Effects of vegetation patterns and grazers on tidal marshes

Increasing grazing pressures threaten grasslands on a global scale. Therefore, it is important to understand the impact of grazers on the functioning of an ecosystem. In this study, we focused on the effects of small and large grazers on the marsh vegetation and important marsh dynamics, such as sediment accretion and soil carbon stocks.
Small grazers (Hare, Barnacle and Brent geese) reduced the vegetation height, but they had very limited effect on important marsh dynamics. Other small grazers (Greylag geese) grub for below-ground storage organs and this creates bare patches. Young successional plant species established in these patches and this increased plant diversity locally. On a landscape scale, we found a reduction in Greylag goose numbers due to natural succession. An increase of the marsh surface reduced their main food source. Cattle had a very large effect; they reduced the vegetation height and increased soil compaction through trampling. This reduced the marsh surface elevation change, while it increased carbon stocks in the soil. Both processes are very important when we consider global climate change: 1) the marsh surface needs to increase in elevation sufficiently, to keep up with an accelerated sea-level rise, and 2) carbon storage in the soil will reduce the CO¬2 concentration in the atmosphere.
Livestock grazing is a well-used management tool to maintain biodiversity in European marshes. Therefore, it is important that the indirect effects, through trampling and soil compaction, are taken into account when we want to use cattle as a management tool in coastal marshes.