Proceedings of the Third International Conference on Invasive ...

<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 ManagementTAKING ADVANTAGE OF SPARTINA’S SPATIAL PATTERN FOR EFFICIENT CONTROLF.S. GREVSTADOlympic Natural Resources Center, University <strong>of</strong> Washington, 2907 Pioneer Road, Long Beach, WA 98631;grevstad@u.washington.eduThe invasion <strong>of</strong> open mudflats by Spartina alterniflora takes on a distinctive spatial pattern. Thispattern <strong>of</strong> spread <strong>of</strong>fers opportunity for strategic placement <strong>of</strong> control efforts. Spartina seedlingsestablish on open mud and <strong>the</strong>n spread vegetatively to form expanding circular patches, which dot<strong>the</strong> mudflats and eventually coalesce into a contiguous monospecific meadow. The invasiontypically begins in <strong>the</strong> upper tide zone and <strong>the</strong>n moves down <strong>the</strong> tidal gradient. Using a spatiallyexplicit model, I simulated <strong>the</strong> spread <strong>of</strong> S. alterniflora and compared various strategies for controlin a situation where only a fraction <strong>of</strong> <strong>the</strong> total infestation could be controlled each year. A strategy<strong>of</strong> killing outlying patches first and <strong>the</strong>n attacking <strong>the</strong> dense meadows (moving up <strong>the</strong> tidal gradient)led to eradication in up to 44% less time and effort than a strategy <strong>of</strong> targeting <strong>the</strong> dense meadowsfirst and outlying patches second (moving down <strong>the</strong> tidal gradient). In <strong>the</strong> control <strong>of</strong> contiguousmeadows located adjacent to <strong>the</strong> shoreline, <strong>the</strong> best strategy was to approach one end <strong>of</strong> <strong>the</strong>infestation, moving across <strong>the</strong> meadow to <strong>the</strong> o<strong>the</strong>r end. Suppression <strong>of</strong> seeds was not an effectivecontrol strategy by itself. In general, effective control strategies were those that first eliminate <strong>the</strong>plant in areas where current or future vegetative growth is greatest. Field application <strong>of</strong> <strong>the</strong>se resultsfor S. alterniflora and similar invasive plants could greatly reduce <strong>the</strong> costs <strong>of</strong> control work andimprove <strong>the</strong> likelihood <strong>of</strong> local or complete eradication.Keywords: Spartina alterniflora, Willapa Bay, spatial pattern, control strategy, control efficiencyINTRODUCTIONWhen resources for control work are limited, <strong>of</strong>ten onlya fraction <strong>of</strong> a weed invasion can be controlled in any givenyear. Under <strong>the</strong>se conditions, <strong>the</strong> spatial pattern <strong>of</strong> a weedinvasion can provide an opportunity for strategic placement<strong>of</strong> control efforts to achieve <strong>the</strong>ir greatest effect.The invasion <strong>of</strong> mudflats by Spartina spp. (cordgrasses)takes on a characteristic pattern. Seedlings establish in openmud and <strong>the</strong>n spread vegetatively to form expanding circularpatches. These initially dot <strong>the</strong> mudflats and can eventuallycoalesce into a contiguous meadow. The invasion typicallybegins in <strong>the</strong> upper intertidal zone and <strong>the</strong>n moves down <strong>the</strong>tidal gradient. A distinct boundary is formed by <strong>the</strong> highernative marsh, which Spartina rarely invades. This study usesa simulation to compare control approaches for this pattern<strong>of</strong> spread. A more detailed version <strong>of</strong> it appears Grevstead2005.METHODSA stochastic grid-based model was developed usingMatlab® s<strong>of</strong>tware to simulate <strong>the</strong> spread <strong>of</strong> Spartinaalterniflora on a mudflat and to compare different controlstrategies. The simulations used a grid dimension <strong>of</strong> 120 by120 cells where each cell was 1 square meter (m 2 ). One axis<strong>of</strong> <strong>the</strong> model space follows a tidal elevation gradient, with<strong>the</strong> upper edge representing <strong>the</strong> native marsh boundary and<strong>the</strong> bottom edge representing <strong>the</strong> lower extent <strong>of</strong> S.alterniflora growth. Cells are considered ei<strong>the</strong>r empty oroccupied by S. alterniflora. They become occupied throughvegetative spread from neighboring cells or by establishment<strong>of</strong> seedlings dispersed from an occupied cell. Parameterestimates were obtained from field data for S. alterniflora inWillapa Bay. Assumptions and parameters <strong>of</strong> <strong>the</strong> modelwere as follows:• Vegetative spread = 0.77 m radial increase per year(measured from aerial photos)• Seeds disperse from occupied sites according to aGaussian distribution ( = 50)• Seedling recruitment declines linearly with tidalelevation (based on Feist 1999)• Seedling recruitment produces a 17% increase in areaper year (based on Murphy 2003)• Once established, clonal patches do not die until treatedOutlying patches first vs. meadows firstThe time and total effort needed to eradicate apopulation was compared for a meadows-first vs. anoutliers-first treatment strategy. In each case a fixed amount<strong>of</strong> Spartina was removed each year starting with a 20-yearoldinvasion (Fig.1). Paired trials were replicated 10 timesfor each <strong>of</strong> three levels <strong>of</strong> yearly effort.Approach direction for meadowsFor a case where <strong>the</strong>re are no outliers but only anoblong meadow adjacent to <strong>the</strong> native marsh boundary, threestrategies were compared: (1) Approaching from <strong>the</strong> lowertide zone (mudflat) and moving toward <strong>the</strong> upper tide zone- 203 -