Coastal Marsh Plants as Ecosystem Engineers
Salt and brackish marshes are the result of the interaction between vegetation, sediments, and the tides. Marsh plants act as ecosystem engineers (i.e., organisms that create, modify, or maintain physical habitat such as trees, kelps, or beavers) by increasing the drag of water that moves across the surface of
FIGURE 3.2 A brightly colored forb, Potentilla pacifica, among a background of the rush, Juncus balticus, in a Pacific Coast salt marsh. (Courtesy of Sally D. Hacker.)
the marsh, causing sediments to settle out and be deposited on the marsh surface . The roots of plants may contribute to this process by binding sediments and reducing erosion, but it is the upright stems of marsh plants that control sediment deposition . Plants vary in their ability to accrete sediment depending on their morphology, density, and even stiffness [5,6]. With the deposition of sediment, plant growth is stimulated, creating a positive feedback between growth and deposition. The ability of a marsh to maintain a constant elevation with sea level is sensitive to plant productivity, sediment supply, and rates of sea-level rise . As sea level rises with climate change, there are concerns about whether coastal marshes will be able to keep pace via sediment accretion.
Beyond accreting sediment, coastal marsh plants are masters at modifying the sediment chemistry via two main mechanisms . First, they can passively shade the soil surface and reduce salt accumulation that occurs when seawater evaporates at the marsh surface. Figure 3.3 shows an example of salt accumulation on the surface of a large patch of salt marsh lacking vegetation. Second, some marsh plants, particularly grasses, rushes, and sedges, can use specialized tissue called aerenchyma to oxygenate their roots and rhizomes (underground stems) when exposed to flooding.
Species Interactions in Coastal Marshes
The distribution of marsh organisms across estuarine and elevation gradients is not only a product of salinity and tidal inundation but also of species interactions. Conceptual models [9,10] and numerous empirical studies in coastal marshes [2,11-13] show that species interactions can shift from mostly competitive (negative) under low stress and high productivity conditions (i.e., higher intertidal zones and more brackish marshes) to mostly facilitative (positive) under high stress and low productivity conditions (i.e., lower intertidal zones and more salty marshes). The main mechanisms behind the facilitative interactions were identified as those described earlier: shading and oxygenation of sediment by neighboring vegetation . One well-studied example showed that positive interactions between the rush, Juncus gerardii, and a number of plants and their insect herbivores controlled the species richness of a New England marsh .
FIGURE 3.3 A high-salinity patch formed when vegetation was disturbed. The white on the surface is salt crystals. (Courtesy of Sally D. Hacker.)
Trophic interactions are also important to the distribution of organisms within coastal marshes. Plants are eaten by a variety of herbivores (e.g., snails, insects, rodents, and ungulates), which, in turn, are fed on by carnivores (e.g., crabs, predatory insects, fish, and even turtles). For a long time, it was assumed that trophic interactions played a minor role in structuring marsh plant communities . However, Silliman and colleagues [15,16] showed that so-called top-down effects of predators could dramatically affect the major marsh builder, the grass Spartina alterniflora. Spartina is indirectly killed by snails (Littoraria irrorata), which scrape the grass surface with their mouthparts and cause wounds that become infected by a fungus. Normally, snail populations are controlled by the predatory effects of the blue crab, Callinectes sapidus; however, recent disease and overharvesting of the crabs has released the snails to their destructive ways. Large diebacks of Spartina have occurred in some parts of the southeast United States due in part to the loss of snail predators.