Testing barrier leaking and negative connectivity in biogeographical urban homogenization

Homogenization is a process which is particularly relevant in urban environments. We focus on the flora of a large mediterranean Spanish city, Madrid, located along two different edaphic biogeographical regions. We test whether biogeographical homogenization, in terms of anthropogenic biogeographical barrier crossing, has been taking place, and to what extent.Secondly, we attempt to shed light on the relationship between urban connectivity and urban homogenization. We use the public parks system in Madrid to test whether these spaces could foster an unnatural increase in species movements, and therefore a ‘leakage’ in the biogeographical barrier, diluting the barriers among territories and providing increased opportunities for homogenization.Biogeographical homogenization is not consistent in urban Madrid. Our results identify the common and generalist flora as the most significant plant set in terms of diluting the barrier. The biogeographical homogenization caused by specialist flora (calcifuge/calcicole) is asymmetrical, with a ratio of 7.72:1 of calcifuge to calcicole. Our results do not support the assumption that more urban connectivity is related to greater urban homogenization. The biogeographical barrier, together with human mediated dispersal and the microheterogeneity of urban green areas are possible factors that may explain this lack of relationship.Biogeographical barriers in cities continue to play a natural role and homogenization is a differential process resulting in distinct effects depending on plant ecology and biogeography. Although urban spaces are highly altered, they host several groups of floras, and they contribute to biogeographical patterns and processes. Hence, there is still room for biological conservation in cities.