Photo David Kleijn Bees, butterflies and other pollinators are struggling in the farmland-dominated countryside. Attempts to make the countryside more appealing to them by planting more flowers has far less effect than expected. Expanding the area of suitable habitat would help pollinators most.
That is the crux of an international study headed by WUR that was published on Thursday in Science. ‘Summarizing, the message is: size, size, size,’ says professor of Plant Ecology and Nature Conservation David Kleijn. ‘Invest in the size of the habitat. Investing in the quality isn’t a quick fix if the size is too small.’
With distinction
The lead author Gabriella Bishop was awarded a distinction last year for her PhD research, which formed the basis for this publication. The article quantifies the effects of the size and quality of semi-natural habitats on pollinator numbers. Semi-natural habitats are non-productive sections of land in agricultural areas, such as ditch edges, brushwood banks, hedgerows and copses.
‘When comparing habitats, we normally look at differences in insect density, without considering the size of the habitat,’ explains Kleijn. ‘That’s an omission. A flower verge of a hundred square metres will obviously have a far smaller impact than one measuring several hectares.’
A flower verge of a hundred square metres will obviously have a far smaller impact than one measuring several hectares
David Kleijn, professor of Plant Ecology and Nature Conservation
His group therefore developed a statistical method to distinguish between the effects on insects of the size and quality of a habitat. The calculations show which of the two factors has more effect on the insect population size. Bishop then applied the method to datasets from around the world and deduced generally applicable patterns.
The result is an eye-opener for nature conservationists. The size of a habitat is far more important than was previously thought, until a certain point is reached, after which the quality becomes more important. But this is not a one-size-fits-all relationship, as the turning point depends on the pollinator.
The turning point for solitary bees, for example, is 16 per cent of the land. When the extent of the semi-natural habitat exceeds that point, quality starts to matter more than size. The turning point for hoverflies is reached at around 5 per cent. Butterflies are most ‘sensitive’ to size, as their turning point is found when semi-natural areas make up 37 per cent.
A future
The turning point for butterflies is never reached in practice in farming areas. However, Kleijn says that doesn’t mean butterflies don’t have a future in agricultural areas. Kleijn: ‘For these insects too, more habitat means more butterflies. But improving the quality of the habitat only makes sense if there is a large area of such habitat.’
Kleijn was also surprised by the results. ‘Before this study, I too thought we should start by boosting the quality of the habitat. This research shows that it won’t have much effect if the size of the habitat is below the turning point. You need to make a huge improvement in quality to achieve the same effect as you get from increasing the size by a few per cent.’
‘You can basically stop bothering about measures on farmland to improve the quality for pollinators,’ concludes Kleijn. ‘A lot of effort and money is currently being invested in measures such as herb-rich grassland on dairy farms and greater crop diversity on arable farms. But these approaches are much less effective than simply reserving a bit more land for nature and compensating farmers for the loss.’
The results of this study therefore offer a starting point for conversations with the agricultural sector
David Kleijn, professor of Plant Ecology and Nature Conservation
According to Kleijn, the findings from the study tie in with what farmers themselves want. ‘If you ask farmers which measure they find most acceptable for boosting biodiversity, they are keenest on turning farmland into new nature. They prefer that to having smaller plots with wider nature verges or introducing greater diversity in the crop rotation. The results of this study therefore offer a starting point for conversations with the agricultural sector.’
An important caveat is that the turning points don’t tell you how much pollinator-friendly land is needed. Kleijn: ‘We indicate the point up to which it pays to invest in increasing the size of the habitat. We can determine the turning points, but not the absolute lower limit for what is needed. The lower limit depends on how many pollinators you want.’
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