Species residency status affects model selection and hypothesis testing in freshwater community ecology

Species occurrences have multiple ecological states that may strongly influence community analysis and inference. This may be especially true in freshwater systems where many animals have complex life cycles with adult dispersal and juvenile resident stages. The effects of ecological state variation on standard empirical approaches are largely unknown. Here, we analysed the effects of natal resident versus non-natal immigrant species occurrence on community-level environmental gradient modelling and spatial-environmental hypothesis testing using adult dragonflies and damselflies as model taxa. Resident and total (resident+immigrant) occurrences of these taxa responded to different sets of environmental variables and resident occurrences reduced model selection uncertainty in 75% of test cases. Effects of environmental gradients, spatial gradients or both were observed in residents but not immigrants, and supported predictions of dispersal limitation and niche-based species sorting often implicated for structuring freshwater communities. These results indicate that resident-only analysis of the dispersal stage should improve multi-model inference and detection of spatial-environmental effects in freshwater community ecology. The species resident-immigrant dichotomy neglects population dynamics and individual variation yet apparently marks an ecologically significant boundary that scales up to influence community-level occurrence patterns.