Despite organic viticulture: Rare bird species threatened by lack of food

Threatened with extinction

The wheatear (Oenanthe oenanthe) has only a few breeding areas left in Germany and is considered to be threatened with extinction. The dry stone walls in Höllenbrand provide good nesting sites for this rare migratory bird, while the surrounding vineyards have long been the basis for its main food: flightless ground beetles and caterpillars. Ornithologist Martin Buchmann counted around 150 breeding pairs in the Gundersheim Höllenbrand in the 1990s. This made the vineyard the largest breeding area for the rare bird in the federal state of Rhineland-Palatinate. Since then, however, the population has been declining. Because the breeding pairs of wheatears are only documented annually up to the year 2000, the German Nature and Biodiversity Conservation Union (Nabu) currently estimates the number of breeding pairs at only around 25. "Open ground is a prerequisite for a sufficient supply of food, preferably raw soil such as that found on military training areas or in vineyards," explains insect researcher Erik Opper, who runs an office for monitoring and mapping.

Together with his colleague Sabine Schwabe, he analysed the food supply of the wheatear in the vineyard last year. Daniel Steffen came up with the idea. The chairman of the Wonnegau district group of the Friends of the Earth Germany (BUND) also works with the insect researcher on other projects. He came across a possible connection between food supply and the wheatear population. "We conducted our study on a voluntary basis without a commission in order to remain independent," explains entomologist Opper, adding: "Whoever pays also takes a stand, but it was important to us to involve all winegrowers." 200 hours of voluntary work went into the recently presented results. "The wheatear forages exclusively on the ground," says Schwabe, explaining the feeding behaviour of the bird, which only feeds on insects. During the two-week rearing period of the nestlings, it is crucial that there are enough soft caterpillars and ground beetle species for the offspring within a radius of up to 50 metres around the nest.

Surprising results

Four wineries were involved in the study: One VDP winery was the only organic winery to work exclusively with copper and sulphur for plant protection. In the trial area, the vegetation was not regularly kept short. The situation was similar at the second winery: Copper and sulphur were applied weekly from mid-May, the trial area was only mown late in the year and only rarely. In the third vineyard, sulphur and copper were only applied in mid-June and at the end of July, with regular mulching. The fourth vineyard was the only vineyard to use glyphosate once at the beginning of April and then copper, sulphur and synthetic plant protection products. The scientists used a 2,500 square metre area with sowing of flowering mixtures and mowing in September as well as a vineyard in neighbouring Westhofen as comparison areas. At the same time, a comparative study was carried out on the occurrence of the wheatear in the Swabian Alb biosphere reserve.

The results of the study were surprising - in many respects. The researchers counted more than 7,740 different insects, 1,347 of which were ground beetles. "The food base for the wheatear is therefore basically there," summarises Erik Opper. They were able to document 44 ground beetle species, including ten endangered species. They also identified many endangered insects. Also surprising: in terms of the quantity and number of different ground beetle species, the area with glyphosate use scored significantly higher than the others. "Maybe you could say it's because of the glyphosate use," says Erik Opper thoughtfully. The use of herbicides means that the soil remains open for a long time and the dead plant material serves as food for ground beetles. "Glyphosate use favours particular species such as ground beetles," explains the researcher, "but this is at the expense of biodiversity." The situation was particularly bad for grasshoppers. The area treated with herbicides also recorded the fewest insect species. Here, the compensation area was able to deliver significantly better figures: All insect species except ground beetles showed significantly more frequent occurrences there. They lack the open ground there.

"We worked with live traps that were checked weekly," Sabine Schwabe explains her approach. The rhythm of the monitoring brought a surprising realisation: in the second week of June - in the middle of the wheatear's breeding season - the ground beetle population collapsed dramatically and did not recover until the end of the growing season. In contrast, the ground beetle population at the reference site in the Swabian Alb increased at this time and remained largely stable until August.

Copper threatens food source

Together with the winegrowers, Erik Opper and Sabine Schwabe set out in search of the causes. One possible cause caught their attention: Plant protection in the vineyards began around a week before the numbers collapsed. "All the winegrowers involved used copper," reports Schwabe. Studies have proven the toxic effect of copper on earthworms. The only study to date on ground beetles from 1995 showed that copper leads to increased mortality of the larvae and reduced motor skills in the adults.

"In discussions with those involved, it became clear that not only has the proportion of organic winegrowers in the bird sanctuary increased, but conventional wineries are also using copper and sulphur more frequently. This could explain why the population has been declining for 15 years," says Erik Opper, cautiously formulating his hypothesis. "If the observation is confirmed, it could shake organic winegrowers," says Andreas Huppert from the Huppert winery involved in the study. The winegrower has gained experience with effective microorganisms as an alternative to conventional plant protection and wants to try working without copper next year. However, he does not want to rule out the possibility that the weather and heat could also have an influence on the population. Adolf Dahlem from the Gundersheim Winegrowers' Association, on the other hand, is convinced that these are the decisive factors: The time of the first plant protection last year coincided with the first heatwave after a cold and wet spring. "This needs to be repeated, taking other parameters into account, in order to be able to make a better comparison," he demands. Dirk Emmich from the Neef-Emmich winery, whose vineyards were also analysed, also believes it is possible that copper is the cause of the decline. "We've known for a long time that copper is a problem - for earthworms or microorganisms in the soil, for example," he says. He thinks it is wrong that nobody is questioning copper as a pesticide.

Sabine Schwabe and Erik Opper are now making preparations for the second round of the ground beetle study in the Höllenbrand. They have expanded the study areas, for example with an area in a Piwi vineyard with up to 80 per cent reduced plant protection and with a conventionally farmed area where synthetic plant protection products have been used for many years, but no copper. The researchers have enlisted the support of the University of Kaiserslautern for the experimental set-up and, in addition to the forms of cultivation, are also incorporating the weather data more closely into the analyses. The winegrowers are supporting the scientists in this endeavour. As Andreas Huppert explains: "I have a strong interest in improving things for the wheatear - and I want to help change things."