Salt marsh die-off is a term that has been used in the US and UK to describe the death of salt marsh cordgrass leading to subsequent degradation of habitat, specifically in the low marsh zones of salt marshes on the coasts of the Western Atlantic. Cordgrass normally anchors sediment in salt marshes; its loss leads to decreased substrate hardness, increased erosion, and collapse of creek banks into the water, ultimately resulting in decreased marsh health and productivity. Die-off can affect several species of cordgrass (genus Spartina), including S. alterniflora, S. densiflora, and S. townsendii. There are several competing hypotheses predicting the causes and mechanisms of salt marsh die-off throughout the western Atlantic. These hypotheses place different emphasis on the effects of top-down or bottom-up processes for salt marsh die-off. Combined with salt marsh dieback of the high marsh, salt marsh die-off is a serious threat to the ecosystem services that marshes provide to local coastal communities.
History of top-down vs. bottom-up In light of their effect on community processes, behaviors, and ecological interactions, consumptive interactions are some of the most widely studied concepts in ecology. Because of this, scientists use food webs to depict all of the food chains and trophic relationships in an ecological community.
Food webs can be controlled by bottom-up or top-down forces, which dictate whether a food web's structure and population dynamics are regulated by nutrients (a supply of fixed carbon) and primary production or by top predators, respectively. Much energy is lost from one trophic level to the next (about 90%); therefore, the success of higher levels is linked to lower ones and their supply of resources (Lindeman 1942). However, the abundance and distribution of organisms in an ecosystem is also affected by densities of consumers, which limit the success of organisms at lower trophic levels and thereby influence the abundance of these organisms (Hairston et al. 1960). Many ecologists argue that bottom-up and top-down control do not play equally critical roles in the structure and dynamics of populations in an ecosystem; however, data suggests that both bottom-up and top-down forces impact the structure of food webs and the spatial and temporal abundance and distribution of organisms (Bertness 2007), although to what extent each plays a role is not fully understood. Historically, an emphasis on bottom-up control in many ecosystems has prevailed in ecological thought, often to the exclusion of consumer control (Strong 1992). Many ecosystems in which consumer control has classically been considered trivial are dominated by plants (e.g., forests, grasslands, and salt marshes) and are usually green in appearance. Hairston and colleagues proposed an opposing view in 1960 that emphasized consumer control. They argued that the “world is green” because higher trophic levels regulate herbivore abundance (Hairston et al. 1960). Critics pointed out that the world is not always green, and that when it is, herbivores do not necessarily play an important role in structuring plant communities (Ehrlich and Birch 1967). Others argued that what is green is not always edible or of sufficiently high quality to allow increases in herbivore populations (Dixon 1966, Murdoch 1966). The debate is ongoing, but the dominant view of ecologists remains that although consumers affect many aspects of plant productivity and ecology, top-down control does not drive the productivity of entire plant ecosystems. More recently, however, examples of conspicuous consumer control of entire ecosystems have emerged in a variety of habitats including lakes (Carpenter et al. 1985), rivers (Power 1992), and marine (Estes and Duggins 1995) habitats. Foundation plant species can be replaced with other species or substratum completely lacking vegetation and insects can defoliate whole mangroves (Feller 2002). A classic example of top-down interactions dictating community structure and function comes from Bob Paine's work in Washington, which established that removal of the starfish Pisaster triggered a trophic cascade in which the blue mussel (Mytilus) populations exploded due to release from predation pressure (Paine 1966) Another influential example of top-down control emerged from Jane Lubchenco's experiments on New England rocky shores, which demonstrated that the herbivorous snail L. littorea exerts control on the diversity and succession of tide pool algal communities (Lubchenco and Menge 1978). One hypothesis that arose from Lubchenco's work (Little and Kitching 1996) was that predation by the green crab (Carcinus maenas) influences rocky shore algal communities by regulating L. littorea abundances. Ecologists cite these examples as evidence that consumer regulation is more potent and predominant than previously recognized.
Historical paradigms of salt marsh theory
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