Some disturbing news has come out recently, in the form of a paper in Biological Conservation, reviewing evidence that insect populations around the world are falling at a shockingly high rate (https://doi.org/10.1016/j.biocon.2019.01.020). In a survey of 73 studies on insect populations world-wide, the authors show that according to the papers surveyed, 41% of all insect species are in decline, with a 2.5% loss in total insect biomass every year.
On land, the biggest losses in biodiversity are in dung beetles, followed by moths, butterflies and bees (the authors note that "the fate of other pollinators such as hoverflies is ... largely unknown"). However, the situation is even worse among aquatic insects (33% of species are endangered vs. 28% of terrestrial species), with major losses among dragonflies, mayflies, stoneflies and caddisflies.
The authors identify the main causes as originating in intensive agriculture during the last century, leading to habitat loss and pollution from pesticides and fertilisers. A smaller role is also played by various biological factors including diseases and the effects of other introduced species, as well as by climate change.
Plummeting insect numbers are of course highly concerning because of their fundamental position in ecosystems, playing a role in pollination, recycling nutrients and decomposition (see also Kevin's recent post) and importantly as food for many vertebrate species.
Species that have evolved as specialists in one ecological niche are the most susceptible to loss. In many cases, these species are being replaced by other generalist species, which some could argue could start to fill the roles vacated by the specialists. But as the authors point out, just as import as the loss of insects in general is the fact that the loss of individual species compromises the long-term resilience of ecosystems.
Some caveats may exist on the conclusions drawn in the paper. In some quarters there has been criticism of the search methodology used to select papers for inclusion in the survey, which may have biased the pool of papers towards ones showing declining numbers only. Also, many major species of insects were not included in the study, due to a lack of data, and vast areas of the globe were not included for the same reason.
In fact, one possible glimmer of hope is that the vast majority of the surveyed studies were based in the highly industrialised and developed countries of North America and Europe, with some evidence from the remaining studies that the rate of decline may be lower in other regions of the world.
In response to the paper, some news sites have offered suggestions for actions that people can take locally to help to alleviate the problem. For two such examples, see here and here.