Science - May 15, 2018

A different approach of agriculture to stop insect decline

Roelof Kleis

If we want to restore insect populations in the Netherlands, farmers will have to drastically change the way they work. The time is ripe, says professor of Nature Conservation and Plant Ecology David Kleijn.

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Kleijn is the main author of a study into the decline of insect populations in the Netherlands. The study was commissioned by the Dutch Ministry of Agriculture, Nature and Food Quality following a German study that caused a great deal of controversy. That long-term study showed that the insect population decreased to merely a quarter of its numbers in thirty years’ time. What is the situation in the Netherlands and what can be done about it was what Minister Carola Schouten wanted to know.

Besides support, the German study was also criticised. Wageningen ecologists were among the people asking questions. Undeserved, Kleijn and his colleagues concluded. The German study is sound. But the situation in the Netherlands is most probably less dramatic, the Wageningen evaluation shows. Despite that conclusion, there is plenty reason to worry, says Kleijn. Even Minister Schouten used the word ‘alarming’ in her letter to the House of Representatives following the results of the Wageningen study.

But Kleijn confirms that the exact severity of the insects’ situation is not clear. The correct data is simply missing. Thanks to many years of monitoring of the ‘cute’ insects such as butterflies and dragonflies, the populations of those groups are well itemised. But data regarding biomass, i.e. the total numbers of insects, is lacking. A study comparable to the one performed in Germany would therefore not be possible in the Netherlands.

Furthermore, the studies that have been carried out are not unanimous. For example, the decline of species in nature reserves seems to have been somewhat halted, but the decline of the species that depend on the agricultural landscape has not. Besides, there is barely any data concerning the latter group. Counting insects is usually done by volunteers, and they prefer to count in the nature reserves – after all, those offer much more to see.

We don’t actually know which group dominates the insect composition in a given habitat.
David Kleijn

According to Kleijn, an additional issue is that until recently, no one was interested in the biomass of insects. ‘That has only changed a few years back, following the discussion about bee mortality and the decline of meadow birds. Call it newly gained insight. We have slowly come to realise that bulk insects are important as well. Better yet: they are very important for the food web.’ Kleijn therefore advises the Minister to better map out insect groups in agricultural areas in particular.

Besides better monitoring, Kleijn thinks a better insight into insect societies is needed. ‘We don’t actually know which group dominates the insect composition in a given habitat. For me personally, this was an eyeopener during our study.’ Another aspect is that it is not clear exactly why insects are declining. But according to Kleijn, the important role of agriculture in this is beyond dispute. He suggests starting to think as soon as possible about recovery measures that farmers could apply.

A drastic change in agriculture is necessary, Kleijn thinks. ‘And everyone pretty much agrees, including most farmers, although – understandably – they would like to be sure that they will be able to earn their bread with the new form of agriculture. The intention is to change to other cultivation systems that would be less invasive. The time is ripe. We all see we are heading for a dead end. And that is a fantastic starting point to make a difference.’

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