Blocking the way: The effect of physical barriers on seed dispersal and colonization

How do new species appear in isolated habitat patches, especially in urban settings where growing space is sparse and plant sources are often few and far between? High levels of disturbance, a common feature of urban settings, often result in more accessible space and greater light availability, and provide prime habitat for invaders to colonize. The composition of these urban flora is determined by several factors, including the endemic species that were present in the original environment, and the addition of new species to an area by human interventions (Williams et al., 2009). Therefore, the floristic composition surrounding a patch will likely have a relatively strong influence on what is able to colonize it. But, in order to successfully colonize a habitat patch, seeds must first be able to make it through to the patch itself.

            A recent study by Cutway & Ehrenfeld (2010) tackles the idea of patch boundaries, by examining the effect of the surrounding urban area on the invasibility of a patch. They compared seed dispersal, through measures of seed rain and seed banks, between wetlands bordered by residential areas, and wetlands bordered by industrial areas. Overall, higher quantities of dispersal through seed rain were found in the residential adjacent wetlands, though there were no differences in total numbers of species found between the two sites. This difference between the residential and industrial wetlands is most likely due to the differing edge structures between these two areas, as seen in Figure 1.  

The wetlands adjacent to residential areas have a much more open edge structure than those adjacent to industrial areas, which are much more dense and overgrown, making it more difficult for seeds to colonize these wetlands compared to the more open ones.

            The Noyce Courtyard is a small, isolated habitat patch that was established in 2007 within the Noyce Science Center at Grinnell College. The Courtyard is completely enclosed, with the walls of Noyce surrounding it for at least two stories on all sides. Although the initial composition of this patch was determined by humans, species that were not original planted have begun to colonize the area. A recent study has worked to investigate these newcomers, with the aim of determining when colonization occurred, as well as how and from where the seeds traveled. Based on the findings of Cutway & Ehrenfeld (2010), the walls of Noyce should have formed an even more effective barrier to seed dispersal into this habitat patch than the dense edges of the industrial wetlands. However, the seedlings of many woody plants, including sycamore (Platanus occidentalis) and white mulberry (Morus alba) can now be found within the Courtyard. The seeds of these two species are dispersed through the air, by wind and by bird. Therefore, although the walls of Noyce may act as a barrier to certain types of seed dispersal, the Courtyard itself is relatively open, providing a potentially favorable habitat for those few seeds that are able to make it through.

References

Cutway, H.B., & Ehrenfeld, J.G. (2010). The influence of urban land use on seed dispersal and wetland invasibility. Plant Ecology, 210(1), 153-167.

Williams, N.S.G., Schwartz, M.W., Vesk, P.A., McCarthy, M.A., Hahs, A.K., Clemants, S.E., . . . McDonnell, M.J. (2009). A Conceptual Framework for Predicting the Effects of Urban Environments on Floras. Journal of Ecology, 97(1), 4-9.