Threats to the insect world are growing

The insect world is dwindling. By 2100, half of all insects could be gone. But there could be gainers too.

LONDON, 30 April, 2020 − The butterflies are quietly flying away, the beetles are buzzing off, and the insect world is shrinking. The Earth’s land-based insects are in steady decline, their numbers falling by around a quarter every three decades.

And although there could be a whole world of reasons for the global loss of a vital class of animals, European scientists have pinpointed at least one, in one location.

Insect food plants are being lost in the Swiss canton of Zurich, and with them, many of the hoverflies, bumblebees, bees and butterflies that depend on them.

Scientists from Germany and Russia report in the journal Science that they examined the bigger story told by data from 166 surveys of insects and arachnids – that is, not just flies but spiders too – across 1,676 sites worldwide, over periods from 1925 to 2018, and many of them of around 20 years.

Largely missed

They found that those insects that based their lives on land rather than water were slipping away at an average of 0.92% per year. “0.92% might not sound like much, but in fact it means 24% fewer insects in 30 years’ time and 50% fewer over 75 years,” said Roel van Klink of the German Centre for Integrative Biodiversity Research and based at the University of Leipzig.

“Insect declines happen in a quiet way and we don’t take much notice from one year to the next. It’s like going back to the place where you grew up. It’s only because you haven’t been there for years that you suddenly realise how much has changed, and all too often not for the better.”

He is not the first to draw attention to insect loss: other groups have warned of dramatic instances of decline and imminent extinction, along with the changes in insect populations and the disappearance of the habitat on which so many species depend.

But the researchers found the decline wasn’t uniform. Those insects – midges and mayflies, for example – that are essentially aquatic were actually increasing in number, on average by more than 1% a year. Flying insects overall however are in decline, and ground-dwellers and grassland insects too are slowly losing the battle for survival, while the numbers of insects in the woodland treetops remain about the same.

“Insect declines happen in a quiet way and we don’t take much notice from one year to the next”

Insect declines in Europe and the US West and Midwest were marked, but those insects that live for part of their lives in water in northern Europe and the western US showed a 38% increase over 30 years: this may reflect national and international attempts to limit pollution of the waterways. In both decline and revival, the scientists at work see the impact of human handling of natural habitat.

“Insect populations are like logs of wood that are pushed under water,” Dr van Klink said. “They want to come up while we keep pushing them down. But we can reduce the pressure so they rise again.

“The freshwater insects have shown us this is possible. It’s just not always easy to identify the causes of declines, and thus the most effective measures to reverse them. And these may also differ between locations.”

But within a day of the publication of the Science analysis, German and Swiss scientists had identified the cause of decline in one closely-observed area. They report in the journal Ecological Applications that over the past century there had been an overall decline in wild food plants for all kinds of insects in the Zurich canton.

Urban spread

Wetlands had shrunk by around 90%, the cities and towns had expanded, intensive farming had meant the loss of meadows and farmland habitats.
With help from 250 volunteers, researchers had made detailed studies of the 1,719 seed plant species in 1km plots of land at 3km intervals across the whole canton, between 2012 and 2017.

They then identified 966 of those plants visited by daytime pollinators, and compared their findings with highly-detailed data assembled about the vegetation of the canton before 1930.

Some specialised groups of insects evolved in partnership with equally specialised insects. The scientists found that, for instance, greater knapweed or Centaurea scabiosa was in decline, which was bad news for those bumblebees, bees and butterflies with tongues long enough to reach the nectar. The poisonous plant aconite, or Aconitum napellus, is pollinated by a bumblebee impervious to its toxin. Once again, the loss of floral variety and insect life even in one much-occupied place may not have been obvious.

“It’s hard for us to imagine what vegetation looked like 100 years ago,” said Michael Kessler, a botanist at the University of Zurich. “But our data showed that about half of all species have experienced significant decline in their abundance, while only about 10% of the species have increased.” − Climate News Network