Wetlands and Marshland Habitat








Wetlands are among the most biologically productive ecosystems. They have unique characteristics, and are generally distinguished from other water bodies or landforms based on their water level and on the types of plants that thrive within them. Specifically, wetlands are characterized as having a water table that stands at or near the land surface for a long enough season each year to support aquatic plants.

Wetlands are the link between the land and the water. They are transition zones where the flow of water, the cycling of nutrients, and the energy of the sun meet to produce a unique ecosystem characterized by hydrology, soils, and vegetation making these areas very important features of a watershed. Plant communities are essential components of healthy wetlands. Their presence prevents flooding of adjacent uplands, and hence they provide a valuable flood control function, they promote settling of particulates from the water, and from allochthonous sources thus effectively contribute to filtering and cleaning of water pollution originating from agricultural runoff. They also protect extensive areas from wind and wave damage (Bird Studies Canada). Wetlands have been identified as carbon sinks, since high primary productivity fixes carbon through photosynthesis. High rates of organic matter uptake and low rates of decomposition are characteristics that result in net storage of carbon in the sediments. However, wetland functions, even among wetlands of similar vegetation vary according to hydrologic characteristics and geomorphic setting. They are highly variable environments which may exhibit dynamic short- and long-term hydroperiods that may in turn alter plant and invertebrate communities (Laubhan et al. 2005).

Wetlands can be divided into four general categories: marshes, swamps, bogs, and fens. 

MARSHES are periodically saturated, flooded, or ponded with water and characterized by herbaceous (soft-stemmed) vegetation adapted to wet soil conditions. A marsh's habitat structure is largely based on the composition and distribution of plant species. Water depth is the key factor that determines the types and extent of vegetation communities in wetlands. These vegetation communities occur in transitional zones along depth gradients within wetlands (Bird Studies Canada).

Marshes are further characterized as tidal marshes and non tidal marshes.

Tidal (coastal) marshes occur along coastlines and are influenced by tides and often by freshwater from runoff, rivers, or ground water. Salt marshes are the most prevalent types of tidal marshes and are characterized by salt tolerant plants such as smooth cordgrass, saltgrass, and glasswort. Salt marshes have one of the highest rates of primary productivity associated with wetland ecosystems because of the inflow of nutrients and organics from surface and/or tidal water. Tidal freshwater marshes are located upstream of estuaries. Tides influence water levels but the water is fresh. The lack of salt stress allows a greater diversity of plants to thrive. Cattail, wild rice, pickerelweed, and arrowhead are common and help support a large and diverse range of bird and fish species, among other wildlife. They are characterized by periodic or permanent shallow water, little or no peat deposition, and mineral soils. They typically derive most of their water from surface waters, including floodwater and runoff, but also do receive ground water inputs (US EPA).

A transition zone from permanently flooded to upland marsh habitat.
LLWLT-lower low water, large tide  HHWMT-higher high water, mean tide
LLWMT-lower low water, mean tide  HHWLT-higher high water, large tide
(source: Quebec Breeding Bird Atlas)
Non tidal (inland) marshes are dominated by herbaceous plants and frequently occur in poorly drained depressions, flood plains, and shallow water areas along the edges of lakes and rivers. Major regions of the United States and Canada that support inland marshes include the Great Lakes coastal marshes, St. Lawrence river watershed, northern Canadian taiga, and the prairie pothole regions, and the Florida Everglades.

In addition, inland marshes may contain many of the following six habitat zones:

Habitat Zone Definition Species Examples
Tree/Shrub (swamp) Dominated by water-tolerant woody plants Willow, dogwood, alder, buttonbush, maple, ash, cottonwood
Wet Meadow (marsh) Transitional between woody and tall emergents. Subject to shallow flooding, often moist to dry Grasses, sedges, forbs
Tall Emergents (marsh) Usually flooded during entire growing season Cattail, reed grass, burreed, bulrush
Sparse Broad-leaved Emergents 
(aquatic community)
Usually shallow, continuously flooded vegetated areas. Roots of plants submerged, but leaves and flowers above water's surface Arrowhead, smartweed, pickerelweed
Floating Aquatic 
(aquatic community)
Zones where plants are either deep-rooted with leaves floating on surface or with free floating non rooted plants. Interspersed through sparse emergents to open water zones Water lily, pondweed
Submerged Aquatic
(aquatic community)
Zones where plants grow entirely underwater. Typically occur in open water patches Wild celery, milfoil, coontail, pondweed
(source: Bird Studies Canada)

