Horizontal and vertical island biogeography on green roofs

Background/Question/Methods

Green roofs, aside from providing benefits such as storm water retention, insulating buildings, and mitigating urban heat islands, also show promise for providing local habitat that can support a diversity of species. From an ecological perspective, green roofs in urban environments can be viewed as green islands surrounded by an urban matrix. Island biogeography theory predicts that the number of species on an island is the outcome of dynamic equilibrium between immigration and extinction. Immigration is affected by the distance of an island from a source of colonists. In the context of green roofs, building height can potentially be a limiting factor for colonization. We used green roofs to study two components of distance - vertical (building height), and horizontal (distance from open areas).  Here, we broadly consider works in this context.  We then focus on a specific ongoing study conducted in the city of Haifa, Israel. We compared arthropod abundance and richness in a collection of 16 pots containing four plant species on 19 rooftop buildings and adjacent ground-level yards. These buildings varied in height and distance from rural areas. 

Results/Conclusions

Only a few studies have compared diversity between green roofs and ground-level habitats and found that green roofs and ground-level fields support many similar species. In our literature review, we also found that only a small number of studies addressed the effects of distance from city edge and building height on insect occurrence on roofs, and usually these studies focused on a small number of roofs.  One study that addressed the height issue found no effect of roof height, on arthropod species richness. In our currently ongoing Haifa study, preliminary results show that approximately two times more insects were caught in yards adjacent to the roofs than on the roofs themselves. This trend is constant both in the city center and near the city edge. In addition, about four times more insects were caught near the city edge than in the city center. Additionally, the effect of distance from species source was also evident vertically. We found a negative correlation between building height and the number on individuals caught. Our results suggest that the distance from open areas and building height can be limiting factors for insect colonization on roofs.