General Impact
Competition between Spartina densiflora and native flora leads to a loss of local plant biodiversity and decline of species specific habitats, which negatively effects local fauna. The increased invasion of S. densiflora can lead to a rise in marsh elevation because of plant presence on mud flats, thus limiting water flow and increasing sedimentation (San Fransisco Estuary Invasive Spartina Project, undated).
S. densiflora adds more complexity to mudflat habitats and it may increase local abundance and diversity of the associated fauna (Dr. Alejandro Bortolus, pers.comm., 2009).
Location Specific Impacts:Gulf of Cádiz (Atlantic Ocean) 
Reduction in native biodiversity: Spartina densiflora occupies all topographic levels of marshland where it outcompetes and replaces the more diverse populations of plant species including Anthrocnemum macrosthachyum, Sarcocornia ramosissima, Halimione portulacoides, Suaeda splendens, Limoniastrum monopetalum, Juncus maritimus, Scirpus maritimus and Typha spp. (Nieva, 2005). Odiel River (Spain)
Reduction in native biodiversity: The continued invasion of Spartina densiflora in the south-western coast of Spain brings it in contact with small cordgrass (Spartina maritima), with which it competes for resources (Castillo, 2000). Humboldt Bay (United States (USA))
Reduction in native biodiversity: Spartina densiflora smothers native Salicornia and Distichylis resulting in open colonization of bare areas (Daehler, 1996).
Threat to endangered species: The expansion of Spartina densiflora into high-elevation salt marshes in Humboldt Bay puts the two rare plants, Cordylanthus maritimus ssp. palustris and Castilleja ambigua ssp. humboldtiensis, in jeopardy. (Tatum, undated). San Francisco Bay (Estuary) (United States (USA))
Habitat alteration: The ability of Spartina densiflora to colonize mudflats causes a loss of foraging habitat for local waterfowl and shorebirds (San Fransisco Estuary Invasive Spartina Project, undated).
Other: The buildup of dense patches of Spartina densiflora leads to a decrease in water flow, in turn leading to an increased rate of sedimentation (San Fransisco Estuary Invasive Spartina Project, undated).
Reduction in native biodiversity: The negative interaction with native plants, such as pickleweed (Salicornia virginica) and California cordgrass (Spartina foliosa) may over time lead to a loss of local plant diversity (San Fransisco Estuary Invasive Spartina Project, undated). Other native species affected in California are Frankenia salina, Limonium californicum, and Jaumea carmosa (University of California, undated).
Threat to endangered species: The invasion of Spartina densiflora affects the habitat of the 'Vulvnerable (VU)' salt marsh harvest mouse (see Reithrodontomys raviventris in IUCN Red List of Threatened Species), and the federally listed California clapper rail (see Rallus longirostris obsoletus for State and Federal status (San Fransisco Estuary Invasive Spartina Project, undated).
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