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Scientists have observed a drastic decline in grasshopper populations on natural areas. The insect decline might be driven by another cause that has received little attention.
In a recent long-term study at the Konza Prairie in Kansas, researchers from the US report a significant decrease in grasshopper populations (Orthoptera). In two studies spanning 16 and 22 years each, grasshopper populations declined by an average of 2.1 to 2.7 percent per year, leading to a reduction of more than a third over the entire study period.
No insecticides, no sealing - insects still die
Particularly alarming is that the declines occurred in an environment where common causes of insect decline - such as pesticides, land sealing, or intensive farming - were excluded. Both grazed and ungrazed areas were affected. Scientists speak of a decline in an almost disturbance-free natural environment.
Surprising: Why CO2 makes plants grow and insects starve
In recent years, the concentration of carbon dioxide (CO2) in the air has increased. Higher CO2 concentrations promote photosynthesis and lead to better plant growth. This was also noticeable in the Konza Prairie. Biomass development increased by 60 percent during the study period. However, the increased biomass development is deceiving: while the amount of plants increased, the concentration of important nutrients in the tissue, such as nitrogen, phosphorus, or sodium, simultaneously decreased. This effect is known as nutrient dilution. It hits plant-eating insects like grasshoppers particularly hard. They have to eat more to meet the same nutrient needs - which often fails. The result: poorer development, reproduction, and ultimately population decline.
Climate also influences the number of insects
The researchers also observed regular fluctuations in grasshopper populations in a five-year cycle. These cycles correlated with climate phenomena such as the El Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific Decadal Oscillation (PDO). These phenomena influence regional weather, plant growth - and ultimately the living conditions for insects.