Tropical insects are highly sensitive to climate change – ScienceDaily

Insects that are adapted to permanently moist environments, such as tropical rainforests, tend not to do well when their surroundings dries up. New research published Wednesday suggests they may be just as averse to torrential rain.

Results of an intensive five-year study conducted in Peru revealed a 50% decrease in arthropod biomass after short periods of drought and increased precipitation. One of only a few studies of this range 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 of climate change, we think of warming, but precipitation patterns will also change, something that insects seem to be particularly sensitive to,” said Felicity Newell, a postdoctoral fellow and former doctoral student. Florida Museum of Natural History. “We are of the opinion that extreme precipitation can have negative effects over very short periods of time.”

The insect apocalypse takes on new dimensions

The discovery of Goldilocks’ preference for the right amount of water appears for the first time against a disturbing background of declining population. Over the past two decades, thousands of studies have documented the decline and extinction of insects on every continent except Antarctica, a pattern some have called the insect apocalypse.

These results paint a stark but incomplete picture. Most of these studies were conducted in temperate, densely populated regions, while the planet’s most biologically diverse ecosystems – the tropics – have received far less scrutiny.

Half of the diversity of insects lies in the tropics, and as a result, scientists know a lot about only a small part of the species of insects that are at risk. This imbalance places severe constraints on understanding how insects deal with the complex problem of climate change.

“One of the biggest challenges is abiotic factors like temperature and precipitation that affect multiple things. They can affect both the growth of new leaves and the arthropods that feed on them. In temperate systems, the two are hard to tell apart because they are often very synchronous,” Newell said.

In temperate regions, seasons continue in a controlled stride. It comes alive and blooms in spring and summer, then fades and sleeps in fall and winter. Near the equator, annual changes are less pronounced. Wet and dry seasons create rhythmic variation, but consistent temperatures allow plants to retain their leaves and tropical ecosystems to remain active year-round.

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

Insects retreat in wet conditions for reasons that remain mysterious

Newell and co-author Ian Osprey spent two and a half years between 2015-2019 conducting fieldwork along the slopes of the Andes in northern Peru. Living and working with local villagers, they collect insects several times a year at sites spanning more than 4,500 feet of elevation. In total, they collected more than 48,000 insects, and compared them to measurements of rainfall and temperature taken throughout the year.

They predicted that the abundance of insects was strongly correlated with the growth of plants. While most trees and shrubs do not lose their leaves in the tropics, the production of small, soft leaves favored by herbivorous insects coincides with the onset of the rainy season. But that is not what they found. Brilliant green growth flux, as explained by satellite data and visual inspections in the field, had little effect on the insects’ biomass.

Instead, rainfall was the single largest indicator of how many insects you might 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 peaks at intermediate rainfall, creating a dynamic equilibrium between extreme moisture and extreme drought.”

Newell and Osprey took things a step further by trying to pinpoint the exact mechanism behind the dips. They conducted drying experiments on insects collected from the field. Most of their specimens found it difficult to handle even a slight drop in humidity. This was especially true for small insects; Their surface-to-volume ratio makes them particularly susceptible to drying out.

However, researchers are at a loss to explain why more than average wet conditions are problematic. Theories range from physical damage received by small insects by being pelted with raindrops to reduced foraging times due to frequent storms. Another idea is that temperatures cooler than prolonged cloud cover may stunt the growth and development of insects.

“One hypothesis is that there are more fungal spores during the rainy season, which could lead to an increased incidence of entomopathogenic fungi,” Newell said. These insect-eating fungal pathogens are common in tropical ecosystems. Infection often results in the death of the insect’s host, but only after its behavior is radically changed to ensure optimal dissemination of the next batch of spores, as is the case for zombie ants.

Whatever the reason, the authors are concerned about what their findings might predict for the insects and animals that depend on them in a rapidly warming world. By combining information collected in the field with 50 years of regional rainfall data, they also developed a predictive model that may help decipher the “black box” of ecosystem function and response. Their model 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,” Newell said. “They fill the ecosystem’s role in pollination and decomposition, and they serve as a food source for many birds and mammals.” “Our predictive model shows that insects respond to extreme precipitation events, but how they respond to long-term climate change remains to be seen.”