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 SpartinaFig. 2 Model results with two species with biomass distributions as above (A) and a rate <strong>of</strong> sea-level rise <strong>of</strong> 0.2 cm/yr. Shown in panel C is <strong>the</strong> predicted biomasstrajectory <strong>of</strong> both species as (B) <strong>the</strong> marsh equilibrates at a higher relative surface elevation <strong>of</strong> about 45 cm above mean sea level (MSL) and a depth belowMHHW <strong>of</strong> about 15 cm.Fig.3. Model results with two species with biomass distributions as in <strong>the</strong> previousexample (Fig. 2A) when <strong>the</strong> rate <strong>of</strong> sea-level rise was increased to 0.8 cm/yr; parameter values were o<strong>the</strong>rwise identical to those in <strong>the</strong> previous example(Fig. 2). Shown in panel B is <strong>the</strong> predicted biomass trajectory <strong>of</strong> both species as(A) <strong>the</strong> marsh surface equilibrates at a relative surface elevation <strong>of</strong> about 25 cmabove mean sea level (MSL) and a depth <strong>of</strong> about 35 cm below MHHW.Phragmites australis into brackish Spartina patens marsh(Windham 1999). Note that competition occurs in this modelindirectly by virtue <strong>of</strong> a change in relative elevation, favoringspecies 2. Species responses are defined exclusively by <strong>the</strong>irhabitat distributions (Eq. 2) and not by direct interference.For example, species 2 displaced species 1 when species 2was given a habitat range wider than species 1, a higheroptimum elevation, and a greater biomass density at itsoptimum elevation. In this example, <strong>the</strong> marsh equilibrated ata higher surface elevation, about 45 cm (Fig. 2B), than in <strong>the</strong>preceding case. The equilibrium surface elevation (Fig. 2B)was significantly higher than <strong>the</strong> optimum elevation forspecies 2 (Fig. 2A), which is a consequence <strong>of</strong> <strong>the</strong> relativelyhigh biomass density <strong>of</strong> species 2. Thus, as <strong>the</strong> amplitude<strong>of</strong> a species’ biomass distribution increases, <strong>the</strong> equilibriumelevation <strong>of</strong> <strong>the</strong> marsh surface will increase.This prediction is important for predicting and understandingchanges in salt marshes following <strong>the</strong> introduction<strong>of</strong> an alien species. An invader with a greater biomass and awider habitat range can raise <strong>the</strong> elevation <strong>of</strong> <strong>the</strong> marsh above<strong>the</strong> range <strong>of</strong> <strong>the</strong> native species, although <strong>the</strong>re are severalqualifications that must be noted. Firstly, <strong>the</strong> model discussedhere is zero- dimensional or plot-scale. In two dimensions, <strong>the</strong>geomorphology <strong>of</strong> <strong>the</strong> marsh landscape will adjust to changesin its relative elevation, and it is possible that <strong>the</strong> resultingtopographic gradients will support a dynamically stable zonation<strong>of</strong> species, i.e. marshes transgress and species migrate andsegregate across topographic gradients. Secondly, <strong>the</strong>re areconditions, such as <strong>the</strong> rate <strong>of</strong> sea-level rise, explored below,that lead to <strong>the</strong> persistence <strong>of</strong> a weaker species.Facultative Behavior and <strong>the</strong> Rate <strong>of</strong> Sea-Level RiseA facultative interaction between species is possible byvirtue <strong>of</strong> <strong>the</strong> additive effects on sediment accretion <strong>of</strong> overlappingspecies distributions. The additive effect on accretionrate can maintain <strong>the</strong> elevation <strong>of</strong> <strong>the</strong> marsh surface withinboth species’ ranges at a high rate <strong>of</strong> sea-level rise. This wasdemonstrated by raising <strong>the</strong> rate <strong>of</strong> sea-level rise from 0.2to 0.8 cm/yr (Fig. 3). The increase in rate <strong>of</strong> sea-level riseresulted in stabilization <strong>of</strong> <strong>the</strong> relative surface elevation atabout 28 cm (Fig. 3A) at an elevation that is greater than <strong>the</strong>optimum that had been specified for species 1 (22.5 cm) andsuboptimal for species 2 (32.5 cm, Fig. 2A). At this elevationboth species were able to coexist (Fig. 3B).- 112 -