An assessment of greenroofs as urban invertebrate habitat

Background/Question/Methods: Greenroofs are an important part of reconciliation ecology, the use of designed habitat to mitigate species loss in urban areas. Despite being more structurally simplistic than ground level habitat, some studies suggest greenroofs are able to support a surprising diversity of invertebrates, including rare and endangered species.  What is the potential of greenroof habitat for supporting urban invertebrate populations?  In order to address this question, invertebrates were collected from pitfall traps on 8 greenroof and paired ground sites in Portland Oregon biweekly from March until September 2014. Four greenroof sites were planted in herbaceous species and included habitat supplementation such as logs and four others were planted primarily in Sedum. 

Results/Conclusions: Data analysis is ongoing, but for spring samples no significant difference in beetle abundance or richness was measured between ground and roof sites.   Still on average, beetle abundance was higher on the ground  at 84.2(±21) compared to an average abundance of 30.2(±10) on greenroofs.  At 12.8(±2.5), average beetle species richness was also higher on the ground compared to 8.4(±1.4) on greenroofs.  Patterns of spider abundance for the whole 2014 field season are the opposite of those for beetles, with the greenroofs supporting a greater spider abundance compared to ground sites. Spider abundance was also much higher (>1500 individuals for the season on one roof) on the greenroofs planted with herbaceous species compared to those planted in Sedum.  Spider identification is ongoing, but samples are dominated by members of the Thomisidae and Lycosidae families.  Ongoing work will integrate these findings to characterize the greenroof foodweb.  The abundance of predatory spiders raises questions about the availability of prey, for example. The beetle species found on the most greenroofs, the introduced Xantholinus linearis, is a detrivore, which raises questions about energy requirements as well as about whether these organisms are using the greenroof as their primary habitat.