Distribution of invasive plants in a spatially structured urban landscape

Plants that are invasive in conservation land also exist as ‘wild’ elements within planned urban landscapes. We investigated the relationship of the abundances of 10 invasive species to variables of urban spatial structure [land use (houses, apartments, commercial/industrial, or vacant), impervious surface, distance from city center, distance from forest preserve, income] at an individual block scale of analysis. Eighty blocks (about 2 ha each) distributed along a 15-mile (24 km) transect from the center of Chicago, IL, USA west to the county line were inventoried for abundance of three woody species, five biennial forbs and two grasses. Nine of the ten invasive species were found, with an average of 2.74 of our 10 species per block. Collectively, abundance was negatively related to percent impervious surface, and presence was negatively related to local income. No invasive species’ abundance was structured by distance and only one of the 10 species, burdock, had any significant correlations with land use. This general lack of connection to urban land use is surprising and suggests most plant species find opportunities for colonization and reproduction independent of land use in the built urban environment. There is also evidence that the built urban environment may be a sink for these invasive plant species, with populations in forest preserves sending more propagules into the built environment than they receive from it.