Spartina alterniflora facilitates rocky shore mussel reefs through a hierarchy of positive interactions.
Altieri, Andrew*,1, Silliman, Brian 2, Bertness, Mark1, 1 Brown University, Providence, RI, United States2 Virginia Institute of Marine Science, Gloucester Point, VA, United States
ABSTRACT- The significance of facilitation in marine communities is increasingly recognized, and is exemplified by foundation species which can increase the distribution and/or abundance of other species by modifying the physical environment. However, it is less well understood how two such engineers may (1) facilitate one another, and (2) interact to facilitate other species. On cobble beaches in New England, beds of cordgrass (Spartina alterniflora) are commonly inhabited by ribbed mussels (Geukensia demissa). Moreover, species such as acorn barnacles (Semibalanus balanoides) that are otherwise restricted to lower tidal heights are abundant within this assemblage. We conducted a series of experiments to test whether cordgrass and mussels facilitate one another and/or barnacles, and to reveal the mechanisms underlying these relationships. In the first experiment, manipulations of mussel densities and the grass canopy within cordgrass beds revealed that cordgrass did facilitate mussels, mussels did not facilitate cordgrass, and that both acted synergistically to facilitate barnacles. Mussels appeared to benefit from attachment to cordgrass, rather than from amelioration of solar stress provided by the canopy, as revealed by their loss from a treatment in which the canopy was removed and replaced by an artificial shade. Barnacle abundances were highest in treatments in which mussels were present to provide substrate, and either a natural canopy or shade ameliorated solar stress. Results of a second experiment confirmed that shade plus the substrate provided by stabilized cobble was sufficient to significantly increase barnacle densities. On cobble beaches of New England, cordgrass facilitates barnacles directly by ameliorating solar stress, and indirectly by anchoring mussels which in turn provide a stable hard substrate. Synergism between foundation species may have a significant effect on the distribution and abundance of species in other stressful systems as well.
Key words: Environmental stress, Foundation species, Positive interactions, Intertidal
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