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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 ManagementDethier 2006). In addition, it can provide substantialaboveground structure not normally present in somehabitats. These structural and biogeochemical modificationshave direct and indirect effects on o<strong>the</strong>r plant and animalspecies as mentioned earlier. In our study system, Englishcordgrass differentially modifies four habitat types, resultingin variable, community-wide consequences (Hacker andDethier 2006). For example, we know that <strong>the</strong> accumulation<strong>of</strong> sediment by cordgrass differs between <strong>the</strong> four habitats,with mudflats experiencing <strong>the</strong> greatest sediment accretion(~30 cm) and low and high salinity marshes, <strong>the</strong> least (~10-15 cm) (Hacker and Dethier 2006). Cordgrass growing incobble beaches accretes much less sediment (~20 cm) thanmudflats. A likely explanation for this difference is waveenergy; water movement is lower in mudflats but higher incobble beaches.In addition to accreting sediment, thus changing <strong>the</strong>structural characteristics <strong>of</strong> <strong>the</strong> substrate, cordgrass causes anumber <strong>of</strong> chemical transformations. We found changes insediment water content, redox potential (a proxy for oxygencontent), and salinity in invaded versus native areasdepending on community type (Hacker and Dethier 2006).For example, cordgrass caused a decline in sediment watercontent in mudflats and high salinity marshes suggesting that<strong>the</strong> elevated root mat had less tidal inundation, betterdrainage, and/or more water uptake by cordgrass. In cobblebeaches, <strong>the</strong> opposite was seen; root mat sediments hadhigher water content than <strong>the</strong> unmodified cobble sediments.In low salinity marshes, <strong>the</strong>re was no change in sedimentwater content with <strong>the</strong> presence <strong>of</strong> cordgrass compared tonative vascular plants. Cordgrass generally increases oxygencontent in <strong>the</strong> sediments <strong>of</strong> all <strong>the</strong> communities. This may berelated, in part, to better drainage seen in some <strong>of</strong> <strong>the</strong>communities but is more likely due to oxygen leakage fromroot aerenchyma (Maricle and Lee 2002). Finally, surfacesalinities were generally lower in sediments with cordgrassalthough <strong>the</strong>y did not change in low salinity sites with nativeplant cover. These results suggest that cordgrass shades <strong>the</strong>sediment surface thus decreasing water evaporation and saltaccumulation compared to unvegetated areas although thishypo<strong>the</strong>sis was untested. We have not investigated cordgrasseffects on nutrient cycling but it is clear that cordgrass is amajor carbon source unlike any o<strong>the</strong>r in <strong>the</strong>se communitiesand likely modifies microbial and macr<strong>of</strong>aunal resource use.English cordgrass modifications have significantconsequences for community structure. We compared nativevascular plant and algal abundance in areas with and withoutcordgrass in all communities (Hacker and Dethier 2006). Inthis study, we found that in mudflats, cobble beaches, andhigh salinity marshes, cordgrass invasion caused an increasein native vascular plant cover and decline in algal cover. Byelevating sediments, increasing oxygen content, anddecreasing salinity, cordgrass clearly provides a moresuitable habitat for native vascular salt marsh plants but aless suitable habitat for algae, which require greater tidalinundation to avoid desiccation. In low salinity marshes,native vascular plants declined precipitously, presumably aconsequence <strong>of</strong> cordgrass competitive dominance. We alsocompared marine invertebrates in mud and cobble sedimentsversus adjacent cordgrass patches. Cobble areas withoutcordgrass had oligochaetes, bivalves, and a variety <strong>of</strong>crustaceans; cobble with cordgrass had less infauna because<strong>of</strong> dense root mat formation. Uninvaded mudflats <strong>of</strong>ten hadabundant clams and a variety <strong>of</strong> polychaete worms, whilethose with cordgrass had fewer infauna but more epifaunalcrustaceans such as amphipods (presumably because <strong>of</strong> <strong>the</strong>three–dimensional structure provided by <strong>the</strong> vegetation).Invertebrate studies (Zipperer 1996; O’Connell 2002) inareas invaded by Spartina alterniflora in Willapa Bay, WA,similarly found