Proceedings of the Third International Conference on Invasive ...

Chapter 2: Spartina Distribution and Spread<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaFigure 5. Model results with two species with biomass distributions as above (A) and a rate <strong>of</strong> sea-level rise <strong>of</strong> 0.8 cm/yr (B). Shown in panel C is <strong>the</strong>predicted biomass trajectory <strong>of</strong> both species (species 2 approaches extinction) as <strong>the</strong> marsh surface equilibrates at a relative elevation <strong>of</strong> about 20 cm (B)above mean sea level (MSL).than a species ‘optimum’ elevation, rising sea level increasesprimary production, which stimulates sedimentation andmaintains equilibrium with MSL. Optimum elevation isin quotes because, while that is <strong>the</strong> elevation that favorsmaximum growth, it borders on instability. Several speciesor communities <strong>of</strong> plants may coexist if <strong>the</strong>y partition <strong>the</strong>habitat space, and if a topographic gradient <strong>of</strong> sufficient slopeexists. Alternatively, coexistence is unlikely following aninvasion by a species with greater niche breadth and higherproductivity as in Fig. 2A S2. Locally, sediment accretionFig. 6. Model results with two species with biomass distributions as in <strong>the</strong>previous example (Fig. 5A). Shown in (B) is <strong>the</strong> predicted biomass trajectory<strong>of</strong> both species (s1 became extinct) as <strong>the</strong> marsh surface equilibrates ata relative elevation <strong>of</strong> about 45 cm (A) above mean sea level (MSL). Therate <strong>of</strong> sea-level rise was decreased to 0.2 cm/yr (A); parameter values wereo<strong>the</strong>rwise identical to those in <strong>the</strong> previous example (Fig. 5).may drive species replacements or succession. Because <strong>of</strong> <strong>the</strong>sensitivity <strong>of</strong> species’ productivities to hydroperiod, anomaliesin mean sea level and astronomically forced changesin MHHW can alter <strong>the</strong> dynamics <strong>of</strong> competing species ondecadal or shorter time scales.Species differ in <strong>the</strong>ir tolerance to flooding, hypoxia, desiccation,and salt stress (Pennings & Callaway 1992, Kuhn &Zedler 1997). Consequently, <strong>the</strong> fundamental distributions<strong>of</strong> plant species within <strong>the</strong> intertidal zone are determined byphysical factors that set upper and lower depth limits, andoptimum depths. The realized distributions may or may notresemble closely <strong>the</strong> fundamental distributions, dependingon <strong>the</strong> strength <strong>of</strong> competitive or facilitative interactionsamong species (Ungar 1998, Bertness 1991). For a givenrate <strong>of</strong> sea-level rise, <strong>the</strong> relative shapes <strong>of</strong> <strong>the</strong>se distributionsultimately determine <strong>the</strong> equilibrium elevation, speciesreplacements and persistence. One species may replaceano<strong>the</strong>r by modifying <strong>the</strong> elevation <strong>of</strong> its habitat to <strong>the</strong> detriment<strong>of</strong> competitors. This is characteristic <strong>of</strong> species withrelatively high biomass densities such as <strong>the</strong> Spartina hybridswarms that have become established in San Francisco Bay(Daehler & Strong 1997, Ayres et al. 2004).These insights are important to our fundamental understating<strong>of</strong> <strong>the</strong> ecology <strong>of</strong> marsh ecosystems and should alsobe relevant to <strong>the</strong> management community responsible forforecasting and controlling <strong>the</strong> course <strong>of</strong> species invasions.On a fundamental level, <strong>the</strong> work demonstrates how processesthat operate at different temporal scales can interactto modify ecosystem structure and function. For example,<strong>the</strong> outcome <strong>of</strong> interspecific competition among marsh macrophytes,a biological process that operates on relative shorttime scales, can be affected by <strong>the</strong> rate <strong>of</strong> sea-level rise, aprocess that occurs on very long time scales.- 114 -