Tropical insects extremely sensitive to changing climates

Insects that are adapted to perennially wet environments, like tropical rainforests, don’t tend to do well when their surroundings dry out. New research published this Wednesday indicates they may be equally averse to heavy rainfall.

The results of an extensive five-year study conducted in Peru revealed a 50% decline in arthropod biomass following short periods of both drought and increased precipitation. One of only a few studies of this scope conducted in the tropics, the findings suggest terrestrial arthropods, a group that includes insects and spiders, will be more susceptible to climate change than previously suspected.

“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,” said Felicity Newell, a postdoctoral associate and former doctoral student with the Florida Museum of Natural History. “We’re seeing that rainfall extremes can have negative effects over very short timescales.”

The insect apocalypse takes on new dimensions

The discovery of a Goldilocks preference for just the right amount of water makes its debut against a worrying backdrop of population declines. Over the last two decades, thousands of studies have documented insect decline and extinction 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 received considerably less scrutiny.

Half of all insect diversity resides in the tropics, and as a result, scientists know a great deal about only a small fraction of imperiled insect species. This imbalance places strict limits on understanding how insects will fare with the complex problem of 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 synchronized,” Newell said.

In temperate zones, the seasons proceed in a tight lock-step. Life stirs and flourishes in spring and summer, then wanes and lies dormant in autumn and winter. Near the equator, the annual changes are less pronounced. Wet and dry seasons create rhythmic variation, but the consistent temperatures allow plants to retain 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 changing climates. For scientists like Newell who want to understand how climate change will affect insect populations, the tropics are the ideal place to study.

Insects decline in wet conditions for reasons that remain obscure

Newell and co-author Ian Ausprey spent a combined two-and-a-half years between 2015-2019 conducting field work along the slopes of the Andes Mountains in northern Peru. Living and working with the residents of local villages, they collected insects multiple times of the year at sites spanning more than 4,500 ft of elevation. In total, they collected more than 48,000 insects, which they compared to rainfall and temperature measurements taken throughout the year.

They expected insect abundance would be strongly linked to the growth of plants. While most trees and shrubs don’t lose their leaves in the tropics, the production of young, supple leaves favored by herbivorous insects coincides with the onset of the rainy season. But this isn’t what they found. The flush of bright green growth, as interpreted by satellite data and by visual inspection in the field, had only a small effect on insect biomass.

Science Daily