Species vary according to whether they benefit from or are harmed by disturbance and intensive human activity. This variation can be quantified by indices of disturbance and unnaturalness. An urban flora was characterized by comparing quadrat data from cities with several large data sets from the countryside. Existing scales of species response to disturbance and unnaturalness, ruderality (a plant's ability to survive in disturbed conditions) and hemeroby (a measure of human impact) were contrasted with derived scales based on the number of associated annuals and aliens and with 'urbanity', defined as the proportion of urban land in the vicinity of each quadrat. Species presence data were available from 26 710 quadrats distributed through Great Britain, with urban sites only in central England. Satellite imagery was used to measure the proportion of urban land cover in the vicinity of each quadrat; 2595 quadrats were located in 1-km squares having at least 40% cover of urban land. The 20 species having highest urbanity were all alien to Britain, comprising 12 neophytes and eight archaeophytes. Of the 20 most frequent species in quadrats situated in 1-km squares with at least 40% urban land cover, 18 were natives. The two exceptions were Artemisia vulgaris, an archaeophyte, and Senecio squalidus, a neophyte. Both ruderal and hemerobic species, as usually defined, include many non-urban arable species. The hemeroby scale of Kowarik (1990), designed for Berlin, does not work well in Britain. The proportion of associated annuals (annuality) and the proportion of associated neophytes (alien richness or xenicity) can be developed into good indices. The annuality scale is very well defined because annuals tend to occur with other annuals. Plants with high annuality are mostly arable weeds. Urban specialists in central England are, with a few exceptions, character-species of the phytosociological classes Artemisietea, Galio-Urticetea and Stellarietea. Most of them have numerous non-urban associates and they do not form a very well defined group. They have intermediate levels of annuality combined with relatively high levels of xenicity. While it is possible to develop indices of hemeroby, urbanity and ruderality, these concepts are relatively complicated. Annuality and xenicity are simpler measures that can complement Ellenberg values, but definitive values for Great Britain would require additional data from southern England.
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