Ecology and Food Webs in Wetlands

(The introductory material on general ecology and food webs is mostly from:

Caduto, M.J. 1985. Pond and Brook. Hanover, N.H.: University Press of New England. 276p.)


What is Ecology?

Energy and the Food Web

Just like any other sort of system, an ecosystem requires energy to operate. Energy for an ecosystem comes from the sun in the form of photons. When a photon of energy from the sun hits a green plant or an algae, it triggers a complicated chemical reaction in the chlorophyll pigments: photosynthesis. This is the only way on earth that living organic matter can be created out of sunlight and except for some bacteria, all living things depend on this energy. Organisms which can create their own organic material from the sun (or some other source as some bacteria can do) are called autotrophs.

Photosynthetic Equation: H2O + CO2 --> O2 + CHO’s

Since plants/algaes are the only organisms that can make organic energy out of the sun’s light, they are considered the primary producers in an ecosystem.

Let’s say that our incoming photons create 100 kilograms of algae in a pond. Why algae since you can’t even see it unless there is a lot? Most animal life in a pond either eats algae directly or eats smaller organisms that eat algae. Thus algae is the producer and everyone else is a consumer. Consumers which get their energy by eating (in other words they do not make their own energy) are heterotrophs. Consumers can be plant eaters (herbivores), meat eaters (carnivores), scavengers which eat dead things or detritus (detritivores), or they can eat just about anything (omnivores—humans, for example, are typically omnivores).

So you have 100 kilograms of algae to pass on to the algae eaters. About 90% of that available energy will be used up by those algae eaters just by their having to live: growth, respiration, energy lost as heat, energy required for movement, etc. So your 100 kilograms of algae can produce 10 kilograms of algae eaters. In other words, only 10% of the energy produced gets passed on from on trophic level (or level in the food web) to the next. Thus the more trophic levels you have, the less energy is available at the top. This is depicted as the trophic pyramid.

(from: Caduto, 1985)


Food Webs in Specific Wetland Types

1. Salt Marsh

(from: Mitsch and Gosselink, 1993)

Producers in a salt marsh include the marsh grasses, Spartina and Juncus mostly, plus various other salt tolerant plants as well as lots of algae. The consumers come in several categories according to their preferred habitat.

a. Aerial Habitat

The aerial habitat is that above the substrate, not that in the air necessarily. Consumers here include spiders and insects that live on plant leaves, periwinkle snails that travel up and down plants, and some of the marsh crabs. This includes all the grazers plus birds common to saltmarshes that do not eat out of the water (red winged blackbirds, marsh wrens, some sparrows)

b. Benthic Habitat

Probably less than 10% of the above ground primary productivity in a saltmarsh actually gets grazed,. Most plant biomass dies and decays and is passed through the detrital food web where the major consumers are bacteria and fungi. These are then consumed by the smallest animals—worms, copepods, rotifers, larval stages of benthic invertebrates, in other words, plankton. The bigger benthic invertebrates are either scavengers (crabs, snails) or filterers (oysters, mussels).

c. Aquatic Habitat

This overlaps with the benthic habitat, but here we’ll put the "higher" trophic levels here, the vertebrates. There are several species of resident fish such as silversides, killifish, and mummichogs, however most marsh fish are transients, typically starting out their life histories in the marsh and then moving on. In fact about 90% of the commercially important fish and shellfish in the southeast Atlantic and Gulf coasts depend on marshes.

d. Avian Consumers

This includes many ducks and wading birds. Like the fish, many of these depend on marshes along their migration routes and are not year round residents.

e. Mammals

Raccoons and muskrats use salt marshes extensively as a food source


2. Mangroves

Because of the salt, there is less variety of producers here, but very high biomass because of the constant tidal influx of nutrients. Mangroves themselves are not required to live in salt, but they are good at it, so they outcompete anything else.

There is a wide diversity of consumers feed in the mangrove forest, especially filter feeders and detritivores. Here again it is the detrital web that accounts for most of the production (see the figure-the wider arrow indicates the larger input). Barnacles and oysters filter feed and fiddler crabs scavenge, among lots of other invertebrates, especially juveniles. These juveniles include the commercially important spiny lobster, shrimp, mullet, and tarpon. These all provide food for a vast array of wading birds.

