Tropical insects are extremely sensitive to changing climates, especially changes in rainfall

Not unlike Goldilocks, insects that have adapted to live in perennially wet environments, like tropical rainforests seem to prefer just the right amount of water.

The results of a five-year study conducted in Peru have revealed that, following short periods of both drought and increased rainfall, there was a 50% decline in arthropod (insect and spider) biomass.

The new study, published in Global Change Biology, is one of only a few of this scope that have been conducted in the tropics. The findings suggest that terrestrial arthropods will be more susceptible to climate change than scientists expected.

“Most of the time when we think about climate change, we think about warming temperatures, but rainfall patterns will change as well, which is something insects seem to be especially sensitive to,” says first author Felicity Newell, a postdoctoral researcher with the Florida Museum of Natural History and the University of Florida in the US.

“We’re seeing that rainfall extremes can have negative effects over very short timescales.”

The “Insect Apocalypse”

A pattern of insect decline and extinction on every continent except Antarctica has been documented over the last two decades. Some scientists have even dubbed the phenomenon an “Insect Apocalypse”.

But most of the studies have been conducted in densely populated temperate regions, whereas the tropics – where half of all insects reside – have received considerably less scrutiny and this limits scientists’ understanding of how insects will respond to climate change.

“One of the biggest challenges is abiotic factors like temperature and rainfall influence multiple things. They can influence both the growth of new leaves and the arthropods that feed on them. In temperate systems, it’s difficult to tease the two apart because they’re often very synchronised,” explains Newell.

But the consistent temperatures in the tropics allow plants to retain their leaves year-round and with a constant supply of food, any large increase or decrease in insect abundance is more likely to be the result of changing climates.

Insects decline in too dry and overly wet conditions

Newell and co-author Ian Ausprey – a postdoctoral researcher at the University of Florida – spent 2.5 years between 2015-2019 conducting field work along the slopes of the Andes Mountains in northern Peru.

In total they collected more than 48,000 insects and compared them to rainfall and temperature measurements.

Against their expectations, instead of insect abundance being strongly linked to plant growth, rainfall was the single greatest predictor of how many insects you might expect to find at a given location.

“Arthropod biomass decreased after three months of dry weather, but it also decreased after three months of exceptionally wet conditions,” Newell says. “Biomass peaked at intermediate rainfall, creating a dynamic balance between too wet and too dry.”

Why do insect populations decline?

By conducting desiccation experiments on insects collected in the field they found that most couldn’t cope well with even a small reduction in humidity experienced during drought. This was especially the case for small insects, as their greater surface-to-volume ration made them more prone to drying out.

However, the team are still at a loss at how explain why wetter-than-average conditions are detrimental to tropical insects. There have a few hypotheses including: physical damage by being pelted with raindrops; decreased foraging times caused by more frequent storms; or cooler temperatures from increased cloud cover that may hinder insect growth and development.

“One hypothesis is there are more fungal spores during the rainy season, which would result in a greater occurrence of entomopathogenic fungi,” says Newell.

Entomopathogenic fungi are a group of pathogenic fungi that live in soil and infect and often kill insects.

The team  also developed a predictive model that suggests that insects will be among the first organisms to respond if conditions continue to shift toward a dangerously unbalanced climate.

“Insects are incredibly diverse and important. They fill the ecosystem roles of pollination and decomposition, and they serve as a food resource for many birds and mammals,” says Newell. “Our predictive model shows that insects respond to rainfall extremes, but how they respond to changing climates over the long term remains to be seen.”