Proceedings of the Third International Conference on Invasive ...

Chapter 4: Spartina Control and Management<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaA. Best casescenarioRestorationB. More likelyscenarioAlternativeFig. 1. Stages <strong>of</strong> invasive species removal (arrow) under A) <strong>the</strong> best casescenario for which <strong>the</strong> invader modification is lost and habitat is restored,and B) <strong>the</strong> more likely scenario for which <strong>the</strong> modification remains andcommunity follows an alternative trajectory. From Hacker and Dethier 2009In this paper, we explore <strong>the</strong> possible consequences <strong>of</strong>removing <strong>the</strong> invasive English cordgrass, Spartina anglica,which has invaded <strong>the</strong> shoreline <strong>of</strong> Puget Sound inWashington State, USA. Spartina anglica colonizescommunities with different species assemblages andphysical conditions, and thus produces variable degrees <strong>of</strong>invader modification. Our goal is to use <strong>the</strong>se patterns <strong>of</strong>differential invasion along with a simple conceptual modelto make predictions about post-removal communitystructure. Based on this analysis, we predict that somecommunities will be readily restored while o<strong>the</strong>rs willfollow alternative states. These states will depend on how<strong>the</strong> modifications and physical disturbance interact toinfluence both species recruitment and communitymaintenance through time.STUDY SYSTEMSpartina anglica was first introduced to Puget Soundfrom England in 1961. It did not become a managementpriority until <strong>the</strong> late 1990s when it had spread to a total <strong>of</strong>3,300 hectares (ha) <strong>of</strong> intertidal habitat at 77 sites (Hacker etal. 2001). Cordgrass is a strong ecosystem modifier thataccretes sediment around its dense root system and changessediment biogeochemistry, which can have importantcommunity-wide consequences (Thompson 1991; Daehlerand Strong 1996). Species affected by <strong>the</strong> invasion includenative and commercial invertebrates (infauna and epifaunasuch as clams and oysters; Zipperer 1996; O’Connell 2002),plants (Hacker and Dethier 2006), and birds (Goss-Custardand Moser 1988; Triplet et al. 2002).Cordgrass grows in a range <strong>of</strong> communities in PugetSound from mudflats and cobble beaches, which arenormally devoid <strong>of</strong> vascular plants, to low and high salinitymarshes, where native vascular plants are <strong>the</strong> mainbiological component (Hacker et al. 2001). Our researchshows that cordgrass invasion, removal, and post-removalpatterns vary dramatically between <strong>the</strong>se communities(Hacker et al. 2001; Reeder and Hacker 2004; Dethier andHacker 2005; Hacker and Dethier 2006), thus suggestingthat its removal will result in different post-removalcommunity structure depending on habitat.Habitat dependent invasion and modification by cordgrassThe abundance and distribution <strong>of</strong> English cordgrassvaries between four habitat types within Puget Sound(Hacker et al. 2001). Low salinity marsh and mudflat siteshave much larger infestations than cobble beach and highsalinity marsh sites. These differences are driven mostly byvariability in physical conditions across <strong>the</strong> four habitattypes although biological interactions play a small role(Dethier and Hacker 2005). In a seed addition experiment(Dethier and Hacker 2005), we found that mudflat and lowsalinity marshes had <strong>the</strong> greatest cordgrass germination,survival, and growth <strong>of</strong> all <strong>the</strong> habitats. In mudflatcommunities, which are naturally devoid <strong>of</strong> vascular plants,seed germination was high and surviving seedlings grewquickly. This growth pattern eventually results in <strong>the</strong>coalescence <strong>of</strong> individual plants and widespread growthacross all <strong>the</strong> intertidal elevations. Low salinity marshes, on<strong>the</strong> o<strong>the</strong>r hand, are dominated by a diverse native vascularplant assemblage. Cordgrass had high seedling germination,survival and growth; this pattern eventually results in largeswards that outcompete native plants and form densemonocultures. In high salinity marsh habitats, cordgrassgrows mainly in sparsely vegetated low intertidal streamchannel areas. Seed additions show high germination andsurvival in low elevation areas without plant neighbors butnot higher in <strong>the</strong> intertidal zone, pointing to <strong>the</strong> importance<strong>of</strong> high salinity and native plant interactions in restrictingcordgrass distribution. Finally, cordgrass is rare in cobblebeach habitats despite <strong>the</strong> lack <strong>of</strong> native vegetation. Hereshifting cobble sediments contribute to <strong>the</strong> low germination,survival, and growth <strong>of</strong> <strong>the</strong> plant.Cordgrass generally modifies its habitat by accretingsediment around its large root system, forming an elevatedroot mat and changing sediment biogeochemistry(Thompson 1991; Maricle and Lee 2002; Hacker and- 212 -