Metabolic insight into bacterial community assembly across ecosystem boundaries

The movement of organisms across habitat boundaries has important consequences for populations, communities, and ecosystems. However, because most species are not well adapted to all habitat types, dispersal into suboptimal habitats could induce physiological changes associated with persistence strategies that influence community assembly. For example, high rates of cross‐boundary dispersal are thought to maintain sink populations of terrestrial bacteria in aquatic habitats, but these bacteria may also persist by lowering their metabolic activity, introducing metabolic heterogeneity that buffers the population against species sorting. To differentiate between these assembly processes, we analyzed bacterial composition along a hydrological flow path from terrestrial soils through an aquatic reservoir by sequencing the active and total (active + inactive) portions of the community. When metabolic heterogeneity was ignored, our data were consistent with views that cross‐boundary dispersal is important for structuring aquatic bacterial communities. In contrast, we found evidence for strong species sorting in the active portion of the aquatic community, suggesting that dispersal may have a weaker effect than persistence strategies on aquatic community assembly. By accounting for metabolic heterogeneity in complex communities, our findings clarify the roles of local‐ and regional‐scale assembly processes in terrestrial‐aquatic meta‐ecosystems. The movement of material and energy across habitat boundaries is important for the structure and function of recipient ecosystems (Polis et al. 2004, Gounand et al. 2018a). These spatial resource subsidies can stabilize population dynamics, alter food web structure, and modify biogeochemical cycles (Polis et al. 2004, Massol et al. 2011). However, in complex landscapes linked by spatial fluxes of resources and organisms, the process of community assembly remains less clear (Gounand et al. 2018a). Meta‐ecosystem theory predicts that poorly adapted species dispersed across ecosystem boundaries will be eliminated from the recipient habitat via species sorting (Massol et al. 2017, Gounand et al. 2018a), unless resource flows sufficiently homogenize the landscape (Gravel et al. 2010). However, if generalist species are capable of tolerating a range of environmental conditions, then cross‐boundary dispersal could affect community assembly in recipient habitats (Haegeman and Loreau 2014). Habitats at the terrestrial‐freshwater interface are ideal for addressing questions about meta‐ecosystem ecology (Gounand et al. 2018b).