Ecological specialization indices for species of the Czech flora

Theoretically the concept of species ecological specialization is very useful, however, practically it is often difficult to quantify due to a lack of relevant environmental data. We introduce the Ecological Specialization Index (ESI), which describes the degree of specialization of a species based on its realized niche along multiple environmental gradients and is conceptually based on the co-occurrence specialization metric theta introduced by Fridley et al. (2007). We estimated ESI for species of the Czech flora occurring in at least 10 vegetation plots stored in the Czech National Phytosociological Database. We prepared three sets of ESI values calculated from three datasets including (i) plots of all vegetation types (ESIw, 1597 species), (ii) only plots of non-forest vegetation (ESInf, 1529 species), and (iii) only plots of forest vegetation (ESIf, 881 species). We also provide the frequency of species in the datasets, since the reliability of the calculated ESI values increases with the species frequency. The use of these ESI values is limited to the Czech Republic, and in the case of less frequent species, the value can be influenced by sampling bias. To facilitate understanding of the ecological meaning of ESI, we related the calculated values of ESIw to several species attributes and applied them in a case study using a local vegetation dataset from a deep river valley. We found that ESI correlates significantly with specialization metrics based on the number of phytosociological associations and habitats in which the focal species occur. The species listed in the national Red List in higher risk categories are on average more specialized than less threatened species. Neophytes tend to be significantly less specialized than archaeophytes and native species. When related to Ellenberg-type indicator values for the Czech Republic, specialists tend to be more shade-tolerant, better adapted to nutrient-poor soils and soils with either a low or high (but not intermediate) pH and to either warm or cold (but not intermediate) habitats. In a case study of herbaceous plants species in a forest understory on river valley slopes, we found that specialists tend to be confined to deeper soils on cooler north-facing slopes, to stony soils in ravine forests and sites with a denser canopy of woody species. In contrast, shallow lithic soils on eroded south-facing slopes and sites with a more open canopy tend to be dominated by generalists. The complete list of ESI values is included in an electronic appendix to this paper.