Scientist takes the same walk 40 years in a row and makes a shocking discovery

In 40 years, the Utrecht ecologist completed the three-kilometre walking route 254 times and each time he counted the hoverflies he encountered during his walk. And those counts paint a disturbing picture.

That is what Barendregt and colleagues write in a research article based on these counts that appears today in the magazine Insect Conservation and Diversity† Barendregt counts show that the number of hoverflies in this place – a forest on the Veluwe – has decreased considerably. In 2021, for example, there were on average 80 percent fewer hover flies counted than forty years earlier, when Barendregt made his first walk in the area in 1982. And it’s not just the absolute numbers that seem to have fallen sharply; diversity has also declined. For example, in 2021 on average 44 percent fewer species were found than in 1982.

Veluwe

As mentioned, the disturbing results are the result of 254 walks that Barendregt made 40 years in a row in the same place. This is a walking route in a mixed deciduous and coniferous forest west of Garderen, a village in the Veluwe. The forest is surrounded by other forests and has changed little in the last 40 years. It makes the rapid decline of gliding in this area somewhat puzzling; in any case, it does not seem to be due to the management of the forest. “Acid rain and nitrogen deposition are possible explanations for the decline, as well as the influx of pesticides,” says Barendregt. “Weather during and prior to the counting days could not explain the decline in gliding, but that does not rule out other effects of climate change.”

explanations

What is also striking is that the decrease is in fits and starts. For example, the number of gliding gliding declined particularly sharply in the period between 1982 and 1990, before stabilizing in the 1990s. Since the beginning of this millennium, the numbers have declined sharply. In the 1980s, especially rare species of hoverflies disappeared. After 2000, species that were still common in the decades before also became rarer.

worrisome

The findings are worrying. Hoverflies play an important role in ecosystems. They are, after bees, the most important pollinators in our country. And some species are avid eaters of aphids.

Germany

Surprisingly, the findings may not be immediate. For example, two previous German studies have already shown that gliding today is having a hard time. “In one study, entomologists in German open grasslands with six insect traps counted the numbers of insects, including hoverflies,” says researcher Eelke Jongejans. “This new study concerns a mixed forest, with very different hoverfly species. The fact that the results are roughly comparable means that the hover fly is just as dramatic in other ecosystems.”

This is the first time – certainly in the Netherlands – that pollinators have been counted in one place for such a consistent and long-term basis. However, that achievement is heavily overshadowed by the results that come from those counts. “The importance of taking the same walk for so long and counting accurately is now clear, but the disturbing findings weigh heavily on my heart,” says Barendregt.