Wetenschap - 14 november 2013

Large grazers bad for breeding birds

tekst:
Albert Sikkema

The large grazers are doing well at the Oostvaardersplassen, but the summer birds are faring poorly. Numbers of breeding pairs and species of summer birds at this nature reserve have declined by one-third since 1997. These were the findings of a Sovon bird study in early November. Alterra researcher Martijn Hammers is not surprised.

Why are the summer birds doing so badly?

‘It’s due to the high intensity of grazing, which has caused the disappearance of suitable vegetation for many species. For birds that like grassland, tangled growth and thickets there are fewer and fewer suitable breeding sites and their nests have a higher risk of being trampled by the large grazers. This explains why the number of breeding pairs of the bluethroat, for example, declined from 281 pairs in 1997 to 39 in 2012.’

Was this predictable?

‘It is a choice. The authority in charge created a wilderness at the Oostvaardersplassen, a kind of ecosystem that is self-sustaining and in which large grazers are used to keep the grass short. This is good for some species, such as geese, but not for a large number of other summer bird species. That’s something you can estimate in advance.’

Is it a bad thing?

‘That depends on your choice, what you want to protect. If you want a nature park with a lot of large grazers, this is a consequence.’

Can you prevent it?

‘You can if you want to. By reducing the number of large grazers, for example, or making areas less accessible to them. That would benefit these summer birds. They thrive better in parts of the Oostvaardersplassen that are open to the public and where fewer large grazers roam, because visitors don’t eat any vegetation or tram­ple any nests. The disturbance they cause is only minor.’

But this is manmade nature, right?

‘In the Netherlands we are skilled at this: making nature conservation areas suitable for certain target species through focused management.


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