Climate change is altering species distributions worldwide. Resident species can facilitate or inhibit range-expanding species, and variation in abiotic and biotic conditions can shift these interactions. Additionally, structural attributes of interacting ecosystem engineers can affect species interactions by physically modifying the environment. I used mangrove expansion into salt marshes to examine how a resident ecosystem engineer (i.e., salt marsh) affects the establishment of its range-expanding competitor (i.e., mangrove) and explore the abiotic and biotic conditions that influence these interactions. I also assessed whether the structural attributes of range-expanding engineers can provide substitutable ecosystem functions relative to resident engineers. Mangrove and salt marsh species both form the foundations of their respective ecosystems, but they provide different growth forms and litter structure. I used surveys, outdoor mesocosm experiments and field experiments to examine how adult mangroves and resident salt marsh both live and dead affect mangrove propagule recruitment and establishment, and how these interactions vary with tidal inundation. The structural attributes of live adult salt marsh facilitated mangrove propagule retention, and retention depended on tidal water levels relative to vegetation heights. Dead salt marsh wrack both facilitated and inhibited mangrove propagule establishment, depending on the spatial position of propagules relative to wrack. The magnitude of these effects depended on inundation; effects were accentuated during harsher conditions. In the field, effects of salt marsh wrack also shifted with mangrove ontogeny. Wrack facilitated mangrove establishment by promoting propagule recruitment, retention, and rooting, but inhibited seedling development by blocking access to light and attracting herbivores. Hurricanes could also have negative, indirect effects on mangrove recruitment by relocating wrack to upland environments. Lastly, marsh and mangrove litter resources were non-substitutable; the lower structural value of mangrove litter relative to salt marsh litter caused shifts in epifaunal communities. Collectively, this work highlights that the structural attributes of resident and range-expanding species can affect both expansion dynamics and associated ecosystem functions; species interaction vary predictably with both abiotic and biotic factors, which can influence where and when resident species will facilitate or inhibit range-expanding species.