Common insect species face the greatest declines

A study from the German Centre for Integrative Biodiversity Research (iDiv) has brought a new perspective to the discussion on insect biodiversity. Contrary to the belief that rare species are most at risk, the research indicates that the most common or abundant insect species are facing the biggest losses. This revelation challenges existing ideas about how changes in insect biodiversity occur.

The iDiv researchers focused on terrestrial insects like beetles, moths, and grasshoppers. They discovered that the declines in locally common species contribute significantly to overall insect declines. The term “common species” refers to insects found in the highest numbers locally, and these vary by location.

Studying the loss of insect species

The research is timely, as it addresses the growing concerns about the dramatic decline in the total number of insects globally. However, detailed trends among locally rare and abundant species over extended periods have remained less understood. 

Roel van Klink, the study’s lead author and a senior scientist at iDiv and Martin Luther University Halle-Wittenberg (MLU), emphasized the need to study these trends.

“It was obvious this needed exploring,” said van Klink. “We had to know whether observations about declines in total abundances of insects differed among common and rare species, and how this translated into changes in the overall insect diversity.”

Key insights from the study

To investigate, the researchers compiled a database from 106 studies, with data spanning 9 to 64 years. One notable study included in their database, focused on ground beetles, began in 1959 in the Netherlands and is still ongoing.

The findings are striking: land-based insects in these studies are declining by an average of 1.5% annually. However, the most abundant species at the start of the studies showed an average decline of about 8% per year, a much steeper rate than that of rarer species. 

The decline in dominant species is not offset by increases in other species, which has significant implications for ecosystems. Abundant species serve as crucial food sources for birds and other insectivores, and their loss indicates substantial changes in food webs and ecosystem functioning.

“Food webs must already be rewiring substantially in response to the decline of the most common species,” explained van Klink. “These species are super important for all kinds of other organisms and for the overall functioning of the ecosystem.”

Local insect species losses

The experts also found that while formerly abundant species are losing the most individuals, less abundant and rare species are not spared, contributing to a decrease in local species numbers. 

Despite some rare species going locally extinct, there was a modest overall decrease in species numbers by just under 0.3% annually. Interestingly, some new species have successfully established themselves, occasionally becoming very abundant, like the invasive Asian Ladybeetle (Harmonia axyridis).

While the study did not explicitly investigate causes, the declines are likely linked to human-related impacts such as climate change and urbanization. 

According to the study authors, further research is needed to determine the underlying causes of these trends. 

Disproportionate impacts

Professor Jonathan Chase, senior author of the study and professor at iDiv and MLU, pointed out that insects seem to be disproportionately affected by human activities compared to other species groups.

“Insects seem to be taking a heavier hit than many other species as humans continue to dominate the planet,” said Professor Chase. “Other studies, including those our team has worked on, have not found such diversity declines at local scales from many other groups of animals and plants.”

The lifeboat effect 

The study’s results are primarily based on data from Europe and North America, meaning they might not represent a global phenomenon. 

“The patterns we observed might be a best-case scenario for quantifying the real impact of people on insects,” said Professor Chase, referring to what scientists call the lifeboat effect. 

“These declines were observed in long-term data from areas that have remained largely intact, sort of like a lifeboat, rather than in areas where massive conversion of natural areas into human-dominated landscapes has occurred, such as malls and parking lots.”

The study is published in the journal Nature. 

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