Ecology of Wetland Ecosystems: Water, Substrate, and Life

Often conjuring images of dank, smelly, mosquito-infested wastelands, upon closer look, wetlands are actually biologically diverse and productive ecosystems. Home to a variety of plant life, including floating pond lilies, cattails, cypress, tamarack, and blue spruce, wetlands support diverse communities of invertebrates, which in turn support a wide variety of birds and other vertebrates. Primary consumers from crustaceans, mollusks, and aquatic insect larvae to muskrats, geese, and deer rely on the abundance of algae, plants, and detritus for food. Wetlands also support a variety of carnivores, including dragonflies, otters, alligators, and osprey. Thus, wetlands of the world maintain biologically diverse communities of ecological and economic value.

What may seem like a relatively straightforward task, developing a precise definition for wetlands presented some difficulty and resulted in many different definitions (Table 1). Part of the difficulty arises from the diversity of wetland types that exist around the world, from salt or brackish water coastal marshes and mangroves to inland freshwater swamps, peatlands, riparian wetlands, and marshes. Furthermore, as transitional areas, wetlands can possess characteristics of both terrestrial and aquatic ecosystems while also possessing characteristics unique unto themselves. Despite the diversity of wetland types, all wetlands share some common features. To be considered a wetland, an area must have:

1. hydrology that results in wet or flooded soils
2. soils that are dominated by anaerobic processes, and
3. biota, particularly rooted vascular plants, that are adapted to life in flooded, anaerobic environments.

Many ecologically and economically important species call wetlands home for at least part of their lives. For instance, commercially important fishes and shellfish, including shrimp, blue crab, oysters, salmon, trout, and seatrout rely on, or are associated with, wetlands. Wetlands are also critical habitat for migratory birds and waterfowl, including ducks, egrets, and geese. In fact, more than one-third of the species listed as threatened or endangered in the United States live solely in wetlands and nearly half use wetlands at some point in their lives (USEPA 1995). As such, many wetlands are often recognized as important conservation or restoration targets.

While covering only 6% of the Earth's surface, wetlands provide a disproportionately high number of ecosystem services, in addition to maintaining biodiversity. For instance, wetlands also mitigate floods, protect coastal areas from storms, improve water quality, recharge groundwater aquifers, serve as sinks, sources, or transformers of materials, and produce food and goods for human use. When evaluating the economic value of these various functions, Costanza et al. (1997) concluded that the economic value provided by wetland ecosystems exceeded that provided by lakes, streams, forests, and grasslands and was second only to that provided by coastal estuaries.

Increasing recognition of the value and importance of wetland ecosystems over the last century led to the creation of laws, regulations, and plans to restore and protect wetlands around the world. In the US, wetlands protection largely falls under the Clean Water Act of 1972, which requires permits for dredging and filling activities in most US wetlands and monitors water quality standards. Initiatives such as the "no-net-loss policy," which was recommended by the National Wetlands Policy Forum in 1988, aim to limit further wetland loss in the US, requiring wetland creation, restoration, or mitigation to offset wetland losses due to human activity. With mitigation, wetlands are created, restored, or enhanced to offset or replace wetland loss due to development. The Ramsar Convention, an international treaty aimed at conserving wetlands, requires member countries to develop national wetland policies, to establish wetland reserves, and to designate one or more wetlands as an area of international importance. All these efforts are designed to protect or conserve wetlands and the ecosystem services they provide.

Because of the predominance of water and anaerobic conditions in wetlands, the organisms living there, especially rooted plants, often exhibit remarkable adaptations to deal with the stresses imposed by flooding. These adaptations, including pressurized gas flow (Figure 3), creation of oxidized root zones, and anaerobic respiration, allow wetland plants to remain productive under otherwise stressful conditions, making wetlands among the most productive ecosystems in the world (Whittaker & Likens 1973). This high primary production, in turn, supports high rates of secondary production, rates that can exceed those of terrestrial ecosystems (Turner 1977).

Figure 3: Pressurized gas flow in the floating-leaved water lily, Nuphar lutea

Pressurized gas flow is one mechanism for overcoming oxygen root deficiency in plants growing in anaerobic wetland soils. Gaseous transport results in the flow of oxygen from the atmosphere to the roots, and carbon dioxide and methane from the roots to the atmosphere.

© 2011 Nature Education Reproduced from Dacey 1981. All rights reserved.

Wetlands exist along soil-moisture gradients, with wetter soils at lower elevations and drier soils at higher elevations. Wetland plant communities develop in response to this environmental gradient based primarily on their individual abilities to tolerate flooding and anaerobic soils but also in response to biotic interactions with other species. Establishment of plant species along an environmental gradient can contribute to sharp plant zonation patterns, as can be seen in coastal wetlands where species separate out along an elevation gradient in response to differences in flooding and salinity (Figure 4).

Figure 4: Typical plant zonation pattern in coastal marshes of the eastern Gulf of Mexico, Spartina alterniflora to Juncus roemerianus

Salt marsh plant communities shift in dominance from the first to the second along an elevation gradient before transitioning into maritime pine uplands in Grand Bay National Estuarine Reserve, Mississippi, USA.

© 2011 Nature Education Courtesy of Julia Cherry. All rights reserved.

The development of these productive and often diverse plant communities fuels complex food webs that not only sustain microbial communities through large inputs of detritus to wetland soils but also support diverse communities of animals that utilize wetlands for part or all of their lives (Figure 5). Detritivores, such as shredding insects and crayfish, can utilize dead plant material as their primary energy source, while others (e.g., marsh periwinkle snails) help process organic matter for subsequent use by other organisms. Herbivory of algae by invertebrates and small fish and of plant biomass by some invertebrates, birds, and mammals (e.g., grasshoppers, geese and muskrats) is a significant energy source for primary consumers in many wetlands. Secondary production by these primary consumers supports higher trophic levels, including predatory insects, fishes, reptiles, amphibians, birds, and mammals.

Figure 5: Example of a food web in a coastal salt marsh

This food web of a marsh in Georgia, USA, lists the important primary producers, herbivores, and carnivores in order of importance.

© 2011 Nature Education Reproduced from Teal 1962. All rights reserved.

As awareness of wetland ecosystem services and values has increased, wetland ecological research also has increased. Wetland ecologists examine interactions between species and their environment, recognizing the important role that hydrology plays in shaping the physicochemical environment and biological communities in wetlands. Within that framework, ecologists can examine a near-endless array of ecological topics, from the physiology of species coping with flooding stress and anoxia to species interactions, to the impacts of and feedbacks to global climate change. The diversity of wetland types, the biodiversity they support, and the numerous functions they provide make wetlands an exciting and rewarding arena in which to explore fundamental ecological questions. And it is this science that informs efforts to manage, restore, and conserve the wetlands of the world.