Insects: Decline on Land, but Recover in Freshwater

Habitat Destruction

Scientists are uncertain about the trends of the decline of land insect populations, and aquatic insect population recoveries. Significantly, however, they can relate natural habitat destruction by urbanization with the fall in terrestrial insect numbers. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment blame land-use change and habitat destruction principally cause global biodiversity loss.

The German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig (iDiv) and their synthesis center (sDiv) facilitated the study reviewed here. As the most thorough examination of the global state of insects, the study illustrates where protection is essential.

A careful compilation of continuous investigations on insect abundance lists the number of terrestrial insects in decline—the global decrease of 0.92% per year equal about 25% over 30 years. Freshwater insects–imagine midges and mayflies–has increased on average by 1.08% each year. Sufficient water protection policies likely account for the increase. Local trends, however, vary profoundly, and the places least human-impacted have shown a more limited decline. 

The journal Science has published this extensive study of insect population change that encompasses 1676 localities. Researchers from (iDiv), Leipzig University (UL), and Martin Luther University Halle-Wittenberg (MLU) devised the program. It fills critical holes in our knowledge of insect declines.”

Insect biodiversity accounts for a large proportion of all biodiversity—over half of the estimated 1.5 million organism species described are classified as insects.

Several published studies show dramatic declines in insect numbers over time. Published data from nature reserves in Western Germany, for example, reported a greater than 75% decrease in biomass over 27 years. After publication in 2017, subsequent studies reported mostly decline, to varying degrees, and, in some cases, even increases. No one had yet compiled the available data on global insect abundance trends.

Current Data

Investigators had not collected data on just how widespread and deep are insect declines. The international scientific team that co-operated on assembling comprehensive data from 166 long-term surveys from 1676 global sites global. They investigated individual insect numbers–not species abundance–between 1925 and 2018.

The aggregate analysis exposed a high variation in patterns even at sites in proximity to each other. For example, in places, such as Germany, the UK, and the US, in places undergoing declines, neighboring locations registered no change or, indeed, some increases. The consolidated trends worldwide did not permit researchers to determine how total insect abundances were changing on average across time.  

The percentage of a country’s population that lives in urban areas.

Modest Annual Insect Decline

They found that for terrestrial insects like butterflies, grasshoppers, and ants, there was an average decrease of 0.92% per year. This seemingly small number, however, indicates 25% fewer in 30 years and 50% fewer over 75 years. These values demonstrate how insect declines proceed modestly, and without a yearly warning.

In the data, we see substantial declines since 2005. The sharpest drops are in the West and Midwest US, and Europe, most significantly in Germany. For Europe broadly, long-term trends are chiefly negative.

The new study confirms decreasing flying insect numbers. However, the majority of insects are less noticeable because they inhabit the soil, forest or woodland canopies, or the water. The researchers also counted historically fewer numbers in the grass and on the ground. The number of insects living in trees has been constant over time.

Insect population windshield survey

Freshwater Insect Recovery

Aquatic insects, like midges and mayflies, had an average annual increase of 1.08%–or a 38% gain over 30 years. In parts of Northern Hemisphere, i.e., Northern Europe, the Western US, and, after 1989, Russia, the positive trend was most marked–an indication that we can reverse negative trends. The World has cleaned up many of its polluted freshwaters in globally.

Two water striders mating

In the US, as Environment America states about its work, “(We take) concrete steps to move us closer to the World we want to live in, from holding illegal water and air polluters accountable to networking local citizens. They serve as champions of the local waters they love.” Such actions have and will lead to the recovery of many aquatic insect populations.

In a greener, healthier world, all of us would treat our air and water as the precious life-giving resources they are, not as dumping grounds for our waste.

The freshwater insects’ revival demonstrates the possibility of recovery. Reversing the trend for currently declining land insect populations now also seems reachable. We must identify, however, the causes of declines to make sufficient progress toward restoration. Accumulating the data for a broad range of species and locations globally is essential for global conservation management. Insect decline is a world problem that will probably be resolved at the local level but with worldwide cooperation and collaboration.

Story Origin:

Materials provided by the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-LeipzigNote: We edit for style and length.

Journal Reference:

  1. Roel van Klink, Diana E. Bowler, Konstantin B. Gongalsky, Ann B. Swengel, Alessandro Gentile, Jonathan M. Chase. Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundancesScience, 2020; 368 (6489): 417-420 DOI: 10.1126/science.aax9931