T he world's human population is becoming increasingly urbanized (UN 2018), posing a global threat to biodiversity and ecosystem functioning (McKinney 2006). This threat is pervasive across terrestrial, freshwater, and marine realms (here, we use the term "realm" rather than "ecosystem" to highlight the domain or sphere of influence and interconnections that terrestrial, riverine, and marine systems have on one another). The cross-realm effects of urbanization are largely driven by habitat modification to create a built environment (eg roads, buildings, stormwater infrastructure, marinas, and so forth; Trombulak and Frissell 2000;Walsh et al. 2005;Dafforn et al. 2015) and associated stressors (eg toxicants, nutrients, non-native species), which collectively threaten, degrade, and/or modify habitats (eg Airoldi et al. 2009;He et al. 2014). Such effects may propagate not only across habitat boundaries within urban realms (Lapoint et al. 2015;Bishop et al. 2017) but also across boundaries among realms (Bugnot et al. 2019) (Figure 1).Populations, communities, and ecosystems are connected via a variety of physical, biological, and chemical processes that operate at a range of spatial and temporal scales (Sheaves 2009;Lapoint et al. 2015;Bishop et al. 2017). Changes in connectivity can have substantial effects on biodiversity, productivity, energy fluxes, and food-web dynamics (Lapoint et al. 2015;Bishop et al. 2017). Urbanization can change the connectivity among realms by introducing barriers or conduits to the flow of species and resources, altering the abundance of organisms and materials that cross boundaries, and fragmenting or modifying habitats (Crook et al. 2015;Bishop et al. 2017), all of which can lead to cross-realm impacts. For example, barrages designed to protect coastal settlements from storm surges and dams that manage water flows can impede fish migrations between freshwater and marine habitats, and can starve coastal sandflats and mudflats of terrestrial sediment supplies, which are transported downstream by river systems (Figure 1; Crook et al. 2015; Bishop et al. 2017). Conversely, deforestation of catchments may increase
