The Luquillo mountains in the El Yunque National Forest.
Photo: Shannon McGee (Flickr)

One of the most well-studied tropical forests on Earth has seen its insect populations crash since the 1970s with effects rippling up the food chain, an alarming new study has found.

Published Monday in the Proceedings of the National Academies of Sciences, the Rensselaer Polytechnic University researcher-led study shows a 10-60 fold decline in the abundance of insects in Puerto Rico’s Luquillo Rainforest, the only tropical forest in the U.S. national forest system. The authors say rising temperatures are the most likely culprit, raising the possibility that the base of the food web could be unraveling elsewhere as climate change turns up the heat on rainforests around the globe.

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Comparing 1976 to 2011-2013, the new study found a massive drop in the weight of insects collected in traps from two long-term research sites. Twenty years of data on the abundance of more than 120 groups of canopy insects revealed a similar decline. The researchers found a concomitant drop in the abundance of insect-eating lizards, frogs, and birds.

This loss of life correlated with steady uptick in mean maximum temperatures at the study sites. Across the two sites, the hottest days have become 2 degrees Celsius hotter since the 1970s.

Correlation does not necessarily imply causation, but the authors aren’t really sure what else could be driving the insect die-offs. The forest has remained largely undisturbed since receiving protection in the early 20th century, and agriculture has been on the decline across the region. Pesticides—which were implicated in a study showing major insect declines in German nature reserves last year—seem an unlikely culprit. Lead author Bradford Lister told Earther that a statistical analysis used to determine causality also pointed to the strong effect of temperature on insect declines.

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“[O]ur initial results are very suggestive [of a climate change effect] but to extrapolate to other rainforest ecosystems we need more data,” he said.

James Cook University tropical ecologist Bill Laurance was less sure about that linkage, noting in an email to Earther that “you can’t get away from the fact that we live in a complicated world with lots of environmental changes happening all at once,” from pollution to habitat disruption.

“The results are certainly alarming—insects are the base or a key part of an incredible wealth of ecological interactions” Laurance wrote. “Are the declines down to global warming? It’s hard to say any for sure.”

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Tropical ecologist Dan Janzen, who has been working in Costa Rican rainforests for decades, said he did not find the results surprising. He told Earther he thought rising temperatures are “massively contributing” to the trend the paper saw, but that it was already “well on its way long before people started noticing climate change.”

More field studies and different lines of data could certainly help shore up the idea that rising temperatures are driving an unraveling of the food web. At the same time, as Lister noted, the findings underscore the need for more research on the effects of climate change across the tropics at large.

Historically, scientists have paid far more attention to how climate change impacts temperate and high-latitude ecosystems. But a growing body of research suggests many tropical species are specialized to a narrow range of temperatures, having never experienced seasonal swings. Tropical mountains, in particular, feature unique microclimates that can be lost entirely as temperatures rise.

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And that’s to say nothing of changes in rainfall and extreme weather events that are affecting tropical forests worldwide. Home to an enormous fraction of Earth’s biodiversity, right now, these ecosystems are flying blind into a hotter, weirder future.