α- and β-diversity in moth communities in salt marshes is driven by grazing management

This study evaluates the effects of long-term sheep grazing in salt marshes on the diversity of moths and derives conclusive management suggestions for the conservation of invertebrate diversity in salt marshes. Study sites were located on the Hamburger Hallig, on the Western coast of Schleswig–Holstein, Germany. Between 2006 and 2009, salt marshes that have been under four levels of livestock density (0, 1–2 sheep/ha, 3–4 sheep/ha, 10 sheep/ha) for over 20 years were sampled using light traps and photoeclectors. Plant and moth species richness were highest under low stocking densities, moth species richness, however, showed no difference between low stocking densities and abandonment. Species richness of moths was only weakly correlated with vegetation parameters (species richness, vegetation height, cover and litter). Using additive diversity partitioning we show that no single grazing treatment harbored all recorded moth species and that grazing increases habitat heterogeneity within each treatment. Additionally, we show that moths react more sensitively to grazing than plants, and that therefore assessments of plant species richness in salt marshes do not allow conclusions on invertebrate diversity. For the evaluation of salt-marsh diversity, a multi-species approach should be favored combining plant and invertebrate assessments. A mosaic of abandoned sites and sites with low and intermediate stocking densities would benefit moth diversity in salt-marsh conservation.