Urban areas provide a unique opportunity to think about keeping landscapes connected. The challenge is not only to maintain functional green spaces, but to make sure that species can move freely between those green spaces when surrounded by a highly human-modified landscape. Three recent papers look at the effects of urbanization on species composition, and provide advice on how to better model connectivity in urban settings.
Urbanization interrupts both habitat connectivity and local environmental conditions, such as temperature or stream incision, which can impede plant dispersal. A recent study sampled riparian vegetation in a highly urbanized area of the eastern United States, and looked at the relative impact of changes in local environmental conditions and habitat connectivity on species diversity.
Urbanization seemed to impede dispersal for some native species, especially short-distance dispersers, and with an increasing effect over time. Exotics species mostly spread from multiple urban sources and along stream channels, and were more influenced by measures of urbanization in the surrounding landscape than native species. Management plans that consider dispersal mode and landscape-level factors would benefit urban plant communities.
The link between landscape metrics and ecosystem function can often be unclear, especially in urban areas where “suitable habitat” can be difficult to define. A study of three cities north of London analyzed the spatial form and characteristics of urban green spaces both in relation to and independent of the surrounding matrix, and looked at how this form and characteristics related to potential ecosystem services.
The study introduces a new way to classify urban form or land use/land cover that breaks free of the potential bias in using pre-existing classifications. Modelling carbon storage, pollinator abundance, and potential soil erosion against patch size showed that ecosystem service provision was lower in small green patches, but increased with patch size until leveling off at around 10 ha. Multivariate landscape analysis is more effective than analyzing landscape metrics individually, and focusing on habitats rather than the matrix may be more useful in understanding how urban patches support ecosystem services.
Do urban forest fragments act like island archipelagos? The theory of island biogeography suggests that forest patches are like islands, in that species richness increases with patch size and increased connectivity to other patches. It’s uncertain, however, how well this idea holds up for forest fragments within an urban landscape, which are likely to be smaller and more isolated than those in a more suburban or rural one.
A recent analysis compares canopy and tree seedling abundance and richness in “baseline” U.S. National Park Service lands to urban forests in western New York State. In both cases, tree seedling richness increased with park area and decreased with the incursion of non-native species. In addition, tree seedling richness in urban forest patches increased with proximity to neighboring forest patches and rural forests, and canopy tree richness increased with proximity to rural forests. The data support the idea that urban forest patches act like island archipelagos.
Lopez BA, D Urban, and PS White. 2018. Nativity and seed dispersal mode influence species’ responses to habitat connectivity and urban environments. Global Ecology and Biogeography. DOI: 10.1111/geb.12760.
Grafius D, R Corstanje, and JA Harris. 2018. Linking ecosystem services, urban form, and green space configuration using multivariate landscape metric analysis. Landscape Ecology 33: 557-573.
Olejniczak MJ, DJ Spiering, DL Potts, and RJ Warren. 2018. Urban forests form isolated archipelagos. Journal of Urban Ecology. DOI: 10.1093/jue/juy007.