Local and regional factors influencing assemblages of dragonflies and damselflies (Odonata) in California and Nevada

Studies of landscape effects on assemblages and distribution of insects are relatively uncommon, largely because of the lack of occurrence data that span broad spatial or temporal scales. Here, we provide a multi-species analysis using generalized linear mixed models to examine the effects of local and regional variables on richness and occurrence rates of Odonata (dragonfly and damselfly) species at 81 sites throughout central California and north-western Nevada, USA. These study sites were located across a range of ecoregions, including the Sierra Nevada Forests, California Mediterranean, Great Basin Shrub Steppe, and Northern Coastal California Forests. Dynamic regional variables in this study, degree-days and precipitation, influenced the richness of dragonflies, but not the less-mobile damselflies. In contrast, local habitat type influenced the richness of damselflies, but not dragonflies. Overall species occurrence was higher during site visits with higher degree-days, especially for highly mobile groups including dragonflies and migratory species. Dragonflies were also positively associated with total precipitation, but migratory species were not. Probability of presence across species was lower in highly urban sites, particularly for habitat specialists. Further, habitat specialists had lower rates of occurrence overall, suggesting that widespread generalist species may increasingly dominate Odonata assemblages. Our study indicates that Odonata in this semi-arid region are responsive to a combination of local and regional environmental variables.