Green roofs are becoming increasingly prevalent in many North American cities as a result of incentive programs, by-laws and construction standards. Aside from the well-documented roof cooling and stormwater management benefits, green roofs also present an opportunity to provide habitat for local flora and fauna. Several studies have documented invertebrate and avian diversity of green roofs, yet only one has attempted to determine whether green roofs can support biodiversity comparable to level-ground urban habitats. In this study, insect richness and abundance were compared between five green roofs and adjacent ground level habitat patches in downtown Halifax, Nova Scotia. Eight pitfall traps were set randomly at each site, collected bi-weekly between May-October 2009 then identified to morphospecies (except where taxonomic expertise was available). Insect composition between site types was compared with the Simpson's Index (D), the Shannon-Weiner Index (H′), and Evenness (Evar). The Jaccard's Index (J) was used to quantify similarity between site types. Each of the indices (except J) was analyzed in linear mixed effects models to document any influence of sampling effort, plant species richness or site area.
No significant differences in richness, abundance or any of the indices were detected in the analyses, suggesting many insect species within the urban landscape may permanently or temporarily occupy green roofs. Of 361 species collected, 189 were common to both green roof and ground level sites, whereas 155 species (recorded only once) were unique to one site type. Only 17 species were collected exclusively from one site type in numbers greater than five specimens. Plant richness had the greatest effect on insect species composition. Interestingly, several unique species were identified from green roofs in large numbers, most notably, Phosphaenus hemipterus (Lampyridae: Coleoptera), an endangered firefly in its native range, and a new record for Otiorhynchus porcatus (Curculionidae: Coleoptera), a generalist weevil. Our data shows that green roofs not designed to provide habitat are spontaneously colonized by many functionally valuable species similar to ground level habitat patches, and that urban environments can provide refuge for uncommon species. As the rate of green roof installation increases, optimizing these constructed habitats for long-term support of insect species will not only improve services such as pollination, pest control and decomposition, but also aesthetic and educational opportunities in “species-poor” cities.