Background and aims – Biotic homogenization, i.e.increasing similarity between communities through time, is promoted by alien species exchange. Biotic homogenization can occur at several spatial scales. Urban environments, with their deep and convergent anthropogenic transformation worldwide, are nodes of intense species exchange. Our objective was to explore biotic homogenization at a global scale, and to achieve this goal we analyzed the effect of alien species on homogenization of two urban floras at an intercontinental scale. Methods – We sampled the spontaneous flora of two mediterranean cities located on two different continents: Almería (Europe) and Ensenada (North America) and analyzed the effect of alien species on the similarity between both urban floras. Key results – Floristic similarity between both cities increased significantly due to alien species establishment. Alien species corresponded mainly to biotic exchanges occurring between both mediterranean regions, especially from Mediterranean Basin to California. Native species from the Mediterranean Basin are the principal group of total exchanged species, and therefore of increased similarity, with very little weight to archaeophytes. Alien species coming from third regions have a minor impact on floristic homogenization between these two cities, and they apparently even have a differentiating effect. Conclusions – The trend of biotic homogenization increasing as a result of biotic exchange at different scales does not seem to be temporally or spatially uniform. It reflects, at least partially, the path of footprints following human expansion history on Earth.

ALIEN SPECIES; HOMOGENIZATION; MEDITERRANEAN REGIONS; URBAN FLORAS