Facilitation of cobble beach plant communities through habitat modification by spartina alterniflora

Single species can have large effects on entire communities through habitat modification and facilitation. I tested the general hypothesis that the intertidal grass Spartina alterniflora facilitates the establishment and persistence of New England cobble beach plant communities by modifying the shoreline environment. This community is dominated by halophytic forbs and is restricted to estuarine cobble beaches bordered by Spartina. Beds of Spartina can reduce mean water velocity by 50% and maximum velocity by nearly an order of magnitude, and they can substantially stabilize the cobble substrate. Specifically, I determined the importance of five life history stages (seed supply, seed germination, seedling emergence, seedling establishment, and adult survival) and four factors (water velocity, substrate stability, herbivory, and soil quality) in limiting lateral plant distribution. A seed addition experiment demonstrated that seedlings could only emerge behind Spartina, suggesting that seedling emergence is the proximate life history stage limiting adult distribution. Seed germination and adult survival do not appear to be limiting stages, at least in an absolute sense. Although seed supply was much greater (∼10–100×) behind Spartina, a substantial number of seeds were caught in seed traps placed between beds, suggesting that seed supply also does not limit absolute plant distribution. These results are supported by the presence of seedlings buried below the substrate surface between beds at the time when seedlings are naturally emerging behind beds. A manipulative field experiment was performed to test the effects of substrate instability, soil quality, and herbivory on seedling emergence between beds. Seeds of two annual cobble beach species (Suaeda linearis and Salicornia europaea) were added to plots behind Spartina and also between beds with and without substrate stabilization manipulations. This treatment was designed to stabilize the substrate in a manner that would not affect water velocity and soil characteristics or prevent access by potential herbivores. When not buffered by Spartina, seedlings of both species were only able to emerge and survive when the substrate was artificially stabilized. These results indicate that Spartina alterniflora facilitates the establishment and persistence of cobble beach plant communities by stabilizing the substrate and enabling seedlings to emerge and survive.