Community modification by a grass invader has differing impacts for marine habitats Sally D. Hacker and Megan N. Dethier Hacker, S. D. and Dethier, M. N. 2006. Community modification by a grass invader has differing impacts for marine habitats. / Oikos 113: 279 /286. There is increasing recognition of invasive species impacts but less is known about how modifications may differ under variable environmental contexts. In particular, it is generally unknown whether impacts of single invasive species can vary among habitats andwhat the consequences of this variability may be. We used a multi-site comparative approach to examine the impacts of a single invader, the marine grass Spartina anglica , on estuarine habitatswith different native species assemblages and physical conditions. We found that range (extent), abundance, and effects on sediment and native plant species vary depending on the habitat invaded. S. anglica has by far the greatest range and abundance in mudflats and low salinity marshes compared to high salinity marshes and cobble beaches. Changes in sediment characteristics also substantially differed among habitats, with invaded areas in some habitats experiencing greater sediment accretion, water content, and salinity than other habitats. In addition, in opposition to the theory that strong invaders decrease species diversity, we found that native plant diversity in our plots increased within invaded areas in some habitats while it declined in others. These variable modifications suggest that single invaders, even species that are considered strong interactors, do not produce the same effect in all habitats. We suggest that understanding impact variability can help predict how invasive species will respond to environmental changes, new habitats, and management strategies. S. D. Hacker, Dept of Zoology, Oregon State Univ., 3029 Cordley Hall, Corvallis, OR 97331, USA (hackers@science.oregonstate.edu). / M. N. Dethier, Dept of Biology, Univ. of Washington, Friday Harbor Laboratories, 620 University Road, Friday Harbor, WA 98250, USA. Invasion biology is a growing field, with a strong focus on the colonization of invasive species and their impacts on native species (Parker et al. 1999, Grosholz 2002). Invasive species of particular concern are those that cause community and ecosystem level effects (Vitousek 1990, Byers et al. 2002, Crooks 2002, Grosholz 2002), by either altering processes such as nutrient cycling (Vitou- sek et al. 1987), disturbance regime (Mack and D’Anto- nio 1998), species interactions (Callaway and Aschehoug 2000), or structural and physical characteristics of the community itself (Crooks 1998). A recent review by Crooks (2002) illustrates the complex ways invasive species can transform communities, resulting in both positive and negative effects for other native and nonindigenous species. These invaders can have signifi- cant impacts especially if they create positive feedbacks that negatively influence native species but benefit their own continued expansion. Despite the increasing recognition of the community impacts of invasive species, less is known about how these modifications may differ depending on the context of the invasion (Parker et al. 1999, Byers et al. 2002). For example, while it is recognized that different species vary in their impact in the same habitat (so called ‘‘weak’’ and Accepted 10 October 2005 Subject Editor: Jane Memmott Copyright # OIKOS 2006 ISSN 0030-1299 OIKOS 113: 279 /286, 2006 OIKOS 113:2 (2006) 279