The Bees among Us: Modelling Occupancy of Solitary Bees

Occupancy modelling has received increasing attention as a tool for differentiating between true absence and non-detection in biodiversity data. This is thought to be particularly useful when a species of interest is spread out over a large area and sampling is constrained. We used occupancy modelling to estimate the probability of three phylogenetically independent pairs of native—introduced species [ Megachile campanulae (Robertson) —Megachile rotundata (Fab.), Megachile pugnata Say— Megachile centuncularis (L.), Osmia pumila Cresson— Osmia caerulescens (L.)] (Apoidea: Megachilidae) being present when repeated sampling did not always find them. Our study occurred along a gradient of urbanization and used nest boxes (bee hotels) set up over three consecutive years. Occupancy modelling discovered different patterns to those obtained by species detection and abundance-based data alone. For example, it predicted that the species that was ranked 4 th in terms of detection actually had the greatest occupancy among all six species. The native M . pugnata had decreased occupancy with increasing building footprint and a similar but not significant pattern was found for the native O . pumila . Two introduced bees ( M . rotundata and M . centuncularis ), and one native ( M . campanulae ) had modelled occupancy values that increased with increasing urbanization. Occupancy probability differed among urban green space types for three of six bee species, with values for two native species ( M . campanulae and O . pumila ) being highest in home gardens and that for the exotic O . caerulescens being highest in community gardens. The combination of occupancy modelling with analysis of habitat variables as an augmentation to detection and abundance-based sampling is suggested to be the best way to ensure that urban habitat management results in the desired outcomes.