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 2: Spartina Distribution and SpreadFig.4. Model results with one species with a biomass distribution as above (A) and a rate <strong>of</strong> sea-level rise <strong>of</strong> 0.8 cm/yr. Shown in panel C is <strong>the</strong> predictedbiomass trajectory <strong>of</strong> species 2 as (B) <strong>the</strong> marsh surface equilibrates at an elevation below MSL and outside <strong>of</strong> species 2’s range. Parameter values wereo<strong>the</strong>rwise identical to those in <strong>the</strong> previous examples (Fig. 2 and 3).The biomass <strong>of</strong> species 2 varied with depth at <strong>the</strong> frequency<strong>of</strong> <strong>the</strong> lunar nodal cycle, but 180º out-<strong>of</strong>-phase, whereas<strong>the</strong> lunar nodal cycle and biomass <strong>of</strong> species 1 were in phase(Fig. 3B). The biomass <strong>of</strong> species 2 decreased with a rise inMHHW, i.e. as equilibrium depth increased, because <strong>the</strong>marsh surface elevation, 28 cm, was suboptimal for growth.Conversely, <strong>the</strong> biomass <strong>of</strong> species 1 increased with a rise inMHHW because surface elevation was super-optimal for itsgrowth. Thus, species can coexist <strong>the</strong>oretically when <strong>the</strong>reis a periodic change in flood duration and when one speciesresponds to <strong>the</strong> change positively and <strong>the</strong> o<strong>the</strong>r negatively.This is one <strong>of</strong> several conditions that promote a facultativeinteraction between species.Facultative interactions among marsh macrophytesinvolving amelioration <strong>of</strong> soil salinity have been described(Bertness & Ewanchuk 2002), and simulation results shownhere suggest that biogeomorphological interactions couldalso be facultative. When two species with overlapping distributions(Fig. 2A) were present, and <strong>the</strong> rate <strong>of</strong> sea-levelrise was raised to 0.8 cm/yr, <strong>the</strong> resulting interaction couldbe described as facultative (Fig. 3), because nei<strong>the</strong>r speciespersists in <strong>the</strong> absence <strong>of</strong> <strong>the</strong> o<strong>the</strong>r (Fig. 4). When species 1was removed from <strong>the</strong> simulation, <strong>the</strong> equilibrium elevationquickly dropped below MSL and below <strong>the</strong> lower limit forspecies 2. Species 1 suffered <strong>the</strong> same fate when species 2was removed. However, at a low rate <strong>of</strong> sea-level rise, 0.2cm/yr, species 1 did not survive (Fig. 2). Thus, <strong>the</strong> outcome<strong>of</strong> competition and <strong>the</strong> emergence <strong>of</strong> facultative behaviordepend on <strong>the</strong> rate <strong>of</strong> sea-level rise.The Rate <strong>of</strong> Sea-Level Rise and Alternative Stable StatesIt can also be demonstrated that <strong>the</strong> marsh will movetoward alternative stable states or habitat preemption by onespecies or ano<strong>the</strong>r, depending on <strong>the</strong> rate <strong>of</strong> sea-level riseand <strong>the</strong> species’ biomass distributions. When <strong>the</strong> rate <strong>of</strong> sealevelrise was 0.8 cm/yr and <strong>the</strong> distribution <strong>of</strong> species 1 wasmodified by raising its maximum biomass to equal that <strong>of</strong>species 2 (Fig. 5A), only species 1 persisted throughout <strong>the</strong>simulation, while species 2 was intermittent (Fig. 5C). Themarsh surface equilibrated at an elevation <strong>of</strong> about 23 cm(Fig. 5B), which was at <strong>the</strong> lower limit <strong>of</strong> species 2 and above<strong>the</strong> optimum elevation for species 1 (Fig. 5A). Keeping <strong>the</strong>species distributions as in Fig. 5A and lowering <strong>the</strong> rate <strong>of</strong>sea-level rise to 0.2 cm/yr resulted in a different outcome. Atthis lower rate <strong>of</strong> sea-level rise, only species 2 persisted (Fig.6C). The marsh surface equilibrated at an elevation <strong>of</strong> about46 cm (Fig. 6A), which is above <strong>the</strong> upper limit <strong>of</strong> species 1and above <strong>the</strong> optimum <strong>of</strong> species 2 (Fig. 5A).CONCLUSIONSThis paper describes a <strong>the</strong>ory <strong>of</strong> biogeomorphology thataddresses how intertidal macrophytes can modify landscapeelevation and affect <strong>the</strong> outcomes <strong>of</strong> species interactionsby means <strong>of</strong> vertical geomorphological displacement.Competition by geomorphogical displacement is indirectand is a function <strong>of</strong> <strong>the</strong> fundamental and realized niches thatdescribe species biomass distributions in <strong>the</strong> tidal frame. Theoutcomes <strong>of</strong> competitive interactions are a great deal morecomplex than described here when <strong>the</strong>re are direct interferencesor facultative interactions <strong>of</strong> <strong>the</strong> sort described byBertness and Ewanchuk (2002). These types <strong>of</strong> interactionscan modify a species’ realized distributions dynamically andwould result in truly complex behavior.Marsh primary production and standing biomass aresensitive to hydroperiod and have positive effects on sedimentaccretion. In that part <strong>of</strong> <strong>the</strong> tidal frame that is higher- 113 -