A transition zone from permanently flooded to upland marsh habitat.
(source: Quebec Breeding Bird Atlas)
SWAMPS are fed primarily by surface water inputs and are dominated by trees and shrubs. Swamps occur in either freshwater or saltwater flood plains. They are characterized by very wet soils during the growing season and standing water during certain times of the year. Swamps are classified as forested, shrub, or mangrove, but here we discuss only the first two. 

Forested swamps are found in broad flood plains of the northeast, southeast, and south-central United States, St. Lawrence valley, and boreal regions of Canada and receive floodwater from nearby rivers and streams. Common deciduous trees found in these areas include bald cypress, water tupelo, swamp white oak, and red maple in the U.S.; balsam poplar, silver maple and black ash in Canada, with cedar, spruces, and the balsam fir representing the conifers in the Canadian boreal forest regions.

Shrub swamps are similar to forested swamps except that shrubby species like buttonbush, swamp rose, willow, dogwood and alder dominate (US EPA). 

BOGS (ombrotrophic peatland) a wetland fed primarily by precipitation, whose soils are nutrient-poor and acidic. The surface of the bog is slightly raised or level with the surrounding terrain. Bogs are dominated by Sphagnum mosses and ericaceous shrubs, (laurel). Wild flowers such as orchids can be found and certain bogs are characterized by wet depressions or pools (Kirby, J and J. Beaulieu, 2006).

FENS (minerotrophic peatland) a wetland that is fed not only by precipitation but also runoff (both groundwater and surface). As a consequence, a fen is generally richer in nutriments and less acid than a bog. The composition of the vegetation varies according to the humidity of the soil and its mineral content. Fens are often found in basins and at the base of slopes, where water and nutriments accumulate. They are characterized by shrubs, cyperaceaes (such as sedge), graminoids (reed grass, Canary grass) and brown mosses (Kirby, J and J. Beaulieu, 2006).

Flora and Fauna Associated with Wetlands
Wetlands are home to a wide variety of plants and animals. They are important habitat for invertebrates, fish, amphibians, reptiles, birds, and mammals. Migrating and breeding birds use wetlands to rest and feed during their cross continental journeys and as nesting cover and to raise their young when they are at home.

Species Group Marsh Habitat Use Species Example
Birds Breeding, feeding, cover, migratory stopover Sora, Black Tern, Least Bittern
Fish Spawning and nursery, feeding, cover Northern Pike, largemouth bass, bluegill
Amphibians Breeding, nursery, feeding, cover Bullfrog, spring peeper, mudpuppy
Invertebrates Breeding, feeding, cover Crayfish, draghonfly, damselfly, midge
Mammals Breeding, feeding, cover Beaver, muskrat, mink
(source: Bird Studies Canada)
The presence of ALGAE in wetlands can easily be overlooked, yet, algal biomass can equal that of submersed macrophytes (Goldsborough 2001), and constitute an important component of primary production and source of dissolved oxygen. In saline systems, the algal biomass within wetland often exceeds the biomass of macrophytes (Zedler 1980) and is of critical importance for invertebrate fauna, and often a first element in a wetland food chain. Epiphytic algae are an important food for many macro invertebrates (Laubhan et al. 2005).

VASCULAR PLANTS can be divided into four life forms:
Submerged plants are rooted in substrate, but the stems and leaves are mostly, if not entirely in water.
Emergent plants grow with their roots in wet soil part or all of their life cycle, but stems and leaves extend through the water column above water surface.
Floating leaf plants are rooted in the submerged substrate, but extend broad floating leaves to the surface.
Floating plants are not rooted, and usually remain suspended within the water column, or floating on the surface of water.

