STRUCTURE, DIVERSITY AND STABILITY OF A SALT-MARSH HEMIEDAPHIC INVERTEBRATE COMMUNITY

In 1990, the soil surface community (hemiedaphon) of a salt marsh ecos ystem in the German Wadden Sea was investigated in order to determine the structure of the community, systems diversity and stability, and h ow this is affected by grazing. Community structure was determined by analysis of species interaction strength, solving binary Lotka-Volterr a equations. Subset interaction diversity (SID') as a structural index regarding both the compartmental structure within the community and t he variety of interactions, and interaction diversity (ID') serving as sum parameter are introduced. In the higher salt marsh (Juncetum gera rdii sites) Shannon-Wiener diversity H' for the ungrazed site reaches 1.35 and SID' 108.5 (ID'=3.22), respectively H'=1.25 and SID'=13.1 (ID '=3.0) for intensively grazed parts. The accuracy of the SID-index in detecting changes within the community exceeds that of H' and ID' by o rders of magnitude. In the lower marsh, the value for SID' is twice as high in the ungrazed field as at intensively grazed sites with hardly any difference in the ID' values. The ratio SID'/ID' is introduced as a measure of intra-community competition ICC. ICC and SID' are the hi gher, the more succession has proceeded. Three model runs with 14, 19 and 21 species reveal that higher numbers of species lead to increased stability in the ungrazed fields. However, total stability is higher in the intensively grazed fields, which are already highly disturbed.