Effects of Horizontal and Vertical Island Biogeography on Green Roofs Arthropod Communities

From an ecological perspective, urban green roofs can be viewed as green islands surrounded by an urban matrix. Island biogeography theory suggests that species richness on an island is the outcome of a dynamic equilibrium between immigration and extinction. Immigration is affected by the distance of an island from a colonizing source. Building height may act as a limiting factor for many species on green roofs. Here, we used green roofs to consider two distance components - vertical (building height), and horizontal (building distance from open areas). We first review the green roof literature on these two spatial aspects. We then focus on a new study conducted in Haifa, Israel. In our review, we found only a small number of studies addressing the effects of distance from city edge and building height on insects' occurrence on roofs, and usually, these studies considered a small number of roofs. In the Haifa study, we compared arthropod abundance and richness by visiting a set of 16 pots (four flowering plant species) placed on 18 rooftops and on adjacent ground-level yards. Yards had significantly more species and there was significant community dissimilarity between roofs and yards. This dissimilarity and the large proportion of unique species appearing exclusively on roofs suggest that green roofs add to the city's biodiversity. Species richness at the city edge was significantly larger than in the city core. These results suggest that the distance from open areas and building height can be limiting factors for insect colonization on roofs.