The soil seed bank of vascular plants rarely reflects the composition of the standing vegetation (Thompson 1992). Of the potential suite of plant species, the abiotic conditions dictate the types and densities of species which germinate and survive to maturity (Van Der Valk 1981). The abiotic factors influencing recruitment are soil temperature, moisture, O2 concentration, photoperiod, quality and quantity of light, and soil and water chemistry (Simpson et al. 1989, Cronk and Fennessy 2001). Submerged plants germinate under flooded conditions, whereas most emergents germinate during draw down (Laubhan et al. 2005).

Wetlands provide many habitat niches for INVERTEBRATES which are important foods for water birds, amphibians, reptiles and fish (Mott et al. 1972). Invertebrate communities can be grouped based on habitat associations into those that occur in:

  1. benthic substrate 
  2. submergent vegetation 
  3. perennial herbaceous vegetation
  4. annual herbaceous vegetation
  5. leaf litter
Differences in invertebrate composition and distribution among wetland types are driven by hydrologic regimes and vegetation structure (Murkin et al. 1992). Short-term water regime, physical, chemical and biological factors affect the occurrence, abundance, growth rate, and reproduction of individual species of invertebrates. Another important factor affecting temporal invertebrate abundance and diversity in wetlands is litter type and availability. After senescence, plant material form litter. Herbaceous litter include stems, leaves and flower structures. Upon deposition at the bottom of wetlands, nutrients and organic matter rapidly leach from litter and concentrate in water column (Yates and Day 1983, Wylie 1985). Fungi, bacteria and invertebrates associated with litter accelerate decomposition and release of nutrients. Litter decomposition rates generally increase when shallowly flooded due to higher temperatures and an increase of gas exchange. Sometimes, the decomposition process may cause anaerobic conditions resulting in subsequent elimination of invertebrate communities in deeper zones. Because the factors controlling decomposition processes change constantly, peaks of invertebrate abundance often are dramatic and short-lived. This cycle of "pulsing" of invertebrate populations is typical in populations which exploit nutrient-rich detrital-based systems. Because they are readily consumed as food, invertebrates form an important link in the transfer of nutrients from decomposing litter to higher invertabrates and vertebrates such as water birds, herptiles and fish (Batema et al. 2005)(Laubhan et al. 2005).

AMPHIBIANS represent a diverse group of organisms which forms an integral component of wetland ecosystem and affects wetland functions and processes. They occupy a variety of positions in the food web and can differentially affect energy flow in wetland systems. During their larval stages, amphibians may regulate primary production through suspension feeding (Dickmann 1968, Seale 1980). As adults, they are both predators, and a high quality food source for predators, and may heavily influence invertebrate populations, thereby indirectly affecting litter decomposition (Burton and Likens 1975)(Laubhan et al. 2005).

Amphibians have complex life cycles (Wilbur 1980). Many species are biphasic, spending most of their adult life in terrestrial habitats using aquatic environments only for mating, oviposition, and larval development. Population sizes naturally undergo wide fluctuations (Semlitsch et al. 1996, Pechmann et al. 2001), and the survival of a population is dependent on the production of large numbers of metamorphs during favourable years, rather than consistently producing some metamorphs every year (Pechmann et al. 1989, Berven 1990). Abiotic and biotic factors, as well as interaction between these factors influence the rate and success of larval growth and development (Semlitsch. 2000a). The length of time larvae spend in aqueous habitat depends on species, and ranges from several weeks up to two years. Similarly, for mating adults it ranges from a couple of days, up to several months (Laubhan et al. 2005).

