Carabid and spider population dynamics on urban green roofs

Green roofs are valuable ecosystems that enhance the biodiversity value of urban landscapes in northern Alberta. Using pitfall traps on green roofs and adjacent ground sites, we show that roof arthropods are characteristic of native grasslands that are threatened in Alberta. Although we found lower abundance of spiders and carabids on roofs, species richness as assessed by rarefaction did not differ between roof and nearby ground sites. Thus, arthropod communities of these extensive green roofs do not seem to be impoverished compared to ground habitats, despite differences in local environmental variables (e.g. substrate depth, surface, vertical isolation). Seasonal distribution of larval and adult captures in pitfall traps, and observation of egg sacs in spiders suggest that a number of species have established reproducing populations on these green roofs. Interestingly, carabid assemblages differed markedly in species composition between roofs and ground sites, but spider assemblages were much more similar. We explain this in relation to differences in dispersal ability between these taxa. Green roofs are likely valuable for urban conservation allowing native species characteristic of native grasslands to permeate through urban landscapes.