For conservation management of grassland ecosystems, an important question is under which conditions large grazers induce compositional and structural variation in plant communities, which is a prerequisite for high biodiversity. Here we used two long-term projects on the mainland salt marshes of the Wadden Sea to test the hypothesis that long-term grazing management with different stocking densities results in plant communities with distinctively different plant species composition and vegetation structure. The two projects took place on a low clayey and a high sandy salt marsh with different stocking densities of sheep: 0, 1.5, 3.5, 4.5 and the initially 10 sheep ha−1, where measurements were collected 11, 15, 19 and 23 years after the start of the project. Moreover, grazers affect abiotic conditions by reducing soil-redox potential and surface elevation, thereby driving composition and structure of salt-marsh vegetation. On the low salt marsh, a continued high stocking density (10 sheep ha−1) resulted in succession from the early-successional Puccinellia maritima community to the late-successional Atriplex portulacoides community. On the high salt marsh, the early-successional Festuca rubra community was maintained under all stocking densities. Cessation of grazing resulted in succession to the Elytrigia atherica community in both salt-marsh types. Intermediate stocking densities (1.5, 3 or 4.5 sheep ha−1) resulted in a mosaic of tall vegetation and patches of lawn, i.e. short-grazed vegetation, where Puccinellia maritima lawn occurred interspersed with patches of the Festuca rubra and tall Elytrigia atherica communities in both salt-marsh types. Effects of grazers were influenced by the presence of watering points near the sea wall. To conclude, our results show how joint interactions between grazers and abiotic conditions drive vegetation diversity and heterogeneity, with implications for ecosystem functions and services such as wildlife biodiversity and coastal protection.
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We thank the participants of the Coastal Ecology Expeditions 1999, 2003, 2005, 2007, 2009 and 2011 of the University of Groningen for their help in collecting data. Dick Visser enhanced the figures.
This study was partially supported by the ‘Waddenfonds’ (Project WF200451).
Communicated by Stijn Temmerm
Communicated by Stijn Temmerm
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Expected mean vegetation height (cm) for (A) low and (B) high salt marsh as functions of the distance to watering points and sheep stocking density based on predictions of the regression model. More specifically, vegetation height peaked at a lower stocking density and decreased more steeply for each unit increase in stocking density in the low than the high salt marsh. Similarly, vegetation height increased at a steeper rate for each unit increase in distance from watering point in the low than high salt marsh. (PDF 314 kb)
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Bakker, J., Schrama, M., Esselink, P. et al. Long-Term Effects of Sheep Grazing in Various Densities on Marsh Properties and Vegetation Dynamics in Two Different Salt-Marsh Zones. Estuaries and Coasts 43, 298–315 (2020). https://doi.org/10.1007/s12237-019-00680-5
- Grazing lawn
- Long-term vegetation dynamics
- Plant-herbivore interaction
- Soil-redox potential
- Surface elevation
- Tall vegetation