An analysis of temporal homogenisation and differentiation in Central European village floras

Agriculture and urbanisation shape biodiversity through extirpation of species and facilitation of species introductions. These processes include changes in the functional composition of species assemblages and can result in taxonomic and functional homogenisation. Especially the spread of non-native species has been discussed as a driver of homogenisation. However, no consensus has been reached so far; instead, both homogenisation and differentiation by non-native species have been shown. This inconsistency can partly be attributed to the lack of temporal data: Most homogenisation studies rely on purely spatial analyses, while homogenisation develops over time. We studied vascular plant species occurrences in 59 villages in the West of Germany in the 1980s and twenty years later. Within this period, the villages experienced changes in agriculture and trends towards urbanisation. We asked whether the villages' floras became more similar to each other within the study period, and whether this process differed between selected plant groups. We based plant groups on leaf traits, life form, species native/non-native status, and mode of introduction. This enabled us to discuss changes in the flora in the context of land-use changes. We used Simpson's index of dissimilarity as a measure of beta-diversity among villages and calculated species turnover and homogenisation in time. Overall, village floras became more similar to each other within the study period. However, neophytes became less similar to each other across villages. Turnover between sampling periods was largest for species promoted by horticulture and for species with helomorphic leaves (suggesting an effect of habitat loss on turnover). Neophytes will likely continue to differentiate floras on regional scales due to on-going and various introductions.