Starlings and swallows disappear in areas with high levels of neonicotinoids (Nature)
9 July 2014
Populations of common insectivorous birds are declining in farmland areas with high levels of the neonicotinoid insecticide imidacloprid. This is shown by an analysis of detailed data on local bird population trends and environmental factors, including imidacloprid concentrations in surface water. The scientific journal Nature published the study, written by biologists at Radboud University Nijmegen and the Sovon Centre for Field Ornithology, on June 9, 2014.
Many farmland bird species are in decline already for years. But there are unexplained local differences in the decreases. Biologists from Nijmegen, the Netherlands, have explained these differences by the amounts of neonicotinoids in the surface water in relation to other factors, i.e. land use features.
“We decided to look at commonly occurring insectivores, such as the starling and the barn swallow“, says Ruud Foppen from the Sovon Centre for Field Ornithology, an organisation that organises and analyses bird counts. “In many parts of the Netherlands breeding birds are counted annually and there are sufficient data available on a number of insectivores for us to analyse trends in their numbers in farmland areas. Most of these birds forage around ditches, the edges of fields, hedgerows and other similar places.” The Dutch bird monitoring network is one of the world’s densest.
In this study, the researchers used measures of water quality taken by District Water Boards. Many insects that are important for birds spend part of their life cycle in water. The biologists compared these data sets with a database that records changes in land use.
The researchers found a clear trend: the higher the concentrations of imidacloprid in the surface water the greater the decline in bird numbers. For the fifteen included bird species in the study, numbers decreased on average with 3.5 percent per year in areas with more than 20 nanograms of imidacloprid per litre. This concentration is exceeded by far in many regions in the Netherlands.
This is the first study that correlates imidacloprid to possible indirect harmful effects, via the food chain, for vertebrates. Imidacloprid is an insecticide that belongs to a class of chemicals known as neonicotinoids. It is the most widely used insecticide in agriculture around the world.
“We looked very thoroughly for other possible factors that might relate to these birds’ decline. Our analysis shows that, based on our data, imidacloprid was by far the best explanatory factor for differences in the trends between areas”, says Professor Hans de Kroon, who supervised the study. This is the first study that demonstrates a correlation between the decline in populations of vertebrate species and imidacloprid concentrations in surface water.” The research was conducted at the Institute for Water and Wetland Research at Radboud University Nijmegen.
The biologists combined the data from the District Water Boards with systematic bird counts taken before and after the introduction of Imidacloprid in 1995. ‘We see that the decline of farmland bird species started before 1995, but the local differences in decline that we have established after the introduction of imidacloprid are not present in the counts before that time,” Ruud Foppen from Sovon says.
Commonly used agricultural pesticide
Imidacloprid is used in agriculture and horticulture to treat seeds and bulbs, and as a crop spray in greenhouses and in the open. It affects insects’ central nervous systems, so that they become disorientated and paralysed, and then die. It has also been linked to a decline in bee numbers, and other invertebrates.
Lack of food?
The researchers do not yet know precisely what causes the decline. Among the possible explanations are a lack of food (insects), eating contaminated insects, or a combination of both. For a few species, eating insecticide coated seeds cannot be excluded as an explanation. It is not clear whether the breeding success is declining or mortality is increasing.
“Neonicotinoids were always regarded as selective toxins. But our results suggest that they may, in fact, influence the entire ecosystem. This study shows the importance of collecting good sets of field data and subsequent rigorous analysis. Thanks to our partnership with organisations such as Sovon, we can discover ecological effects that would otherwise be overlooked,” says De Kroon.