Functional redundancy of a non-native foundation species (eelgrass, Zostera japonica) across intertidal stress gradients

Non-native species foundation species can alter ecosystems in both positive and negative ways. The creation of habitat can be beneficial to native species when they provide a limiting resource or in a stressful environment. Yet this creation of habitat can also be detrimental by replacing native species and/or facilitating the presence of more non-native species. In Willapa Bay, WA, a non-native foundation species, Zostera japonica, co-exists with the native foundation species Zostera marina. Zostera japonica persists at the higher intertidal in monocultures, the two species overlap in the mid intertidal, and Z. marina persists in monocultures in the low intertidal. Epifaunal invertebrates, the organisms that live on eelgrass blades, connect eelgrass to higher trophic levels. Through a series of transplants and removals, I used this zonation pattern to ask if the two species can fulfill a similar functional role in respect to epifaunal invertebrates (functional redundancy), and if this was due to the identity of the foundation species or a response to the stress gradient of the intertidal. My results suggest that the epifaunal invertebrate community is responding more to the physiological stress gradient, and the functional redundancy of the two species depends on the location they are found. Z. japonica is expanding the range of vegetated habitat into to the physiologically stressful high zone, which supports a different community. This experiment highlights that the impacts of non- native species are highly localized and that abiotic and biotic factors are important to trophic interactions.