Lush and green – too much fertiliser harms birds

Wood Warblers favour woods with sparse to moderate grassy ground cover. © Alex Grendelmeier

Switzerland is a country of verdant landscapes. Fields and meadows are richly fertilised, and mires and woodlands receive more nutrient input than is good for them. In consequence, the habitat of many bird species that depend on low, patchy vegetation for foraging or nesting is degraded.

Fertiliser is applied to increase crop yields. But plants do not absorb the entire amount. A considerable quantity of nitrogen diffuses into the air: in 2005, 65 % of nitrogen emissions into the air came from agriculture, 22 % from traffic, 10 % from trade and industry, and 3 % from private households. Most airborne nitrogen returns to the soil, fertilising surfaces where no such treatment is desired. Today, these immissions often exceed by far the so-called critical load of 5–25 kg of nitrogen/ha and year that is considered acceptable for most ecosystems. An annual nitrogen surplus of 190 000 t was calculated in 1994; the figure is expected to drop to 145 000 t per year in 2020 (if the present trend continues). So-called nitrogen efficiency will have increased from 22 to 30 %. Nevertheless, despite these signs of progress, the nitrogen surplus in Switzerland is huge.

Swiss lowlands most affected by excessive fertilisation

Levels of nitrogen deposition differ from region to region. They are highest in the eastern parts of the Plateau and in the colline zone of the Pre-Alps, but high values are recorded in the remaining lowland areas too.

While the critical loads of nitrogen differ depending on the habitat, they are exceeded almost everywhere in Switzerland: 100 % of raised bogs, 90 % of forests, 84 % of fens and 42 % of dry grassland contain harmful levels of nitrogen.

Comparison of the number of species per atlas square (10 × 10 km, below 600 m on the Central Plateau and in the Jura) with average annual nitrogen deposition from the air (in kg/ha and year). The number of species decreases as nitrogen input increases.

Substantial impact on birds

The surplus quantities of nitrogen that are released into the environment are considered one of the main causes for the decline in biodiversity in central Europe. They have serious consequences for species composition and vegetation structure, and have an indirect impact on breeding birds, as illustrated by the following two examples:

Where woodruff or wood-rush once formed patchy ground vegetation, undergrowth in «over-fertilised woods» is now dominated by species that tolerate high nutrient levels, such as brambles and nettles. The Wood Warbler avoids this type of vegetation when establishing its territory. The increase in nutrients may also have a negative impact on the Western Bonelli's Warbler, a species that occupies nutrient-poor woodland.
Where nitrogen deposition from the air is high, plant diversity is smaller than at other, comparable sites. This is because the number of competitive species increases, crowding out smaller plants that are specialists of nutrient-poor soils. Insect abundance is lower in nutrient-rich, species-poor meadows, affecting many farmland birds. In addition, dense vegetation growth makes it difficult for ground-foraging birds to access insects. Several species with declining populations, such as Eurasian Wryneck, Eurasian Skylark, Woodlark, Red-backed Shrike and Common Redstart rely on low, patchy vegetation for foraging.

At the landscape scale, excessive fertilisation leads to the homogenisation of flora. Analyses of atlas data indicate a similar effect on bird communities. We selected all atlas squares (10 × 10 km) below 600 m on the Central Plateau and the Jura and compared the average nitrogen deposition from the air per hectare with the recorded number of species. Over-fertilised atlas squares supported fewer breeding bird species than squares with low nitrogen input: the number of species declined by 11 species per 10 kg/ha of additional nitrogen.

Distribution change since 1993–1996 of five species that require patches of bare ground for foraging (Eurasian Wryneck, Eurasian Skylark, Woodlark, Red-backed Shrike and Common Redstart). The map combines the distribution change maps of all five species.

Positive effect of nutrient reduction in lakes

Lakes and rivers present a different situation than grassland and forests. The limiting nutrient in freshwater lakes is not nitrogen but phosphorus. Just a few decades ago, many Swiss lakes were so loaded with phosphorus from wastewater and agriculture that ecosystems were close to collapse. The situation has since greatly improved thanks to the expanded sewage treatment system, the ban on phosphates in detergents, and the introduction of buffer zones. As a result, reedbeds and especially submerged vegetation such as pondweed and stoneworts have recovered. Birds that breed in reedbeds have benefited, as has the Red-crested Pochard, which feeds predominantly on stoneworts. Its wintering population has increased considerably in recent years. The species has become a more common breeder as well, increasing its numbers fivefold from 1993–1996 to 2013–2016.

Due to fertilisation with nitrogen from the air, the undergrowth is dominated by species that thrive in nutrient-rich soil, such as brambles; the Wood Warbler avoids this type of environment.

keine Übersetzung benötigt: Roman Graf

Recommended citation of the Atlas online:
Knaus, P., S. Antoniazza, S. Wechsler, J. Guélat, M. Kéry, N. Strebel & T. Sattler (2018): Swiss Breeding Bird Atlas 2013–2016. Distribution and population trends of birds in Switzerland and Liechtenstein. Swiss Ornithological Institute, Sempach.

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