Tropical insects are extremely sensitive to climate change

The results of a comprehensive five-year study conducted in Peru showed a 50% decline in arthropod biomass after brief periods of drought and increased rainfall. One of only a few studies of this magnitude conducted in the tropics, the results suggest that terrestrial arthropods, a group that includes insects and spiders, will be more vulnerable to climate change than previously thought.

“Most of the time when we think about climate change, we think about warming temperatures, but precipitation patterns will also change, which insects appear to be particularly sensitive to,” said Felicity Newell, a postdoctoral researcher and former graduate student at the Florida Museum of Natural History. “We see that extreme rainfall over very short periods of time can have negative effects.”

The insect apocalypse takes on new dimensions

The discovery of a Goldilocks penchant for just the right amount of water makes its debut against a worrying backdrop of population decline. Over the past two decades, thousands of studies have documented insect declines and extinctions on every continent except Antarctica, a pattern some have dubbed the insect apocalypse.

These results paint a stark but incomplete picture. Most of these studies have been conducted in densely populated temperate regions, while the planet’s most biodiverse ecosystems – the tropics – have been studied in much less detail.

Half of all insect diversity lives in the tropics, so scientists know a great deal about just a small fraction of endangered insect species. This imbalance severely limits our understanding of how insects will deal with the complex problem of climate change.

“One of the biggest challenges are abiotic factors like temperature and precipitation, which affect several things. They can affect both the growth of new leaves and the arthropods that feed on them. In temperate systems, it’s difficult to separate the two because they’re often very synchronized,” Newell said.

In temperate zones, the seasons move in close step. Life thrives and flourishes in spring and summer, then slows down and rests in fall and winter. Annual changes are less pronounced near the equator. Rainy and dry seasons create rhythmic fluctuations, but the constant temperatures allow plants to keep their leaves and tropical ecosystems to remain active year-round.

With a constant supply of plant food, any large increase or decrease in insect abundance is more likely to be the result of climate change. For scientists like Newell who want to understand how climate change is affecting insect populations, the tropics are an ideal place to study.

Insects decrease when wet for unknown reasons

Newell and co-author Ian Ausprey together spent two and a half years between 2015 and 2019 conducting fieldwork along the slopes of the Andes Mountains in northern Peru. They lived and worked alongside the residents of the surrounding villages, collecting insects several times a year from sites spanning more than 4,500 feet in elevation. In total, they collected more than 48,000 insects, which they compared with precipitation and temperature measurements throughout the year.

They expected insect abundance to be strongly linked to plant growth. While most trees and shrubs in the tropics do not shed their leaves, the production of young, supple leaves favored by herbivorous insects coincides with the onset of the rainy season. But that’s not what they found. The flowering of the light green growth, interpreted by satellite data and by visual inspection in the field, had little effect on insect biomass.

Instead, rainfall was the single best indicator of how many bugs you could expect to find in a given location.

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

Newell and Ausprey went a step further by attempting to determine the precise mechanism behind the declines. They performed desiccation experiments on insects collected in the field. Most of their specimens found it difficult to tolerate even a small decrease in humidity. This was especially true for small insects; their larger surface area to volume ratio makes them particularly susceptible to desiccation.

However, the researchers don’t know why above-average humidity conditions are problematic. Theories range from the physical damage small insects suffer when raindrops are thrown at them to reduced foraging times caused by more frequent storms. Another idea posits that cooler temperatures due to prolonged cloud cover could hinder insect growth and development.

“One hypothesis is that there are more fungal spores during the rainy season, which would lead to greater emergence of entomopathogenic fungi,” Newell said. Such fungal pathogens that prey on insects are widespread in tropical ecosystems. Infection often results in the death of the insect host, but only after its behavior has been radically altered to ensure optimal dispersal for the next batch of spores, as is the case with the zombie ant.

Whatever the reason, the authors worry about what their findings might mean for insects and the animals that depend on them in a rapidly warming world. By combining their field-collected information with 50 years of regional precipitation data, they also developed a predictive model that could help unravel the “black box” of ecosystem function and response. Their model suggests that insects will be among the first organisms to respond as conditions shift further toward a dangerously imbalanced climate.

“Insects are incredibly diverse and important. They perform the ecosystem’s duties of pollination and decomposition, and serve as a food source for many birds and mammals,” Newell said. “Our forecast model shows that insects respond to extreme rainfall, but how they respond to climate change in the long term remains to be seen.”