Invertebrates on green roofs in Sydney

Rapid urbanisation is a leading cause of habitat loss, worldwide. Green roofs are thought to provide habitat benefits for a range of species, and support biodiversity conservation objectives in cities. Yet, this study is the first to properly quantify the added habitat value of green roofs over conventional bare rooftops. Drawing on classical ecological theory, this study assesses the factors which influence invertebrate diversity and composition on bare and green roofs in urban Sydney. Green roofs with at least 30% green cover are capable of supporting resident populations with up to twice the abundance and three times the variety of invertebrates compared to bare roofs. Bare roofs may provide a peculiar kind of habitat which favours predators or scavengers, but contain mainly transient individuals. The habitat value of green roofs is limited by immigration and resource provision, with large (>490 m2), structurally diverse, and well-connected green roofs hosting the greatest abundance and richness of invertebrates. Low-mobility taxa may be unable to colonise green roofs without human-mediated translocation. The findings of this study suggest that green roof implementation should consider: 1. landscape context, 2. roof size, 3. vegetation characteristics, 4. maintenance (including translocation of species of conservation concern or ecological value).