Effects of built landscape on taxonomic homogenization: Two case studies of private gardens in the French Mediterranean

•Landscape metrics influenced floristic similarity.•Built-up density reduced floristic similarity between gardens.•The effect of exotic species on floristic similarity depended of built-up density.•Homogenization changed over time, due to historical soil seed bank and exotic species. Urbanization can promote the replacement of native species by exotic species resulting in an increase in community's similarity over time. This process is called biotic homogenization, which is usually studied at large scale from species lists. Our paper addresses the effects of urbanization on plant community at local scale in areas where urban policies are implemented. We focus on private gardens as they are the most common components of green spaces in European urbanized areas. They are also a place where exotic species are introduced. Observations were made on spontaneous flora sampled from gardens in two study sites of the French Mediterranean: a large city and a village located in an urbanizing rural area. We evaluated how urban landscape influences floristic similarity and how exotic species affect homogenization. We divided each study site in three built-up density zones. As the urbanization process of both sites did not take place at the same time, we were able to assess the effect of time on floristic similarity. Results indicate that floristic similarity is less important in high dense built-up areas than in low dense ones. Exotic species tend to reduce floristic similarity in city centers and increase floristic similarity in low dense built-up areas in both sites. Landscape metrics calculated in built-up areas surrounding the gardens influence floristic similarity. Lastly, we found that urban planning in terms of built-up density affects biodiversity distribution and that private gardens can provide landscape connectivity within urban areas.