that certain taxa are excluded by cordgrass(esp. polychaetes and bivalves) while o<strong>the</strong>rs are increased(esp. dipteran larvae and spiders). Although we have notquantitatively measured possible changes in epifaunalcommunities, we have observed a general increase in highermarsh epifauna such as grasshoppers, spiders, snails, mice,and marsh wrens within cordgrass meadows and a decline inshorebirds, some species <strong>of</strong> snails, mussels, and oysters.Cordgrass removal and consequences for post-removalrestorationRemoval <strong>of</strong> English cordgrass, involving mowing andherbicide applications, began in 1997 and has caused amodest 10-20% decline as <strong>of</strong> 2002 (Hacker et al. 2001;Dethier and Hacker 2004). Although local eradication hasoccurred at some sites with minimal treatment repetition,most sites have required repeated removal spanning multipleyears to achieve eradication. To investigate <strong>the</strong> factorshindering removal success, we conducted a multiple sitestudy that linked removal data with ecological factors andremoval methodologies. We found that removal successdepended on removal regime and community type (Dethierand Hacker 2004). The bulk <strong>of</strong> cordgrass decline was due toconsistent, multi-year removal (once per year) in certaincommunities. For example, cobble beaches, high salinitymarshes, and mudflats showed <strong>the</strong> most promising responseto consistent removal even when years <strong>of</strong> removal effortwere similar to low salinity marshes. When removal wasintermittent ( 2 years missed), low salinity marshes andmudflats were <strong>the</strong> least responsive to removal.The pattern <strong>of</strong> regrowth with removal regime points to<strong>the</strong> resiliency <strong>of</strong> cordgrass. If photosyn<strong>the</strong>sis is notcontinually interrupted to slowly kill <strong>the</strong> plant, removalsuccess is compromised due to its ability to regrow. In a set<strong>of</strong> manipulative experiments, we found that biomass gainsunder intermittent removal greatly outweigh losses accruedunder consistent removal both because <strong>of</strong> <strong>the</strong> highlyproductive nature <strong>of</strong> <strong>the</strong> species, <strong>the</strong> benefits <strong>of</strong> competitiverelease, and <strong>the</strong> habitat modifications that facilitate fur<strong>the</strong>rgrowth (Reeder and Hacker 2004).- 213 -
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 SpartinaPreliminary study <strong>of</strong> post–removal community structuresuggests that <strong>the</strong> response largely depends on howmodifications created by <strong>the</strong> invader interact with habitattype (Reeder and Hacker 2004). In this study, all sitesexhibited similar community responses following cordgrassremoval. Native vascular plants increased with consistentremoval in all communities, but declined under intermittentremoval. In addition, <strong>the</strong> plant assemblages in <strong>the</strong> two saltmarsh communities were different after cordgrass removalthan <strong>the</strong>ir native counterparts. As a result, post-removalcommunity structure and habitat restoration is more complexthan simply removing cordgrass and anticipating a reversal<strong>of</strong> <strong>the</strong> invasion process.IMPLICATIONS AND FUTURE DIRECTIONSIn this system, we found that removing invasive S.anglica resulted in an increase in native vascular plantsirrespective <strong>of</strong> habitat type (Reeder and Hacker 2004).Although <strong>the</strong>se plants are <strong>the</strong> dominant species in salt marshcommunities, <strong>the</strong>y are uncommon in mudflat and cobblebeach communities, and thus do not represent a restoredpost-removal state. Instead, <strong>the</strong> legacy effects <strong>of</strong> <strong>the</strong>cordgrass infestation produced alternative short termoutcomes, which may or may not continue for some time.Legacy effects include elevated, stabilized sediments andaltered biogeochemical processes that make conditionsbetter for vascular plants and poorer for macroalgae andinfauna. We have observed a similar pattern <strong>of</strong> vascularplant colonization on relic cordgrass root mats both in <strong>the</strong>U.S. (Willapa Bay, Washington) and in o<strong>the</strong>r countries (Baiede Somme, France and Invercargill, New Zealand)suggesting that <strong>the</strong> effect is widespread (personalobservation). Interestingly, in <strong>the</strong> Baie de Somme, nativevegetation that colonizes