Should we fear “insectageddon”?

The world’s insect population is facing mass extinction by the end of the century, according to some experts. That would have catastrophic implications. Is it true? Simon Wilson reports.

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The ecosystem will suffer if insect numbers fall

What has happened?

A shocking academic paper, published in the journal Biological Conservation in February, issued a disturbing warning that the world's insect population is facing mass extinction by the end of the century if the current dramatic rate of decline continues. The peer-reviewed paper, co-authored by Francisco Snchez-Bayo of the University of Sydney and Kris Wyckhuys of the China Academy of Agricultural Sciences in Beijing, is a global scientific review of 73 previous academic studies of the subject (the first such meta-analysis).

The review concluded that a "sixth major extinction event" is under way on Earth, that 40% of insect species are likely to disappear within decades, and that the total biomass of insects is currently falling by 2.5% a year. If that can't be reversed, the "repercussions this will have for the planet's ecosystems are catastrophic to say the least".

What's causing the decline?

"The main cause of the decline is agricultural intensification," according to Snchez-Bayo, an environmental scientist. "That means the elimination of all trees and shrubs that normally surround the fields, so there are plain, bare fields that are treated with synthetic fertilisers and pesticides." According to the report's authors, the demise of insects appears to have begun around the start of the 20th century and accelerated during the 1950s and 1960s.

In the last two decades, the decline has reached "alarming proportions" a phenomenon they ascribe to new classes of insecticides, including neonicotinoids and fipronil, which have been particularly damaging as they are used routinely and have long-lasting sterilising effects on the soil. Wider habitat destruction and climate change are also factors, the experts say.

Why are insects so important?

Because they are easily the most abundant animals on the planet there could be tens of millions of species, on some estimates and they are essential to the functioning of all ecosystems, whether as food for other creatures, pollinators, or recyclers of nutrients. If insects disappeared, the result would be a "bottom-up trophic cascade", affecting the whole food chain and wiping out higher animals. Falling insect abundance in Central America's tropical forest, for example, has been accompanied by parallel falls in the numbers of insect-eating frogs, lizards and birds.

In Britain, farmland birds big eaters of insects have halved in number since 1970 (some species, such as the grey partridge and spotted flycatcher, have declined by more than 80%; the beetle-eating red-backed shrike became extinct in Britain in the 1990s). A 2014 UK government report estimated the annual "value" of insect pollinators in this country alone is £603m. A 2010 academic study estimated that crop pollination is worth $153bn annually to the global economy; others put the figure at $500bn.

Does everyone agree there's a crisis?

Not everyone agrees that the Biological Conservation study presents a convincing case, and some critics have argued that it is radically flawed. The most basic issue is that this is a survey of the literature, which by its own admission surveys only those studies reporting a decline in insect numbers. This "unidirectional methodology" means that it is necessarily partial, and 73 studies isn't a particularly large dataset anyway. Second, it has a heavy bias towards studies carried out in Europe and the US, where modern agricultural methods are almost universal.

By contrast, there have been no surveys to date of wild insect numbers in India, China, Siberia, the Middle East or Australia, and only a single study in each of South America, sub-Saharan Africa and southeast Asia. That's a big gap, since these areas include almost all the tropics where the majority of insect species are thought to live. In other words, the 73-paper survey is worrying, but it would be unwise to extrapolate too far based on its findings. And overhyping the issue could be counter-productive, if it causes us to ignore it.

What other evidence is there to go on?

Two other large-scale recent studies lend scientific credence to what entomologists call the "windshield phenomenon" the widely reported anecdotal observation that people are seeing far fewer insects smashed on the windscreens of their cars nowadays than they did ten, 20 or 30 years ago.

One of the studies drew on data from a long-established society of naturalists in Krefeld, Germany. Members amassed 27 years of data on numbers of flying insects captured using unchanged methods (malaise traps) and sampling spots; academic analysis of the results calculated an overall decrease in insect biomass of more than 75%.

Another similar study, in the Luquillo forest of Puerto Rico, comparing data from the 1970s and the early 2010s, found a collapse in arthropod biomass of more than 90%. These are just two of scores of studies that have found declines in different measures of insect abundance in the order of 50% to 80% from Borneo to California to Britain.

So there are reasons to be fearful?

Indeed. For one thing, the recorded declines in insect abundance are greater than for other taxonomic ranks, such as birds or plants, with almost all species and subspecies that have been studied affected. Second, there are so many influences contributing to the decline, including habitat loss, intensive farming, pesticide use, and the spread of diseases and parasites. Third, dwindling numbers have the potential to disrupt ecosystems dramatically, say ecologists, even where that decline falls short of extinction.

It is "hard to argue that insect decline is yet wreaking significant economic damage", reckons The Economist. But that's no reason to be sanguine. Given the relative paucity of reliable data, it may be too soon to declare a global insect emergency that could trigger an ecosystem collapse. But it would be "reckless to find out by actually triggering one".

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