Proceedings of the Third International Conference on Invasive ...

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<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 4: Spartina Control and ManagementWHERE DO WE GO FROM HERE?ALTERNATIVE CONTROL AND RESTORATIONTRAJECTORIES FOR A MARINE GRASS (SPARTINA ANGLICA)INVADER IN DIFFERENTHABITAT TYPESS.D. HACKER 1 AND M.N. DETHIER 21 Department <strong>of</strong> Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, Oregon 97331;hackers@science.oregonstate.edu2 Department <strong>of</strong> Biology, University <strong>of</strong> Washington, Friday Harbor Laboratories, 620 University Road, Friday Harbor,Washington 98250; mdethier@u.washington.eduLittle is known about <strong>the</strong> effects <strong>of</strong> removing invasive species and subsequent consequences forcommunity restoration. Invasive species removal can have positive effects for some communitiesbut may cause unexpected changes that lead <strong>the</strong> system to an alternative state. The consequences <strong>of</strong>invasive species removal are likely to be context-dependent with restoration occurring readily undersome situations but not o<strong>the</strong>rs. English cordgrass, Spartina anglica, has invaded large areas <strong>of</strong>protected shoreline in <strong>the</strong> Puget Sound, Washington State, USA, and is <strong>the</strong> target <strong>of</strong> intensiveremoval efforts. It invades and modifies a variety <strong>of</strong> habitat types, from unvegetated mudflats andcobble beaches to established low and high salinity native marshes. It binds sediment around itsdense root system and changes biogeochemical processes, all <strong>of</strong> which can have significantconsequences for shorebirds, infauna, and commercial aquaculture. Cordgrass invasion,modification, and removal varies among <strong>the</strong> different habitat types but post-removal colonizationpredictably results in colonization <strong>of</strong> native vascular plants. Although <strong>the</strong>se plants are <strong>the</strong> dominantspecies in salt marsh communities, <strong>the</strong>y are uncommon in mudflat and cobble beach communities,and thus do not represent a restored post-removal state. Instead, <strong>the</strong> legacy effects <strong>of</strong> cordgrassproduce alternative outcomes. We hypo<strong>the</strong>size, based on <strong>the</strong> interaction between recruitment,physical disturbance (water movement) and sediment accretion, that cobble beach and high salinitymarshes will be restored but that mudflats and low salinity marshes will retain <strong>the</strong> legacy <strong>of</strong> <strong>the</strong>invasion for <strong>the</strong> long term.Keywords: English cordgrass, Spartina anglica, habitat modification, salt marsh, mudflat, cobblebeach, restoration, alternative states, Puget Sound, Washington.INTRODUCTIONIn recent years <strong>the</strong>re has been a strong focus on <strong>the</strong>impacts <strong>of</strong> invasive species both at community andecosystem levels (Parker et al. 1999; Ruiz et al. 1999;Grosholz 2002). Some <strong>of</strong> <strong>the</strong>se invasive species, known asfoundation, dominant, or ecosystem engineering species(Jones et al. 1994; Power et al. 1996; Bruno and Bertness2001; Crooks 2002), can transform communities, resulting inboth positive and negative effects for native as well asnonindigenous species. These species can have a largeinfluence on community structure relative to <strong>the</strong>ir biomass;<strong>the</strong>y alter ecological processes in multiple ways and <strong>of</strong>tencreate positive feedbacks that benefit <strong>the</strong>ir continuedexpansion and impact.Much less is known about <strong>the</strong> consequences <strong>of</strong>removing invasive species, especially dominant orfoundation species (Hobbs and Humphries 1995; Myers etal. 2000; Zavalata et al. 2001, Hacker and Dethier 2009).Invasive species removal can have positive effects for somecommunities, with restoration occurring soon after removal(Fig. 1A; Myers et al. 2000). However, in many cases, <strong>the</strong>results have been mixed, with unexpected and widespreadimpacts on natural communities (Zavalata et al. 2001;D’Antonio and Meyerson 2002). Communities may notsimply return to <strong>the</strong>ir former state in a straightforwardreversal <strong>of</strong> <strong>the</strong> invasion process but, instead, could be somodified by <strong>the</strong> invasion that <strong>the</strong>y are not easily restored(Fig. 1B; Hobbs and Humphries 1995; D’Antonio andMeyerson 2002). These modifications are likely to vary indegree, depending on characteristics <strong>of</strong> <strong>the</strong> invader, <strong>the</strong>invaded community, and <strong>the</strong> time since invasion, but couldprevent full recovery after <strong>the</strong> invader is removed. Just how<strong>the</strong> ‘legacy’ <strong>of</strong> <strong>the</strong> invasion influences post-removalcommunity structure is poorly understood; yet suchunderstanding is critical to be able to confidently predictwhe<strong>the</strong>r management goals, originally intended to restore<strong>the</strong> integrity <strong>of</strong> highly invaded communities, will be met byremoval alone or whe<strong>the</strong>r additional measures may berequired. Given <strong>the</strong> numerous removal programs underway,development <strong>of</strong> testable <strong>the</strong>ories that predict post-removalcommunity dynamics are needed to better understand <strong>the</strong>benefits and risks <strong>of</strong> removing invaders that have largemodifying effects (Hacker and Dethier 2009).- 211 -
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 -
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FORWARD & ACKNOWLEDGEMENTSThe <stro
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Community Spartina Education and St
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CHAPTER ONESpartina Biology
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Chapter 1: Spartina Biology
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