Abstract. The main question to be dealt with in the papers published in this Special Feature is to which extent plant species richness can be applied as a parameter in restoration projects to qualify the ecosystem's state. Before considering this problem, it should be recognized that this approach illuminates only one side of the coin; the other side is touched by the opposite question, asking which plant species are essential components of an ecosystem. These two approaches towards the relationship between species richness and ecosystem functioning are not mutually exclusive, but should not be confused either. In view of ecosystem functioning certain species may be considered redundant, while in view of evolutionary processes certain ecosystem processes may be considered redundant. Where do the two approaches meet and when should they be separated? This paper touches upon this question by referring to the dual hierarchy of ecological systems.