“We knew that something was amiss in the first couple days,” said Brad Lister. “We were driving into the forest and at the same time both Andres and I said: ‘Where are all the birds?’ There was nothing.”

His return to the Luquillo rainforest in Puerto Rico after 35 years was to reveal an appalling discovery. The insect population that once provided plentiful food for birds throughout the mountainous national park had collapsed. On the ground, 98% had gone. Up in the leafy canopy, 80% had vanished. The most likely culprit by far is global warming.

“It was just astonishing,” Lister said. “Before, both the sticky ground plates and canopy plates would be covered with insects. You’d be there for hours picking them off the plates at night. But now the plates would come down after 12 hours in the tropical forest with a couple of lonely insects trapped or none at all.”

“It was a true collapse of the insect populations in that rainforest,” he said. “We began to realise this is terrible – a very, very disturbing result.”

“It was not insects that drew Lister to the Luquillo rainforest for the first time in the mid-1970s. “I was interested in competition among the anoles lizards,” he said. “They’re the most diverse group of vertebrates in the world and even by that time had become a paradigm for ecology and evolutionary studies.”

“The forest immediately captivated Lister, a lecturer at Rensselaer Polytechnic University in the US. “It was and still is the most beautiful forest I have ever been in. It’s almost enchanted. There’s the lush verdant forest and cascading waterfalls, and along the roadsides there are carpets of multicoloured flowers. It’s a phantasmagoric landscape.”

“It was important to measure insect numbers, as these are the lizards’ main food, but at the time he thought nothing more of it. Returning to the national park decades later, however, the difference was startling.

“One of the things I noticed in the forest was a lack of butterflies,” he said. “They used to be all along the roadside, especially after the rain stopped, hundreds upon hundreds of them. But we couldn’t see one butterfly.”

“Since Lister’s first visits to Luquillo, other scientists had predicted that tropical insects, having evolved in a very stable climate, would be much more sensitive to climate warming. “If you go a little bit past the thermal optimum for tropical insects, their fitness just plummets,” he said.

“As the data came in, the predictions were confirmed in startling fashion. “The number of hot spells, temperatures above 29C, have increased tremendously,” he said. “It went from zero in the 1970s up to something like 44% of the days.” Factors important elsewhere in the world, such as destruction of habitat and pesticide use, could not explain the plummeting insect populations in Luquillo, which has long been a protected area.”

Text: Insect collapse: ‘We are destroying our life support systems’, The Guardian.