The myth of communities : determining ecological quality of surface waters using macroinvertebrate community patterns

摘要

Macroinvertebrate community patterns are often analysed and related to the ecological quality or conservation value of a water body, using an ecological typology or assessment system. The goal of this study was to determine the effect on the development or application of a typology or assessment system of different choices concerning (1) taxonomic adjustment of the data, (2) the data that are included in the analyses and the community variables focussed on, and (3) the techniques chosen. It appeared that it is very difficult to distinguish and characterise macroinvertebrate communities unambiguously. In developing a typology or assessment system the results depend on the taxonomic level used and on the completeness of the data. Also, the choice of the technique influences the final result. In applying a typology or assessment system it is of importance to use the same taxonomic level as the data used for development and to include all taxa collected. Even taxa with low abundances or small distribution ranges appeared to be important. The number of rare taxa was indicative for a high ecological quality, in contrast to the total number of taxa in a sample. Communities composed of a high number of characteristic species were easier to distinguish than communities composed of more generalists. In conclusion, community analysis is not an objective process, because of the large effect of small technical changes. This is also caused by the fact that any classification of species assemblages is artificial. A community only exists of a combination of populations at a moment on a site and it is impossible to collect the complete community in a sample. Communities are a continuum along an environmental gradient. Species are adapted to their environment (which is characterised by stability, favourability and impairment) by their life tactics. Therefore, water management should rather focus on these life tactics in relation to the environment to investigate the stressor(s) present and the possibilities for restoration. To establish the conservation value of a site, the number of rare species can be used.