One of the basic tools to quantify biodiversity is based on the use of surrogate species. Such groups of species are easily assessed and may reflect the diversity of other important and less easily detectable taxa (cross-taxon surrogacy). Among these key groups of species, the vascular plant flora has great potential in determining diversity of other groups, since in terrestrial ecosystems it constitutes the bulk of total biomass and provides the physical structure for other organisms. The cross-taxon congruence of species diversity (species richness and species composition) among vascular plants, bryophytes, lichens, fungi, oligochaetes, butterflies and birds was investigated with special attention to testing the potential role of vegetation as surrogate for the other studied taxa. The 271 ha Nature Reserve "Bosco di S. Agnese" (Tuscany, Italy) characterised by evergreen Mediterranean woodlands, scrublands, garigues, xeric grasslands and cultivations was used as study area. A multi-scale sampling design, based on a restricted random selection of plots, was used to get information about vascular plants, bryophytes, lichens, fungi, oligochaetes, butterflies and birds. The congruence in species richness and composition among the different taxa was tested by Spearman rank correlation coefficient. Most of the pair-wise comparisons showed weak and statistically not significant correlations for both species richness and compositional data, indicating a limited cross-taxon congruence. Species richness of vascular plants was significantly correlated only to that of birds and butterflies. Compositional patterns were congruent only between vascular plants and butterflies. The results of this study indicated that neither vascular plants nor any other taxonomic group is a valid surrogate in this nature reserve. (C) 2009 Elsevier GmbH. All rights reserved.