Hierarchical Organization via a Facilitation Cascade in Intertidal Cordgrass Bed Communities

Abstract

It has recently been proposed that many communities are structured by a hierarchy of interactions in which facilitation by foundation species is of primary importance. We conducted the first explicit experimental test of this hypothesis by investigating the organization of positive interactions on New England cobblestone beaches. In this midintertidal community, wave-generated substrate instability and solar stress largely limit marine organisms to the shelter of cordgrass beds. Cordgrass, which can establish and persist without the aid of other foundation species, facilitates a dense assemblage of inhabitants (e.g., mussels, snails, seaweeds) with roots/rhizomes that stabilize substrate and a dense canopy that baffles waves and provides shade. Within the cordgrass bed community, ribbed mussels further enhance physical conditions and densities of other species (e.g., amphipods, barnacles) by providing crevice space and hard substrate. We conclude that cordgrass bed communities are hierarchically organized: secondary interactions (e.g., facilitation by ribbed mussels) play a key role within an assemblage dependent on primary facilitation by the independently successful foundation species cordgrass. Our results identify emergent indirect positive interactions in the form of facilitation cascades, have broad implications for conservation, and help unify existing models of community organization that were developed without considering the fundamental role of positive interactions.

Facilitation cascade drives positive relationship between native biodiversity and invasion success

“…Based on our observation that the most diverse and heavily invaded sites had cordgrass, and in accordance with recent conceptual and observational studies suggesting the potential importance of positive interactions for the correlated success of native and invasive species (Simberloff 1986, Levine and D'Antonio 1999, Richardson et al 2000, Stachowicz and Byrnes 2006, we conducted the following three experiments on the shores of the NB-NERR to test the hypothesis that a facilitation cascade (sensu Altieri et al 2007) in cordgrass beds enhances both native diversity and invader abundance, leading to their large-scale association.…”

Marine Phytophthora species can hamper conservation and restoration of vegetated coastal ecosystems

“…flood protection, carbon and nutrient storage, biodiversity enhancement) [25][26][27][28]. In addition, these vegetated coastal ecosystems are globally disappearing [29,30], and costly restoration efforts, with various success rates, are being undertaken to halt and revert these losses.…”

Distinct habitat types arise along a continuous hydrodynamic stress gradient due to interplay of competition and facilitation

“…Once Spartina alterniflora is established, its role as a foundation species and ecosystem engineer facilitates the establishment of plant communities behind (Bruno 2000) and invertebrate communities within (Altieri et al 2007) the bed. This successional development of the coastal environment ultimately climaxes in salt-marsh systems, unless harsh hydrodynamic conditions prevent the peat accumulation necessary for marsh development.…”

“…In our study system, the hydrodynamic conditions along the environmental gradient effectively determine prevailing species interactions, which combine with positive feedbacks to generate distinct habitats from a common pool of potential component species. The shorelines are thus hierarchically organised (Altieri et al 2007), where environmental stress in the form of wave-driven substrate instability determines first where the foundation species S. alterniflora can establish, which in turn determines the nature of secondary interactions among the plants facilitated by S. alterniflora. Alternative hypotheses for variation in community type across the hydrodynamic gradient, such as seed dispersal, have been examined previously in our study system and have been shown not to limit distributions of shoreline forbs (Rand 2000, Bruno 2002.…”