Development of salt marsh arthropod fauna after opening a summer polder

Abstract

Opening summer polders as a measure to re-establish salt marshes in former grassland areas has come into discussion during the recent years. In the Leybucht (Lower Saxony) the summer dike of Hauener Hooge was partly levelled down in October 1994 as a measure of compensation for the construction project ‘Küstenschutz Leybucht’. The renaturation project includes accompanying ecological studies to check the success of the measure, carried out by the Forschungsstelle Küste and containing investigations of vegetation and arthropod fauna.

By means of carabid beetles, amphipods, and some spider groups, the development of salt marsh arthropod fauna has been studied since 1995. Arthropods were caught with pitfall traps from April to October of 1995–1997 and 1999.

First results indicate a rapid change in arthropod community due to an increased seawater influence. The abundance of the salt marsh amphipodOrchestia gammarellus increased considerably during the study period, the spider community (Lycosidae, Tetragnathidae) shifted to more hygrophilous species.

The carabid community was changing in a more differentiated way due to soil level and inundation frequency. Exclusively at the higher study sites, salt marsh species spread out, while many grassland species were still preserved until 1997. The process of species loss and declining abundances in carabids, discernible at the lower study sites already during the first years, accelerated considerably in the whole area after exceptionally numerous inundations in the summer of 1998. A carabid community of lower salt marshes with small species numbers keeps developing at most study sites, clearly dependent on soil level.