Decline in insect numbers largely due to loss of more abundant species, researchers say

iDiv researchers examined long-term trends in terrestrial insects, such as beetles, moths and grasshoppers, and found that declines in once-common species contributed most to local declines insects. Common or abundant insect species are those species found locally in greatest numbers, but their species differ in different locations. The results of the study, published in Naturechallenge the idea that changes in insect biodiversity result from the disappearance of rarer species.

The study follows recent alarm bells about insect loss, as researchers note a dramatic decline in total insect numbers in many parts of the world. However, little is known about general trends in locally rare and abundant species over long periods of time.

“It was obvious that this needed to be explored,” says Roel van Klink, lead author of the study and senior scientist at iDiv and MLU. “We needed to know whether observations of declines in total insect abundance differed between common and rare species, and how this translated into changes in overall insect diversity.”

The most common species are losers

Van Klink and his colleagues set out to better understand trends in insect numbers by delving into previous studies. They compiled a database of insect communities using data collected over periods of between nine and 64 years from 106 studies. For example, a Dutch study of beetles was started in 1959 and continues today.

With this updated database, the researchers confirmed that despite variations between data, overall, terrestrial insects from these long-term surveys are declining by 1.5% each year. To better understand this trend, they compared the trends of species in different abundance categories and found that the most abundant species early in the time series had the greatest average decline – about 8% per year – while rarer species declined less.

It is important to note that losses of previously dominant species have not been compensated by increases in other species, which has far-reaching consequences: abundant species provide a staple food for birds and other insectivorous animals , which makes them essential to ecosystems.

“Food webs are already having to restructure considerably in response to the decline of the most common species,” says van Klink. “These species are extremely important to all kinds of other organisms and to the overall functioning of the ecosystem.”

Winners and losers

The analysis clearly shows that once-abundant species consistently lose the most individuals compared to less abundant insect species. However, less abundant and rare species also suffer losses, leading to a decline in the number of local species. The study found a slight decline in the total number of species, of just under 0.3% per year. This decline indicates that in addition to significant losses of common species, some rare species are locally endangered.

At the head are the newcomers who have managed to establish themselves successfully. Most of these new arrivals remain locally rare and replace other formerly rare insects, but they sometimes become very abundant. An example of this is the invasive Asian ladybug (Harmonia axyridis), now common throughout Europe, the Americas and South Africa.

According to the paper’s authors, more research is needed to determine the underlying causes of these trends. Although this study did not explicitly investigate possible causes, these declines are likely linked to recent human-caused impacts, such as climate change and urbanization, which are considered major drivers of biodiversity loss.

“Insects appear to be hit harder than many other species as humans continue to dominate the planet,” says Professor Jonathan Chase, lead author of the study and professor at iDiv and MLU. “Other studies, including those our team has worked on, have not found such a decline in local-scale diversity in many other animal and plant groups.”

Although the study results are striking, these trends are heavily skewed compared to data on insect communities in Europe and North America. As such, they should not be interpreted as a global phenomenon. Chase adds: “The patterns we observed may be the best possible scenario for quantifying the real impact of humans on insects,” referring to what scientists have called the lifeboat effect.

“These declines have been observed in long-term data from areas that have remained largely untouched, much like a lifeboat, rather than in areas where massive conversion of natural spaces into landscapes dominated by man took place, like shopping centers and parking lots.”