Moths to millipedes: Why are arthropods declining in untouched forests?

Thinking of the rainforest, you probably picture parrots, orchids, maybe a lazy jaguar on a branch. What you don’t see – because most of us never notice them – are the multitudes of insects, spiders, millipedes, and other arthropods that make the rainforest run.

These little laborers till the soil, pollinate flowers, recycle dead leaves, and serve as breakfast, lunch, and dinner for just about everything that hops, flies, or slithers.

For years, scientists in Europe and North America have sounded alarms that insect numbers are sliding. But no one knew whether the planet’s richest wildlife real estate – the tropical belt – was suffering the same fate. A new study argues it is, and that should set off alarms well beyond the equator.

A global detective story

The research stitched together results from more than 80 long-term projects in undisturbed rainforests stretching from the Amazon to Borneo.

No chainsaws, no pesticides, no plantations – just old-growth jungle. By stacking decades of moth counts here and spider surveys there, the team could spot patterns that any single project would miss.

Those patterns are, unfortunately, grim. Whether you look at butterflies, beetles, or orb-weaving spiders, the story is the same: fewer species, fewer individuals.

Leaf-eating caterpillars are chewing less foliage these days, and the tiny cleanup crews that shred fallen leaves are also dwindling. Even in forests so remote you need days of hiking or a helicopter to reach them, life in miniature is thinning out.

Adam Sharp, the University of Hong Kong (HKU) data scientist who wrangled the spreadsheets, didn’t sugarcoat it: “To find such large declines over many studies is really bad news.”

“Our results strongly suggest that the immense biodiversity of tropical forest arthropods is immediately threatened,” he said.

Uncut forests still losing arthropods

Because these field sites are virtually untouched, the usual villain – direct habitat destruction – doesn’t explain the loss. Instead, the researchers traced the problem to the sky.

Tropical weather is famously moody thanks to the El Niño–Southern Oscillation (ENSO), the swing between hot, dry El Niño years and cooler, wetter La Niña years. Historically, that back-and-forth kept communities in balance. Some insects love soggy conditions, while others thrive in drought, but neither side ever totally disappears.

Climate change is messing with that rhythm. El Niño events are arriving more often and hitting harder, stretching dry seasons and scorching understory plants.

Arthropod groups that prefer the gentler La Niña phase are getting pummeled each time the pendulum sticks on the hot, dry side.

“In these tropical forests that haven’t otherwise been physically modified by humans, we can rule out habitat loss, pesticides, pollution, and various other threats,” said lead author Mike Boyle, a biologist at HKU. “In these places, El Niño seems to be the prime suspect.”

Arthropods hold the forest together

It may be hard to feel sentimental about a stick insect, but your well-being is more entangled with them than you might guess. Insects and their kin keep nutrient cycles turning.

Arthropods are the garbage disposal that prevents the rainforest from choking on their own leaves. They pollinate cacao trees and coffee bushes. They are the protein bars that power baby birds and hunting frogs. Lose enough, and you don’t just lose species – you hobble entire ecosystems.

“Arthropods are essential components of functioning ecosystems, carrying out vital processes including decomposition, herbivory, and pollination,” said study co-author Louise Ashton.

When those processes sputter, forests store less carbon, streams change course, and the dominoes keep falling.

Local lessons from global loss

Roger Kitching at Griffith University helped lead the Australian part of the study and sees a lesson for his own backyard.

The Wet Tropics of Queensland and the lush forests of northern New South Wales haven’t been logged in decades, yet nobody is really counting their insects.

“The crucial message for Australia is the need to monitor the biodiversity in our rainforests – revisiting previous surveys is the key,” said Kitching.

Long-running datasets like the ones underpinning this study are important because they reveal slow, silent shifts that annual snapshots miss. Without them, declines can cruise under the radar until they’re drastic – and harder to fix.

Helping the rainforest’s smallest workers

First, keep forests intact and healthy. Dense canopies maintain cooler, moister microclimates that offer insects a refuge when El Niño roasts the region. Restoring battered edges and stamping out invasive plants add extra buffers.

Second, get serious about monitoring. New gadgets – automated camera traps that “listen” for insect wingbeats, DNA snippets caught in river water – could track populations without needing an army of taxonomists.

Most importantly, tackle the root problem. The runaway greenhouse gases heating up oceans are also increasing El Niño’s effects. Curbing emissions slows that feedback loop and gives forests – and their six-legged stewards – a fighting chance.

Rainforest arthropods survived asteroid strikes and ice ages, but the speed of modern climate change is a different beast. Their decline isn’t just a tropical tragedy – it’s a preview of broader ecological unraveling.

The bugs may be small, but their absence would leave a very big hole in the biosphere – and in the services humans quietly rely on every day.

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