Structure of the invertebrate fauna in salt marshes of the Wadden Sea coast of Schleswig-Holstein influenced by sheep-grazing

Abstract

Results of investigations on the influence of five different sheep grazing intensities on the invertebrate fauna of two mainland salt marsh sites of the German Wadden Sea coast are presented for the years 1990 and 1991. The investigation of the invertebrate fauna has been carried out since 1989 in thePuccinellia maritima zone, and theFestuca-Puccinellia as well as theFestuca-Armeria zones, with trapping transects arranged along an inundation gradient. Apart from specific biotic effects, grazing causes changes in environmental characteristics. Effects on microclimate comprise higher ranges of variance in soil-surface temperature on grazed sites. Decreasing food resources caused by grazing bring disadvantages to herbivores, the major part of the invertebrate fauna, due to merotope destruction (e. g. inflorescences ofAster tripolium) and the decline of host plant stands (e. g.A. tripolium, Plantago ssp.). Flower visitors and pollen feeding species that depend onA. tripolium have become extinct. Increasing food resources, caused by grazing, lead to higher population densities of a few specialized grass-feeding and surface-grazing invertebrates (e. g.Mayetiola ssp.,Psammotettix putoni, Bledius tricornis). Soil characteristics in the lower salt marsh have not been altered significantly by grazing; hence, the direct effect of grazing and trampling leads to a decrease in population density of many species such asAssiminea grayana, Orchestia gammarellus and collembolans. The biomass and abundance of detritivores and many herbivores increased from 1990 to 1991 on the totally grazed fields, whereas predators diminished in numbers at the same time. A descriptive model is presented, involving grazing, winter temperature, and precipitation as basic factors.