A new study has suggested that the global decline in insect populations is far more complicated than previously thought by scientists.
While previous research have shown a sharp fall in insect populations globally, with losses of up to 25% per decade, the new study indicates that the data is actually more complex and varied.
Authors of the research, the largest carried out to date, said that while land-dwelling insects are definitely declining, bugs living in freshwater are actually increasing.
The decline in insect numbers and what's actually happening
Insects are among the most abundant and diverse species on earth and they play critical roles, including soil aeration, pollination and recycling of nutrients. Reports of the fast and widespread decline in their populations have caused scientists to worry.
A case study conducted at 63 nature protection areas in Germany revealed an alarming decline in insect populations, with around a 75% decrease over a span of 27 years. Several studies have followed but were mostly specific to a region or a species.
The new study, which examined data from 166 long-term surveys across 1,676 sites, tried to give a more highly nuanced and variable understanding of what's really happening to insects globally.
Results showed that the populations of insects such as butterflies, ants and grasshoppers are falling by 0.92% annually or around 9% per decade, which is significantly lower than many published numbers.
Lead author Dr. Roel Van Klink from the German Center for Integrative Biodiversity Research said: "That is extremely serious, over 30 years it means a quarter less insects. And because it's a mean, there are places where it is much worse than that."
Windscreen Phenomenon
According to the researchers, numerous people perceive the decline in insect numbers based on the so-called "windscreen phenomenon", where you find fewer dead insects splattered on cars.
They confirmed that this was actually the case. Professor Jonathan Chase, another author from the German Center for Integrative Biodiversity Research, mentioned: "Many insects can fly, and it's those that get smashed by car windshields. Our analysis shows that flying insects have indeed decreased on average."
Prof. Chase continued: "However, the majority of insects are less conspicuous and live out of sight - in the soil, in tree canopies or in the water."
Freshwater bugs thriving
While many land-based species are declining, the new study found that insects living near fresh water, such as midges and mayflies, are actually growing by 1.08% per year. This was evident in northern Europe, in the western US and since the 1990s in Russia.
The researchers attributed this trend to legislation that has helped cleaned up polluted rivers and lakes.
Dr. Van Klink pointed out: "They are just a fraction of land based insects, not more than 10%. The area of freshwater we have on earth is just a small percentage of the total land mass, so the numbers of freshwater insects will never be able to compensate for the terrestrial insects."
The scientists argued that while there is no single reason for insect declines, they find natural habitat destruction due to urbanization as a key factor.
The 2019 Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment was one of the major research studies that echoed this sentiment on habitat destruction.
Co-author Ann Swengel said: "We've seen so much decline, including on many protected sites. But we've also observed some sites where butterflies are continuing to do well."
"It takes lots of years and lots of data to understand both the failures and the successes, species by species and site by site," she added.
