The biggest assessment of global insect abundances to date shows a worrying drop of almost 25% in the last 30 years, with accelerating declines in Europe that shocked scientists.

The analysis combined 166 long-term surveys from almost 1,700 sites and found that some species were bucking the overall downward trend. In particular, freshwater insects have been increasing by 11% each decade following action to clean up polluted rivers and lakes. However, this group represent only about 10% of insect species and do not pollinate crops.

Researchers said insects remained critically understudied in many regions, with little or no data from South America, south Asia and Africa. Rapid destruction of wild habitats in these places for farming and urbanisation is likely to be significantly reducing insect populations, they said.

Insects are by far the most varied and abundant animals, outweighing humanity by 17 times, and are essential to the ecosystems humanity depends upon. They pollinate plants, are food for other creatures and recycle nature’s waste.

The previous largest assessment, based on 73 studies, led scientists to warn of “catastrophic consequences for the survival of mankind” if insect losses were not halted. Its estimated rate of decline was more than double that in the new study. Other experts estimate 50% of insects have been lost in the last 50 years.

Recent analyses from some locations have found collapses in insect abundance, such as 75% in Germany and 98% in Puerto Rico. The new, much broader study found a lower rate of losses. However, Roel van Klink, of the German Centre for Integrative Biodiversity Research in Leipzig, who led the research, said: “This 24% is definitely something to be concerned about. It’s a quarter less than when I was a kid. One thing people should always remember is that we really depend on insects for our food.”

The research, published in the journal Science, also examined how the rate of loss was changing over time. “Europe seems to be getting worse now – that is striking and shocking. But why that is, we don’t know,” said van Klink. In North America, the declines are flattening off, but at a low level.

Elsewhere, data is much more sparse. “But we know from our results that the expansion of cities is bad for insects because every place used to be more natural habitat – it is not rocket science,” said van Klink. “This is happening in east Asia and Africa at a rapid rate. In South America, there is the destruction of the Amazon. There’s absolutely no question this is bad for insects and all the other animals there. But we just don’t have the data.”

Van Klink said the research showed that insects were faring only slightly better in nature reserves than outside protected areas. “We found that very striking and a bit shocking – it means something’s going wrong there.”

Losses of insects are driven by habitat destruction, pesticides and light pollution. The impact of the climate crisis was not clear in the research, despite obvious local examples. Van Klink said changes in heat and rain could harm some species while boosting others, even in the same location.

But he highlighted another study showing that rising carbon dioxide levels are reducing the nutrients in plants and significantly cutting grasshopper abundances on prairies in Kansas, US. ”That is absolutely shocking, because that could be happening all over the world.”

Prof Dave Goulson, of the University of Sussex, who was not involved in the new analysis, said: “People should be as concerned as ever about insects. It is great news that some aquatic insects seem to be increasing, probably from a very low level. But the bulk of insects are terrestrial and this new study confirms what was already clear: they have been declining for many decades.”

Matt Shardlow, the head of the conservation charity Buglife, said: “Many insect species are threatened with extinction and this study shows insect abundance is also declining at an unsustainable rate. While the estimate in this study is lower than some, it is still very steep. Massive abundance declines in flying insects remains a developing ecological disaster.

In a comment article in Science, Maria Dornelas, of the University of St Andrews, and Gergana Daskalova, of the University of Edinburgh, said the new study was the largest and most complete meta-analysis to date. “Embracing nuance allows us to balance accurate reporting of worrying losses with hopeful examples of wins,” they said.

Van Klink said: “We definitely have a lot of reason for concern, but I don’t think it’s too late. The increase in freshwater species makes us at least hopeful that if we put the right legislation in place, we can reverse these trends.”