ground in panels covered with a sticky adhesive and lifted several more panels about a meter into the canopy. Researchers also swept nets over the scrub hundreds of times, collecting the critters crawling through the vegetation.
Each technique showed that biomass (the dry weight of all captured invertebrates) had decreased significantly from 1976 to the present. The sweep sample’s biomass was reduced to a quarter or an eighth of what it had been. Between January 1977 and January 2013, the catch rate in the sticky bottom traps dropped 60-fold.
“Everything’s collapsing,” Lister said. The most common rainforest invertebrates—the moths, the butterflies, the grasshoppers, the spiders, and others—are all much rarer.
“Holy shit,” Wagner said of the 60-fold loss.
Louisiana State University entomologist Timothy Schowalter, who is not the author of the most recent report, has studied this forest since the 1990s. The new research is consistent with his data as well as European biomass studies. “To document these trends, these long-term sites with consistent sampling over a long period of time are required,” he said. “I find their data quite compelling.”
The study authors also caught anole lizards in the rainforest, which eat arthropods. They compared these numbers to counts from the 1970s. Anol biomass decreased by more than 30 percent. Some species of anoles have disappeared from the inner forest altogether.
Insectivorous frogs and birds also crashed. Another research team used nets to trap birds in 1990 and again in 2005. Catches fell by about 50 percent. Garcia and Lister analyzed the data with the insectivores in mind. The Ruddy Quail Pigeon eating fruits and seeds had no population change. A bright green bird named Puerto Rican Tody, which eats almost exclusively beetles, decreased by 90 percent.
The food web appears to have been wiped out from the ground up. It’s believable that the authors link the cascade to arthropod loss, Schowalter said, because “there are all these different taxa that are showing the same trends — the insectivorous birds, frogs and lizards — but you don’t see those.” among seed-eating birds.”
Lister and Garcia attribute this crash to climate. During the same 40-year period as the arthropod crash, the average maximum temperature in the rainforest rose 4 degrees Fahrenheit. Temperatures in the tropics stay within a narrow range. The invertebrates living there are also adapted to these temperatures and get along badly outside of them; Beetles cannot regulate their internal heat.
The United Nations climate panel has issued a report warning of an unprecedented rise in temperatures between 2030 and 2052 if global warming continues. (Video: Reuters)
A recent analysis of climate change and insects, published in August in the journal Science, predicts a decline in tropical insect populations, according to one of the study’s authors, Scott Merrill, who studies crop pests at the University of Vermont. In temperate regions farther from the equator, where insects can survive a wider range of temperatures, agricultural pests will gobble up more food as metabolism increases, Merrill and his co-authors warned. But beyond a certain thermal threshold, insects stop laying eggs, he said, and their internal chemistry breaks down.
The authors of a 2017 study on disappearing flying insects in Germany suggested other possible culprits, including pesticides and habitat loss. Arthropods also struggle with pathogens and invasive species around the world.
“It’s confusing and I’m scared to death that it’s actually death from a thousand cuts,” Wagner said. “One of the scariest parts about it is that we don’t have an obvious smoking gun here.” In his opinion, a particular danger for these arthropods is not the temperatures, but drought and lack of rain.
Lister pointed out that pesticide use in Puerto Rico has fallen by more than 80 percent since 1969. He doesn’t know what else could be to blame. The study authors used a more recent analytical method invented by an economics professor at Fordham University to assess the role of heat. “It allows you to put a probability on variable X causing variable Y,” Lister said. “So we did that, and then we got five of our six populations as best we could to support the heat that’s causing these declines in frog and insect numbers.”
The authors sorted out the effects of weather like hurricanes and still saw a consistent trend, Schowalter said, making a compelling case for climate.
“If anything, I think their findings and caveats are understated. The severity of their findings and impact on other animals, particularly vertebrates, is hyper-alarming,” Wagner said. However, he is not convinced that climate change is the global driver of insect mortality. “The decline in insects in northern Europe precedes climate change there,” he said. “Similarly, in New England, some noticeable declines began in the 1950s.”
Regardless of the cause, all scientists agreed that more people should pay attention to the Bugpocalypse.
“It’s a very scary thing,” said Merrill, following a “bleak, grim” UN report that estimates the world has little more than a decade to get climate change under control. But “we can all improve,” he said, by using more fuel-efficient cars and turning off unused electronics. The Portland, Oregon-based Xerces Society, a nonprofit environmental group that advocates for insect conservation, recommends creating a garden with native plants that bloom year-round.
“Unfortunately, we have deaf ears in Washington,” said Schowalter. But those ears will eventually listen, he said, because our food supply is in jeopardy.
35 percent of the world’s crops must be pollinated by bees, wasps and other animals. And arthropods are more than just pollinators. They are the little sentinels of the planet, toiling in unnoticed or avoided corners. They eat rotten wood and eat carrion. “And none of us wants more cadavers,” Schowalter said. According to a 2006 estimate, wild insects provide a six-legged labor force worth $57 billion in the United States each year.
The loss of insects and arthropods could further tear the rainforest’s food web, Lister warned, causing plant species without pollinators to become extinct. “If the tropical forests disappear, it will be another catastrophic failure of the entire Earth system,” he said, “that will affect people in almost unimaginable ways.”
The United Nations Intergovernmental Panel on Climate Change said there was no precedent for the sweeping changes needed to control the planet’s warming. (Video: Danielle Kunitz, Osman Malik/Washington Post)
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