PhD ceremony Mr. F.S. Mandema: Grazing as a nature management tool. An experimental study of the effects of different livestock species and stocking densities on salt-marsh birds

PhD ceremony: Mr. F.S. Mandema

Dissertation: Grazing as a nature management tool. An experimental study of the effects of different livestock species and stocking densities on salt-marsh birds

Promotor(s): prof. J.P. Bakker, prof. J.M. Tinbergen

Faculty: Mathematics and Natural Sciences

The core question Freek Mandema posed in his thesis is how to apply grazing as a tool for nature management for birds both in terms of the choice of livestock species and in the choice of stocking densities. He studied this question by setting up a unique large-scale experiment with five different grazing treatments.

Grazing with high densities of livestock, such as with 1 horse/ha may lead to a homogeneous short sward (Bakker 1989). This type of sward is suitable for geese and may attract larger numbers of geese to salt marshes in autumn. Grazing a salt marsh with a high stocking density (1 horse/ha), however, increases the risk of trampling birds nests in spring and leads to less pronounced micropatterns in the vegetation. The latter is also true for 1 cattle/ha.

A rotational grazing treatment offers interesting perspectives to combine salt-marsh management goals in a single grazing treatment. The years with grazing help in the creation of in field heterogeneity i.e. vegetation structure, while the years without grazing provide disturbance-free habitat in which no bird's nests will be trampled by livestock. In general, Mandema recommends to apply different grazing treatments to different salt marshes simultaneously. This will lead to large scale heterogeneity. Each grazing regime has its specific effects leading to the suggestion that a combination of different grazing regimes in one conservation area may give the best opportunities for nature management of birds in salt marshes.