Closer to understanding the reasons for the 'insect apocalypse'

Many of them, such as bees or butterflies, are excellent pollinators, carrying the precious yellow seeds from one flower to another. Others, like dung beetles or termites, decompose dead matter and, in doing so, release essential nutrients, which return to the soil, making it more fertile. Others also feed on or are parasitic on other insects we consider pests, such as ladybugs, which eat aphids, or some parasitoid wasps, which lay their eggs in caterpillar larvae. And others, finally, like ants, are subterranean, digging tunnels in the soil and thus aerating it, allowing water and air to better reach plant roots. Not to mention that a large number of them are the main food source for a wide variety of animals, from birds to mammals, reptiles, amphibians, and perhaps soon, humans as well.

Insects, in short, are essential for maintaining the health of the Earth's ecosystems. But they are disappearing, and at a rapid pace. Scientists refer to this crisis as the "insect apocalypse," a term they use to describe the massive, global decline in their populations around the world, something that appears to have worsened in recent decades. A rapid decline, therefore, that threatens to become a true global catastrophe.

Therefore, understanding the root causes of this disappearance has become a priority for science. We need to understand whether the "Apocalypse" is due exclusively, as many believe, to "modern" factors, such as the conversion of natural spaces into agricultural areas, the massive use of pesticides and fertilizers, climate change, or the introduction of invasive species, or whether this trend toward the decline of species and individuals can be measured on a longer time scale.

As part of that effort, a team of researchers from the Okinawa Institute of Science and Technology (OIST) has turned to a community genomics approach to explore ant populations in the Fiji archipelago. This approach could serve as a model for understanding insect biodiversity trends. By studying the genomics of collections held in a large number of museums, they were able to trace the ants' evolutionary relationships to explore how they arrived on the islands and reconstruct the population history of the different species. The work has just been published in Science .

More often than desirable, major conservation efforts focus on studying the biodiversity of large (and photogenic) animals that live in easier-to-study areas, such as temperate habitats. However, despite their importance, the rapid population declines of many insects are generally merely a suspicion. This is something that the new study, which is based on a specific insect population and for which the researchers chose an island system, goes some way to remedy.

According to Evan Economo of the University of Maryland and one of the study's lead authors, "It can be difficult to estimate historical changes in insect populations because, with few exceptions, we haven't directly monitored those populations over time. We took a novel approach to this problem by analyzing the genomes of many species in parallel from recently collected museum specimens. The genomes contain evidence of whether populations are growing or declining, allowing us to reconstruct community-wide changes."

What Evan Economo and his colleagues found is alarming: 79% of endemic species (those found only in Fiji) are in decline, on a timescale that correlates with the arrival of humans to the islands. On the other hand, ant species recently brought to the islands by humans are experiencing explosive population growth. The researchers also found that the largest proportion of declines has occurred in the last few hundred years, coinciding with European contact, colonization, global trade, and the introduction of modern agricultural techniques.

By focusing on the Fiji archipelago, the researchers were able to gain a broad view of population changes and the colonization history of nearly all of the region's different ant species. "As closed and isolated ecosystems," explains Cong Liu, the first author of the article, "islands are expected to feel the effects of human impact more quickly, so they're a bit like a canary in a coal mine."

Given the difficulty of directly studying populations in their environment, something that is extremely complex and demanding in tropical climates, the study's authors opted to use museum collections, the result of decades of fieldwork, which, of course, also included those they themselves had built in previous work.

Because DNA degrades over time, the team had to use special sequencing methods to compare small fragments of DNA. In their study, the researchers sequenced genome samples from thousands of ants from more than 100 different species. Using that data, they identified 65 different instances (colonization events) in which new ant species arrived on the island. These instances ranged from natural colonization (i.e., the arrival of ants without human involvement) millions of years ago to recent human introductions after Fiji became part of global trade networks.

Based on this history, scientists applied their population genetic models to identify the increase (or decrease) in the populations of different groups of ants across the Fiji archipelago. This is how they discovered the decline of endemic species and the dramatic increases in the population of non-native species in recent years.

The main reason, according to the article, is that endemic island species often develop traits that make them sensitive to environmental changes, including the arrival of harmful new species. "Most recorded extinctions," Liu adds, "have historically occurred in island systems."

The researchers hope their work can serve as inspiration for future research seeking to expand scientific understanding of insect populations, something that will be extremely useful for future conservation efforts. "This study," explains Alexander Mikheyev of the Australian National University and also lead author of the research, "also highlights the importance of biodiversity and museum collections. As our scientific toolbox expands, there is more and more information we can capture from biodiversity collections, so it is essential that we continue to invest in and maintain these vital resources."