Science1 publisher2 min readPublished
Unmowed dike strips hold most pollinators for only about four days, Radboud fieldwork finds
Radboud University researchers counted 30,000 flower visits on Dutch dikes and found insects crowd unmowed strips for about four days, then mostly leave. Lead author Constant Swinkels says councils should vary mowing dates across whole landscapes to keep rare species.
The Scientist · Science desk
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What happened
- Dutch dikes are usually mown twice a year, and councils increasingly cut them in phases that leave strips of vegetation for insects to feed and live in.
- The fieldwork compared dikes in the Ooijpolder near Nijmegen where about 70% of the plants had been mown with dikes that were left undisturbed.
- Immediately after mowing, the remaining strips drew twice as many flower visits and far more insects, according to Swinkels.
- Rare species coped poorly with partial mowing, and only strong bees such as honeybees stayed once the crowd had gone.
- The paper, published in Insect Conservation and Diversity, reports a significant difference between mowed and unmowed dikes.
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Why it matters
- constraint Dike safety keeps mowing compulsory, so the only way councils can help rare pollinators is by choosing when and where each cut happens.
- exposure Rare wild pollinators do worst under phased mowing, so dikes managed this way can end up supporting mainly honeybees and other strong bees after each cut.
- decision A council that judges phased mowing by how busy the strips look just after a cut is measuring the part of the response that fades within about four days.
The species identifications matter more here than the visit total. Constant Swinkels, an ecologist at Radboud University, and 40 students identified 5,000 wild pollinators down to species level [13][8]. Counted by visits alone, the first days after a cut would have looked like a gain [5]. The species list shows which insects were still there once the rush ended [6][7].
The partial cut left roughly 30% of the plants standing [14]. The undisturbed dikes are the control. They let the change on the cut banks be attributed to the mowing [1]. The press summary does not say how many dikes were in each group, or where the insects that left ended up. It places the study dikes in the Ooijpolder near Nijmegen [1].
Swinkels still prefers phased mowing to a full cut. "Although partial mowing is better than mowing everything, it is not a silver bullet. There are many species that still fly away, even with partial mowing, in search of a safe haven," he said [10]. Letting dikes grow out is not an option, for safety reasons. "If you don't mow, trees or sturdy shrubs such as blackberries may start to grow. You don't want that, because their roots are so thick that they can cause dikes to collapse," he said [3].
His proposed fix works across many sites at once. "We need to take a landscape-level view of these kinds of problems," he said. He suggested councils check when the grassy area next to a dike is in bloom and set their mowing around it, and offered a simpler rule: "Or mow one dike early and another late. Insects need somewhere to go." [11] On losing habitat for a while, he said: "A small section of dike disappearing isn't such a big problem, as long as the insects can find another safe area." [12]
That remedy is Swinkels's argument from the results. The experiment as described compared partially cut dikes with untouched ones [1]. I think the field result is sound for what it measured [15]. Staggering mowing dates across neighbouring dikes is a sensible next design. It would need its own count of rare species before a council could rely on it.
What to watch
- A field trial of staggered mowing dates across neighbouring dikes, with rare species counted, would test Swinkels's proposed fix directly.
- Counts from dikes outside the Ooijpolder would show whether the four-day window holds in other Dutch landscapes.
- Any change by Dutch municipalities to their phased mowing schedules in response to the paper.