Sometimes wildlife likes it dirty. When farmers in Northern Ireland cut the amount of fertiliser leaking from their land into Lough Neagh, the UK鈥檚 biggest lake, the upshot was a dramatic decline in the number of migrating birds spending winter there.
By reducing pollution, farmers have improved the lake鈥檚 water quality, says Irena Tom谩nkov谩 of Queens University Belfast. 鈥淏ut the unexpected consequence is fewer invertebrates and as a result less duck food.鈥�
That鈥檚 because there were fewer nutrients in the lake to feed the insects and snails on the muddy lake bottom 鈥� critters that visiting birds like , tufted ducks and goldeneye feast on.
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So the birds, which used to visit Northern Ireland to escape the freezing winters in northern and eastern Europe, have been staying away. The number of diving ducks spending the winter on the lake has fallen from 100,000 during the 1990s to fewer than 25,000 in 2010.
Cleaner sewage
It had been suggested that the decline in wintering birds was due to warmer winters 鈥� the birds don鈥檛 need to come so far because lakes nearer to home don鈥檛 ice over.
Tom谩nkov谩 agrees that warmer winters across Europe may play a role. But, suspecting something else may be to blame, she looked at how plankton and invertebrates like snails and insects, which the diving birds eat, changed between 1997 and 2010 鈥� immediately after the decline in birds. She found the drop in food coincided with the departure of the birds. All this happened at the same time as farmers reduced the amount of fertiliser run-off as a result of a government programme to tackle pollution.
The birds of Lough Neagh aren鈥檛 the first to suffer in this way. The Firth of Forth in Scotland lost populations of the same birds after a new sewage treatment works reduced the amount of nutrient-rich sewage entering the estuary, says Tom谩nkov谩. And ornithologists have reported the same thing in cleaned-up English estuaries like the Tyne and Thames.
The findings make life difficult for the Northern Ireland government. The lake, which covers 392 square kilometres, is an internationally recognised wetland under the Ramsar Convention, with the visiting birds a particular feature. The government is also required to maintain the lake鈥檚 鈥渇avourable conservation status鈥� under the European Union鈥檚 Habitats Directive. But the government鈥檚 assiduous efforts to encourage farmers to reduce the artificial fertiliser pouring into the lake haven鈥檛 been to the liking of all species.
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