Morphological traits and density of foundation species modulate a facilitation cascade in Australian mangroves

Facilitation cascades are critical to the maintenance of biodiversity in a variety of habitats. Through a series of two experiments, we examined how the morphological traits and density of interacting foundation species influence the establishment and persistence of a facilitation cascade in temperate Australian mangrove forests. In this system, mangrove pneumatophores trap the free-living alga, Hormosira banksii, which, in turn, supports dense and diverse assemblages of epifaunal mollusks. The first experiment, which manipulated pneumatophore height and density, revealed that these two traits each had additive negative effects on the establishment, but additive positive effects on the persistence of the cascade. High densities of tall pneumatophores initially served as a physical barrier to algal colonization of pneumatophore plots, but over the longer term enhanced the retention of algae. The increased algal biomass, in turn, facilitating epifaunal colonization. The second experiment demonstrated that the retention of algae by pneumatophores was influenced more by algal thallus length than vesicle diameter, and this effect occurred independent of pneumatophore height. Our study has extended facilitation theory by showing that the morphological traits and density of basal and intermediary facilitators influence both the establishment and persistence of facilitation cascades. Hence, attempts to use foundation species as a tool for restoration will require an understanding not only of the interactions among these, but also of the key traits that modify interrelationships.