Zonation of ground beetles (Coleoptera: Carabidae) and spiders (Araneida) in salt marshes at the North and the Baltic Sea and the impact of the predicted sea level increase

The ground beetles and spiders of two salt marshes at the German North and Baltic Sea coast were investigated by pitfall traps in 1997 and 1998. While the sites at the North Sea coast are tidal salt marshes, the salt marshes at the Baltic Sea are not influenced by tides. Pitfall traps were installed in a gradient from 20 to 150 cm above MHT (mean high tide: 157 cm + NN, NN: 500 cm above 0 at Amsterdam gauge) at the North Sea coast or NN at the Baltic Sea coast at six or seven sampling elevations, each with five replicates. Conductivity, water content, organic substance, frequency or duration of floodings, sand content and pH of the soil were determined. The flooding regime is the major factor controlling the zonation of invertebrates. Two and three invertebrate assemblages at the North and Baltic Sea, respectively, were distinguished. These corresponded well with the vegetational zones. The border between the two zones was at 60-80 cm above MHT at the North Sea. The three zones at the Baltic Sea extended between 20 and 30 cm, 40 to 80 cm and 100 to 150 cm above NN. The elevation of the mean abundance of species above MHT or NN was calculated. A tide simulation experiment resulted in a shifting population and in an increasing activity under a tidal regime as predicted for the global climate change conditions in 2050. From the actual elevation of the mean abundance, the habitat size of salt marsh species was calculated for a moderate and worse scenario of global climate change. Habitat reduction becomes highest for species of the lower salt marsh zone. Under worse conditions the gradiental length of habitat will only amount to a maximum of 20 m at the slopes of the dikes.