cordgrass removal areas, inparticular <strong>the</strong> salt marsh plants Salicornia and Atriplex, areharvested and sold as a specialty food. However, in mostcases, long-term vascular plant colonization, especially inmudflat habitats, is likely to prolong <strong>the</strong> main managementissues surrounding invasive cordgrass (i.e. reducing habitatfor shorebirds, infauna, and commercially importantshellfish).Given <strong>the</strong> potential that removal <strong>of</strong> cordgrass can resultin alternative community structure trajectories, wedeveloped a conceptual model that helps predict <strong>the</strong> possibleoutcomes <strong>of</strong> cordgrass removal (Hacker and Dethier 2009).It is based on alternative stable state <strong>the</strong>ory which explains<strong>the</strong> observation that different species assemblages can occurin <strong>the</strong> same general locality at different times (or differentlocalities at <strong>the</strong> same time) because historical events orcontingencies play an important role in creating communitystructure (Lewinton 1969; Su<strong>the</strong>rland 1974; Petraitis andLatham 1999). We suggest that it can provide a usefulframework for identifying <strong>the</strong> processes important to postremovalcommunity structure by identifying <strong>the</strong> factors thatAlternative,post-removal stateMAINTENANCETRANSITIONSA. Cobble Beach and Mud flat CommunitiesMaintenance <strong>of</strong> cordgrasssediment accretion,biogeochemicalprocessesVascular PlantsDominateDecreasealgal, infaunalrecruitmentIncrease vascularplant, epifaunalrecruitmentAlgalRecruitmentVascular PlantRecruitmentWater MovementB. Low and High Salinity Marsh CommunitiesHigher IntertidalVascular PlantsDominateMaintenance <strong>of</strong> cordgrasssediment accretion,biogeochemicalprocessesDecreaseoriginal plantrecruitmentIncreasehigh intertidal plantrecruitmentOriginal PlantRecruitmentHigher IntertidalPlant RecruitmentWater MovementRestored,post-removal stateMAINTENANCEAlgae, InfaunaDominateLoss <strong>of</strong> cordgrasssediment accretion,biogeochemicalprocessesIncreasealgal, infaunalrecruitmentDecrease vascularplant, epifaunalrecruitmentOriginalVascular PlantsDominateLoss <strong>of</strong> cordgrasssediment accretion,biogeochemicalprocessesIncreaseoriginal plantrecruitmentDecreasehigh intertidal plantrecruitmentFig. 2. Depiction <strong>of</strong> <strong>the</strong> hypo<strong>the</strong>sized processes controlling alternative vs.restored community structure for ei<strong>the</strong>r A) cobble beach and mudflathabitats (original communities lack native vascular plants) or B) high andlow salinity marshes (original communities dominated by native vascularplants). Modified from Hacker and Dethier (2009).could lead a community toward or away from a restoredstate.In our model (Hacker and Dethier 2009 based onPetraitis and Latham 1999) <strong>the</strong>re are two community states:1) a restored state defined as <strong>the</strong> replacement <strong>of</strong> <strong>the</strong> lostcommunity assemblage, and its function, after <strong>the</strong> invader isremoved, and 2) an alternative state defined as one in whicha new species assemblage colonizes and persists; it couldalso include reinvasion by <strong>the</strong> original invading species.There are transitional processes that include disturbance andstress, recruitment <strong>of</strong> species, and biological interactions. Inaddition, <strong>the</strong>re are positive feedback processes in which <strong>the</strong>existing species assemblage acts to reinforce and maintainits current structure and function.Applying <strong>the</strong>se ideas to post-removal communitystructure, we can hypo<strong>the</strong>size what may happen to <strong>the</strong> fourhabitats when cordgrass is removed (Hacker and Dethier2009). We predict that cobble beaches will assume arestored state due to <strong>the</strong> interaction <strong>of</strong> both transition andmaintenance processes as outlined in Fig. 2. If we assumethat vascular plant recruitment occurs in cobble beaches, butplant density is low due to high water movement, <strong>the</strong>n analternative state can only be produced if vascular plants can- 214 -
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FORWARD & ACKNOWLEDGEMENTSThe <stro
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TABLE OF CONTENTSForward & Acknowle
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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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