A study published October 18, 2017 in the PLOS one magazine, describes an alarming scenario: Over 75 percent of flying insects have disappeared in the last 27 years.
Researchers measured total biomass of flying insects caught in traps, which were located in 63 nature protection areas in Germany. Although protected areas were chosen to control other variables, the locations represent a variety of Western European low altitude human dominated landscapes.
Insects were collected between 1989 and 2016 to quantify the decline in total insects biomass. The data do not exhibit changes within a certain species, nor do they quantify the decline for a specific location. But, as the authors point out, the total loss of biomass quantifies the food loss for predators, like birds, who feed on a certain amount of insects rather than on a certain species.
Furthermore, the researchers could show a seasonal change. While the seasonal average decline (weighted by seasonal occurrence) was 76 percent, the mid-summer decline, when the number of insects is highest, raised to 82 percent.
Weather, land use and habitat data were gathered to correct the interference of these variables with the existence of insects. But the researchers could not find a strong correlation between habitat changes and the decline of insects. „Given the major decline in insect biomass of about 80%, much stronger relationships would have been expected if changes in habitat and land use were the driving forces,“ the authors wrote.
They assume climate variables like droughts or lack of sunshine, which both hadn’t been measured, as well as agricultural intensification could explain the dramatic decline. „Part of the explanation could therefore be that the protected areas (serving as insect sources) are affected and drained by the agricultural fields in the broader surroundings (serving as sinks or even as ecological traps).“
Intense agriculture, characterized by (increased) use of pesticides and fertilizers, year-round tillage and monocultural farming, is already known to cause an overall decline in plant-, insect- and bird-diversity (see Benton et al., 2002; Thomas et al., 2004; Hallmann et al., 2014) .
But the researchers also admit that „large-scale factors must be involved“, which have yet to be uncovered.