The length of hydroperiod is considered to be among the most critical factors associated with habitat condition for amphibians as it influences floristic composition and structure, and therefore the likelihood of larvae achieving metamorphosis. Hydroperiod also influences the distribution and abundance of predators (Schneider and Frost 1996, Wellborn et al. 1996). If wetlands dry prior to metamorphosis, larvae die. Likewise, increased water depth and wetland permanence results in higher survival of predators such as fish and salamanders (Skelly 1995, Wellborn et al. 1996). Consequently most amphibians occupy wetlands during only a portion of the entire hydrologic cycle (Skelly 1997). Most pond breeding amphibians reside in terrestrial habitats within 200 m of wetland during non breeding season (Madison 1997). They can not migrate more than 200-300 meters due to physiological constraints such as desiccation (Grover 2000, Semlitsch 2000a). A suitable terrestrial, and aquatic habitat, including connecting corridors between wetlands are therefore necessary to ensure population persistence and to prevent isolated local populations from extinction (Skelly et al. 1999, Semlitsch 2000a)(Laubhan et al. 2005).

From all wildlife dependent on wetlands, comparatively much less attention has been given to REPTILES such as snakes and turtles. Today, reptiles may be experiencing even greater declines than amphibians world wide (Gibbons et al. 2000). Wetland-dependent reptiles have diverse life history strategies and habitat needs. Some species complete their entire life cycle in wetlands, and sometimes in a single wetland, whereas other species require a diversity of wetland or upland environments to complete their annual cycle (Johnson 2000a, Rowe 2003). The protection of hibernacula for both, snakes and turtles is important for the survival of these species. Many species of turtles over winter in mud of wetlands and snakes often use crayfish, or other burrows and stump holes near wetlands to over winter (Ernst et al. 1994, Kingsbury & Coppola 2000). Some snakes over winter singly (Kingsbury & Coppola 2000), whereas, others over winter in dense aggregations that can include multiple species of snakes as well as other vertebrate taxa (Carpenter 1953). As with amphibians, water birds, and other wetland-dependent wildlife, effective conservation and management requires an understanding of the life history needs of the organism to provide the necessary habitat resources at the appropriate time and an acceptable spatial scale (Laubhan et al. 2005).

WATERBIRDS are a diverse group that includes waterfowl (swans, geese, and ducks), loons, pelicans, grebes, cranes, rails, shorebirds and wading birds among others (Weller 1999). Beside water birds, many song birds also are dependent upon wetland resources for breeding, wintering, and migration (Weller 1999, Shutler et al 2000). Some species, such as willow flycatcher (Empidimax traillii), are considered facultative wetland birds and spend as much time foraging in uplands as around wetlands (Weller 1999). Others, such as swamp sparrow (Melospiza georgiana) are obligate wetland species which depends upon wetlands for its entire life cycle (Mowbray 1997, Petit 1999)(Laubhan et al. 2005).

The use of wetlands by foraging water birds is largely affected by type, quality, distribution and availability of suitable food and cover. In general, diet varies greatly among species and includes amphibians, reptiles, fish, invertebrates, mammals and plant foods. Waterfowl consume a wide variety of plant and animal foods such as tubers, seeds, stems, leaves as well as invertebrates and vertebrates such as fish. However, proportions vary with species and annual cycle of events. For example, diets of diving ducks contain larger proportion of animal foods than the diets of dabbling ducks (Sedinger 1992), and mergansers eat mostly fish (Krapu and Reinecke 1992). The diet of other water birds is diverse. Grebes and herons consume mostly fish, but also take invertebrates, reptiles and amphibians (Laubhan and Roelle 2001). Shorebirds consume primarily invertebrates (Helmers 1992), but seeds also contribute to their diet (Skagen & Oman 1996). Rails also consume varied amounts of seeds and invertebrates, but seeds generally constitute greater portion of their diet during fall and winter (Rundle and Sayre 1983, Meanley 1992). Finally, cranes, geese and some dabbling ducks often consume foods in terrestrial habitats (Laubhan et al. 2005).

