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.

中文翻译：

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不同密度放牧绵羊对两个不同盐沼地带沼泽性质和植被动态的长期影响

对于草原生态系统的养护管理，一个重要的问题是，在什么条件下大型放牧者会在植物群落中引起组成和结构变化，这是高生物多样性的前提。在这里，我们在瓦登海的大陆盐沼上使用了两个长期项目，以检验以下假设：不同放养密度的长期放牧管理导致植物群落的植物种类组成和植被结构明显不同。这两个项目分别在低粘土质和高沙质盐沼上进行，绵羊的放养密度不同：0、1.5、3.5、4.5，最初的10只绵羊ha ^-1，在项目开始后的11、15、19和23年收集测量数据。此外，放牧者通过降低土壤氧化还原电位和表面高度来影响非生物条件，从而驱动盐沼植被的组成和结构。在低盐沼上，持续的高放养密度（10绵羊公顷^-1）导致了从早期成功的滨海普氏菌群落过渡到后期成功的滨藜属马齿community群落。在高盐沼上，在所有放养密度下都维持了早期的Festuca rubra群落。停止放牧导致继承了Elytrigia atherica两种盐沼类型的群落。中间放养密度（1.5、3或4.5绵羊公顷^-1）导致了高大的植被和草坪斑块（即短草植被）的镶嵌，其中的Puccinellia maritima草坪散布着Festuca rubra和高大的Elytrigia atherica群落两种盐沼类型。食草者的影响受到海堤附近水位的影响。总而言之，我们的结果表明，放牧者与非生物条件之间的共同相互作用如何驱动植被的多样性和异质性，从而对生态系统功能和服务（如野生生物多样性和沿海保护）产生影响。