Increase in non-native species richness leads to biotic homogenization in vacant lots of a highly urbanized landscape

Vacant lots, unused lands covered by spontaneous vegetation, are widespread in urban landscapes. Although they are of less conservation value than natural ecosystems, they can provide important ecological services, such as maintaining air quality and mental health, and offering recreational space. Nevertheless, the effects of urbanization on the plant species richness and beta diversity of such lots remain poorly understood. We evaluated how plant species richness and beta diversity differ in vacant herbaceous lots 1) between two urban areas (Montreal and Quebec, Canada) characterized by contrasting urbanization intensity, and 2) along a rural-urban gradient within both landscapes. We inventoried 69 vacant lots in Montreal and 62 in Quebec distributed along three local levels of urbanization (established using NDVI values in a 500 m buffer zone around each lot). We found a lower beta diversity for plant communities in vacant lots in the most urbanized landscape (Montreal) compared to the least urbanized one (Quebec), but no effect of local level of urbanization intensity in either of the landscapes. Montreal was richer than Quebec in regard to non-native species, while native richness was similar between the two landscapes. Non-native richness was higher than that of natives at each local level of urbanization intensity, and the highest at moderate urbanization intensity. Finally, non-native species were functionally more homogenous than natives both within and between landscapes. Overall, our results suggest that biotic homogenization occurr only in very urbanized landscapes and that this homogenization results from non-native species invasion without local native extirpation.