Patch-size dependent habitat modification and facilitation on New England cobble beaches by Spartina alterniflora

Abstract

Most marine habitats are generated by the presence of habitat-modifying species. However, little is know about many aspects of this process, such as how individual- and population-level traits of habitat modifiers affect their ability to reduce environmental stress and thus facilitate other species. An important habitat modifier in New England is the intertidal grass Spartina alterniflora which facilitates the establishment and persistence of cobble beach plant communities by reducing wave-related disturbance. The objectives of this study were to (1) quantify the modification of cobble beach habitats by S. alterniflora, (2) determine how this process is related to S. alterniflora bed traits, and (3) determine why small patches of S. alterniflora generally remain unoccupied by cobble beach plants. Our results demonstrate that S. alterniflora substantially reduces flow-related physical disturbance on cobble beaches. Behind S. alterniflora, mean flow velocity was reduced by 40-60% and substrate stability was dramatically increased compared to portions of the shoreline not bordered by this species. These comparative results were supported by a S. alterniflora shoot removal experiment, which resulted in a 33% increase in average flow velocity and an 85% increase in substrate instability relative to control areas. There was a strong inverse logarithmic relationship between bed length and both average flow velocity and substrate instability behind S. alterniflora. Most S. alterniflora beds were small and bed length was significantly related to the presence of one or more cobble beach plant species. Only 13% of beds <25 m and 40% of beds 30-40 m in length were occupied, in contrast to an occupancy rate of 87% for beds >40 m long. Seeds of two annual cobble beach species (Suaeda linearis and Salicornia europaea) were added to plots behind large (>100 m in length) and small S. alterniflora (<25 m) beds with and without a substrate stabilization manipulation. Seedlings of both species only emerged and established behind small beds when the substrate was stabilized. These results indicate that smaller S. alterniflora patches are usually unoccupied because they do not stabilize the substrate to a degree that meets the establishment requirements of seedlings. Thus, both habitat modification and facilitation by S. alterniflora are patch-size dependent. The conditionality of this facilitation appears to generate a pattern of patchy yet predictable population and community distribution at a landscape spatial scale.

Multiple Mutualists Provide Complementary Benefits to Their Seaweed Host

“…Because the hosts in question (corals, seaweeds) often form the habitat on which the rest of the community depends, mutualist diversity may be important for the maintenance of these ecosystems. Habitat loss and fragmentation as a result of human activities could reduce the density of host foundation species below that required to maintain populations of associated species (e.g., Bruno and Kennedy 2000). When the loss of these associated species matter to the fitness of the host, the resulting feedback could accelerate the decline of the ecosystem based on the foundation species.…”

Switching from negative to positive density-dependence among populations of a cobble beach plant

“…Cobble beaches are common on moderately protected shorelines of New England, with the seaward edge often fringed by beds of the cordgrass Spartina alterniXora that becomes almost fully submerged at high tide. When large enough (>40 m long; Bruno and Kennedy 2000), beds of S. alterniXora are present, they modify the shoreline environment by buVering against wave disturbance, thereby stabilizing cobbles behind the bed. This stabilization facilitates the establishment and persistence of a forb community comprising up to 12 annual and perennial species that are otherwise exceedingly rare (usually nonexistent) in the absence of S. alterniXora beds (Bruno 2000;Bruno and Kennedy 2000).…”

Alteration of the configuration of bioengineers affects associated taxa

“…In addition to potential differences in the relative strength of various engineers (Borthagaray & Carranza 2007, Wallentinus & Nyberg 2007, differences among habitats created by the same type of engineer may occur (Bruno & Kennedy 2000, Flecker & Taylor 2004, People 2006. Such variation may be attributed to the configuration (i.e.…”