Fears of an “insect apocalypse” caught the world's attention with a study a few years back that found a 75 percent decrease in flying insects in Germany over 27 years. Last year, a paper claimed the problem was global. Although the disappearance of fleas and mosquitoes might sound appealing, insects are so essential for pollination and as the basis of the food chain, the rate of such losses could be catastrophic for other life. A new study indicates the trend is not as widespread as claimed, but still represents local catastrophes that could be spreading.
Last year's paper, published in Biological Conservation, found a 2.5 percent annual decline over the last quarter of a century across 73 long-term studies of insect biomass. The authors blamed agriculture, particularly the use of pesticides, writing that “the conclusion is clear: unless we change our ways of producing food, insects as a whole will go down the path of extinction in a few decades.”
However, the paper faced swift criticism for being heavily skewed towards Europe and North America. So few studies were included from tropical regions, there was no way to know if they were representative. Dr Roel van Klink of the German Center for Integrative Biodiversity Research has responded in Science, analyzing 166 long-term studies from 1,676 locations in 41 countries, and finds a far more varied pattern.
“Insects are the most ubiquitous and diverse animals on the planet,” van Klink and co-authors note, making the question of their survival a serious one for us all.
Some studies of insect trends count the number found at a site, while others measure the combined weight of those collected. A few helpfully do both. Some past meta-analyses have looked at one measurement or the other, but by including both van Klink was able to get a larger and more representative pool of data.
On average, van Klink found a decline of 0.92 percent per year for terrestrial insects, including those that fly, mainly over land. Although this is less than half the rate of previous reports, van Klink said in a statement that "0.92 percent may not sound like much, but in fact it means 24 percent fewer insects in 30 years' time and 50 percent fewer over 75 years.” The paper identifies habitat destruction, rather than pesticides or climate change, as the prime cause.
On the other hand, the trend for freshwater species is surprisingly in the other direction – and of similar size. With most of the studies having started in the 1980s and 90s, this appears to reflect the clean-up of once heavily polluted rivers and lakes, demonstrating what is possible with effort.
Although van Klink's sample was larger, it suffers from the same problem as previous meta-analyses. The overwhelming bulk of the studies were from Europe and the United States – there just haven't been the long-term studies over most of the planet to really know what is going on.
Moreover, the trends in Europe have accelerated since 2005, a bad sign for regions still to industrialize.