For many species of birds nutritional requirements vary among annual life cycle events (migration, breeding) therefore many species shift food types to meet their changing demands. Factors influencing availability of foods and foraging efficiency include water depth, vegetation structure, and vegetation distribution (Weller 1999, Bancroft et al. 2002).  Many species including rails, shorebirds, herons, and ibises forage by standing on a substrate, thus foraging locations are constrained by water depth. Small water birds such as shorebirds and rails require shallow flooded habitat, whereas herons are capable capturing foods in deep water. Locating and capturing fish and invertebrates is more difficult in higher water depths, however, therefore feeding efficiency of wading birds increases as wetlands get dewatered and fish and macro invertebrates become concentrated (Kushlan 2000). Water depth must be less than the neck length of swans, and dabbling ducks. In contrast, grebes can forage depths to 6 meters because they forage while diving (Storer and Nuechterlein 1992). Subtle differences in foraging habitat use exist among species in the same taxonomic group, but in general, optimal foraging habitat for most shorebirds is characterized by < 25% cover of short vegetation (Holmes & Pitelka 1998). Cranes, herons and ibises also forage in these habitats, but in addition herons and ibises can forage in taller, denser vegetation (Kushlan & Bildstein 1992). Most waterfowl can exploit wide range of habitat. In contrast, bitterns forage in marshes with densely vegetated habitats for concealment (Gibbs et al. 1992), and rails prefer dense emergent cover interspersed with openings (Meanley 1992)(Laubhan et al. 2005).

Breeding requirements are complex. Suitable nest sites must be located, and sufficient foods of adequate quality must be available in order to lay and incubate eggs. Following hatching, suitable foraging habitat must also be present for young to survive. The primary requisites of breeding habitat for water birds include appropriate interspersion of suitable nesting, brood rearing, and foraging areas (Laubhan and Roelle 2001). Most shorebirds nest on the ground, typically on elevated areas near water (Helmers 1992). Dabbling ducks also nest on the ground, however, in vegetation that provides concealment. Grebes, american coot (Fulica americana) and most diving ducks generally nest over water. Grebes in stands of emergent cover surrounded by large expanse of open water. In contrast, nests of diving ducks are restricted to emergent vegetation. Rails and bitterns also nest in flooded areas characterized by tall, dense emergent cover interspersed with sparsely vegetated sites. Least bittern (Ixobrychus exilis) and clapper rail (Rallus longirostris) however, also nest in shrubs (Weller 1961). The nests are placed in dense vegetation, while more open areas are used as foraging sites by young (Davidson 1992, Eddleman and Conway 1998). Herons nest in variety of habitats including trees, shrubs, and occasionally in emergent vegetation on the ground. Nests often occur in single- or mixed-species colonies near, or over water. However, specific locations often are influenced by distribution of of foraging habitats and predators (Davis & Kushlan 1994)(Laubhan et al. 2005).

Collectively, water birds are highly mobile and many species are migratory. Thus, habitat selection must be considered on a macro- and micro-habitat scale. Wetland complexes are generally needed to provide necessary resources within and between seasons (Plissner et al. 2000, Haig et al. 2002). Numerous factors influence avian use of individual wetlands, including size, topographic complexity and dominant vegetation (Laubhan and Roelle 2001). Reductions in wetland density or size can influence avian richness and abundance (Shutler et al. 2000), and reduce nest density and nesting success of many species (Hunter et al. 1993, Grover & Baldassarre 1995). In addition, changes in habitat surrounding wetlands are also important. Juxtaposition of forest habitat to wetlands is important, especially for many amphibians which breed in wetlands and spend the remainder of their life cycle in the forest. Cavities in large trees occurring adjacent to wetlands are important as nest sites for wood ducks (Aix sponsa) and hooded mergansers (Lophodytes cucullatus). They also serve as den sites for number of mammals. Cavities in smaller trees are important to host of smaller birds and mammals (Laubhan et al. 2005).

Issues of scale are complex, but types and interspersion of wetlands are much relevant to wetland conservation and management activities (Laubhan et al. 2005).

Wetland Conservation and Management

Wetlands are one of the most valuable and fragile components of a watershed. Yet, long regarded as wastelands, for many years they were filled and drained for agriculture and development. As a result, wetland loss had a serious impact on biodiversity and many wildlife species. Habitat degradation since the 1970s has been a leading cause of species extinction. Now we are learning that wetlands are crucial to the health of our waters, and they are now recognized as important features in the landscape that provide numerous beneficial services for people and for fish and wildlife. Some of these services, or functions, include protecting and improving water quality, providing fish and wildlife habitat, storing floodwaters, and maintaining surface water flow during dry periods. These beneficial services, considered valuable to societies world wide, are the result of the inherent and unique natural characteristics of wetlands.

