Physical factors vs. biotic resistance in controlling the invasion of an estuarine marsh grass

We examined the relative roles of biotic resistance and abiotic factors in controlling the establishment and early survival of an invasive marine grass, Spartina anglica, in the Pacific Northwest. S. anglica can invade diverse estuarine habitats, including established salt marshes, mudflats, and cobble beaches; these vary in native species assemblages and in abiotic conditions, especially sediment salinity, water content, and composition. In four habitat types in northern Puget Sound, we conducted a seed addition experiment to quantify germination and early growth. In vegetated habitats we also manipulated plant neighbors. Seed germination and early growth varied with habitat and zone, but physical factors played a more important role than species interactions. Seedlings in low-salinity marshes and mudflats grew almost 10 times larger than those,in coarse-grained or high-salinity habitats. Patterns of seedling persistence among habitats matched those of natural seedling abundances and mirrored the degree of invasion in different habitat types in the region. The lack of biotic resistance may be due to the stressful nature of saline systems or the lack of species that can compete with or consume the invader. In such systems, managers should focus on how physical vulnerabilities of the habitat can provide invasion windows of opportunity.