Tropicalization of the barrier islands of the northern Gulf of Mexico: A comparison of herbivory and decomposition rates between smooth cordgrass (Spartina alterniflora) and black mangrove (Avicennia germinans)

The expansion of black mangrove Avicennia germinans into historically smooth cordgrass Spartina alterniflora-dominated marshes with warming temperatures heralds the migration of the marsh-mangrove ecotone northward in the northern Gulf of Mexico. With this shift, Avicennia is expected to outcompete Spartina where it is able to establish, offering another prevalent food source to first order consumers. In this study, we find Avicennia leaves to be more preferable to chewing herbivores, but simultaneously, chewing herbivores cause more damage to Spartina leaves. Despite higher nitrogen content, Avicennia leaves decomposed slower than Spartina leaves, perhaps due to other leaf constituents or a different microbial community, for other studies have found the opposite in decomposition rates of the two species’ leaf tissue. This study provides insights into basic trophic process, herbivory and decomposition, at the initial stages of black mangrove colonization into Spartina salt marsh.