Bird populations can be used as a reliable biomarker in assessing the health and status of wetlands, as marsh birds rely on healthy and diverse marsh habitats. Any reduction in marsh habitat diversity, loss of certain habitat components, or loss of natural hydrologic regimes could have marked effects on specific bird populations. For instance, marshes where dense monotypic stands of cattail develop, become unattractive habitat for many marsh bird species. Providing ideal marsh habitat for marsh birds and other wetland inhabitants typically involves establishing a structurally diverse wetland. Some marsh birds need patches of tall, dense emergents, some need wet meadow habitats, while others need certain amount of woody habitat. The number, size and varying water depth level of open water patches, and how they are interspersed among the stands of vegetation is also very important (Bird Studies Canada).

HYDROLOGY is the single most important factor affecting the composition and structure of marshes. It influences chemical and physical properties of wetlands and ultimately affects the biota. Therefore, water level management can be an effective method for controlling and restoring the density and diversity of vegetation in marshes. Vegetation, in turn, creates nesting habitat for birds and protective cover for birds, mammals, and herpetofauna. In addition to water depth, the duration, frequency, timing and extent of water level fluctuations determine the composition of marsh plant communities. Components influencing the hydroperiod include those that affect the amount of water entering and leaving a wetland, and can be expressed by a relationship termed - water budget:

water volume=[precipitation]+[surface inflow]+[ground water inflow]-[evaporation]-[transpiration]-[surface water outflow]-[ground water outflow]
In addition, in coastal wetlands, tidal fluctuations contribute to the hydroperiod, whereas changes in surface water dynamics are controlling influence in riparian wetlands. In some wetlands, the influence of ground water dynamics can not be overlooked in terms of its effect on the geochemistry, nutrient cycling and soil condition. These abiotic conditions in turn influence biotic components of wetlands including composition, distribution and productivity of wetland vegetation (Bedinger 1979, Squires and Van Der Valk 1992) and invertebrate community composition and structure (Eakin et al. 1976, Kadlec 1982, Comstock & Ehleringer 1992). Ultimately the vertebrate use of wetlands is indirectly and directly affected by the hydrologic regime (Laubhan et al. 2005).

Seasonally or annually variable water levels tend to result in greater overall wetland plant diversity than do static water levels. Water level management can be extremely effective means to flood out dense vegetation. Alternatively, water levels can be lowered to temporarily dry out, or draw down a marsh for plant rejuvenation. Though generally beneficial for marsh birds, overly extensive dyking may have negative effects on coastal marsh fish communities. Similar effects can be derived from managing muskrat and/or beaver populations. These animals can be valuable tools for wetland management. Beavers cause periodic flooding and draining of water bodies, which is beneficial to marsh habitats. Muskrats create small open pools of water and loafing areas (Bird Studies Canada).

SEDIMENT is a major pollutant of wetlands, lakes, estuaries and reservoirs. Sediment quality is also an environmental concern because it may act as both, a sink and source for water quality constituents. The magnitude and type of sediments deposited greatly influence hydrology, water quality and hence, vegetation structure and wildlife habitat quality (Hupp 2000, Johnson 2000b). As with many nutrients, sedimentation occurs naturally in many wetlands, however, excessive sedimentation caused by anthropogenic influences such as agriculture, development or channelization of streams can alter hydrologic and geomorphic processes throughout the entire watershed and can result in dramatic habitat alterations in a relatively short time (Laubhan et al. 2005). 

Ideal Marsh

When it to comes to marsh habitats, bigger is better. A 10-hectare marsh, or a complex of 10 small marshes that are closely associated is much more valuable to wildlife than are 10 one-hectare marshes that are widely separated and non associated. The ideal combination is a complex containing a large marsh in conjunction with many small ones. As stipulated above, structural diversity, together with a variety of water levels present is the second most important attribute of a wetland most useful for wildlife (Bird Studies Canada).