Effect of foundation species composition and oil exposure on wetland communities across multiple trophic levels

Abstract

Climate-driven range shifts can result in altered mixtures of foundation species that can affect ecosystem structure and function. Higher diversity mixed assemblages may moderate disturbance impacts, yet this prediction has rarely been tested. The Deepwater Horizon oil spill provided an opportunity to examine the effects of salt marsh foundation species identity and composition on the response of both plant and faunal communities to oil disturbance. In the northern Gulf of Mexico, salt marshes are typically dominated by the foundation species smooth cordgrass, Spartina alterniflora. However, the sub-tropical black mangrove, Avicennia germinans, is expanding northward and increasingly growing with, and even replacing, Spartina. Based on a series of field surveys in the Chandeleur Islands, Louisiana, we found that oil exposure had few impacts on the plant community 5 yr post-oiling, though oil disturbance shifted the interaction among foundation species, increasing Avicennia’s competitive advantage and likely enhancing mangrove expansion. In contrast, abundance of the epibenthic animal community was reduced by ~30% and community composition was altered at oiled sites. Mixed foundation species assemblages provided few reductions in oil impacts within either trophic level. Strong habitat associations of epibenthic taxa, coupled with oil effects on individual foundation species, likely suppressed any advantage of mixed vegetation on the associated fauna. Our survey highlights that plant and epibenthic animal responses to disturbance can be decoupled, emphasizing the importance of examining both simultaneously to better understand and predict long-term responses.