Biotic homogenization and the replacement of native species with alien species in an urban landscape

Urbanization and its ecological consequences shape global biodiversity patterns.  Alien species introductions are most prevalent in urban areas and often lead to the extirpation of native species and homogenization of the biota.  This study examined the biotic homogenization of urban floras and the replacement of native species by alien species in the New York metropolitan area, North America’s major urban region.  We analyzed the relationship between species patterns and urban land cover using land use/land cover data and the woody species flora for 297 25 km² blocks covering a rural to urban gradient in northern New Jersey.  We show that alien species richness significantly increased with increasing urban land cover.  Native species richness significantly decreased with increasing urban land cover.  Total species richness changed little along the rural to urban gradient, demonstrating clear replacement of the native flora with alien species as urban land cover increased.  These results may indicate community saturation at the landscape scale.  An analysis of community similarity among rural, suburban, urban, and urban core blocks showed a significant increase in community similarity among blocks as urban land cover increased.  In addition, the 60 most urban blocks were significantly more similar in composition than the 60 most rural blocks.  These results demonstrate biotic homogenization of the woody flora with increasing urban land cover.  This study has important conservation and restoration applications for metropolitan regions.