(from: Mitsch and Gosselink, 1993)

3. Tidal Freshwater Marshes

a. Producers

Like in a saltmarsh or mangrove, the plants in a tidal freshwater marsh are arranged in zones according to how wet they like to be. Pondweed, waterweed, and pickerel weed dominate the submerged areas. In the higher areas you have a mixed aquatic community of cattails, wild rice and cordgrass, among others. Since most of the varieties you find here would grow in just about any marsh, their distribution depends on the flooding pattern and which plants can germinate best (or at all) underwater. Then there’s competition due to shading or chemical defense. Cattails, for example, release chemicals which inhibit seed germination of many other plants.

Algae has less biomass that these vascular plants, but it has a higher turnover rate and since again most production in this wetland is based on the detrital food web, the algae is very important.

b. Consumers

Here the food web is predominantly detrital with the benthic invertebrates being the important link. The smallest eat bacteria and nematodes (microscopic worms) and probably make up most of the living biomass of the anaerobic sediments. As these eat bacteria, they package it into bitesize pieces for the next consumers up the line which would be amoebas and other plankton which are then eaten by snails, worms, and insect larvae.

i. Nekton

If "benthic" refers to what’s on the bottom, and plankton is everything microscopic and drifting in the water, nekton is everything else in the water, in this case, fish. Typical fish are sunfish, bass, crappies, minnows, shiners, carp, and catfish. You also get some estuarine fish such as killifish, mummichogs and anchovies. There are also anadromous fish (those which live in the ocean but spawn in freshwater) such as shad, herring, and striped bass. Occasionally you find juveniles of saltwater species such as spot, flounder, croaker, and drum. They can physically withstand the freshwater at these early life stages and presumably take advantage of the relative lack of predators and the food availability—or they just get washed in.

ii. Birds

Of the wetland types, the tidal freshwater wetlands have the largest and most diverse population of birds. Waterfowl, wading birds, ducks, gulls, birds of prey, shorebirds, plus birds typical of a forest or shrubby area all find satisfactory habitat here.

iii. Reptiles

Population is dominated by turtles, snakes, and alligators.

iv. Mammals

These include deer, rabbits, muskrats, beavers, otters, and nutria (the latter are not found in coastal Georgia, S.C. or Florida although they are common elsewhere in the southeast)

4. Freshwater Marshes

Reed grasses, cattails, rice, sedges, and ragweed are common to most temperate freshwater marshes, although distribution varies with latitude. These are distributed in zones according to how flooded they get.

(from: Mitsch and Gosselink, 1993)

Remember that it is these wetlands that are the most affected by "alien species," those introduced from elsewhere. The water hyacinth, for example is a problem in the southeast but on the up side, it is very good at water filtering and then holding onto the excess nutrients or chemicals. Thus it's often suggested as a valuable addition to natural wastewater treatment programs. Hydrilla and purple loosestrife are two other trouble-makers in our area. Note that these are not necessarily "bad" but since they lack natural controls they tend to overtake habitats and outcompete the natives. This can mean trouble for the consumers that depended on those natives.

As with the producers, there is high diversity among the consumers, often because these marshes serve as oases in the middle of seas of farmlands.

The decomposers are extremely important here, as is the case with other wetlands, but little is known about them.

The most abundant invertebrates are true flies, including mosquitoes.

With a high diversity of invertebrates there is a high diversity of birds. Note in the figure below how the birds share the habitat (or in ecological terms, partition the resources). Muskrats are probably the most typical mammal although lots of others may pass through.

Fish diversity depends on how big an area we're talking about and how wet it stays.

(from: Mitsch and Gosselink, 1993)

5. Peatlands

The dominant plant in a peatland is, of course, moss, especially Sphagnum moss. It grows in cushionlike spongy mats with very high water content. Sphagnum moss can hold 15-23 times its dry weight in water. The moss only grows actively at the surface, and the lower layers die off and decompose into peat. Other plants may include heathers, cranberry, blueberry, some pines, spruce, and tamarack trees.

Remember that these wetlands have the lowest productivity and the lowest nutrient levels of all the wetlands. For this reason several carnivorous plants are found here. They get their extra nutrients from insects. These include pitcher plants, sundews, and venus flytraps.

Animal density is fairly low in these wetlands because they are acidic and not much eats moss. They do serve as trails and shelters for large mammals.

Lots of birds pass through these areas on their migration routes and there are several that absolutely depend on them: sandhill cranes, short-eared and great gray owls, sharp-tailed sparrow.

6. Southern Deepwater Swamps

Cypress and tupelo trees dominate these ecosystems and there may or may not be an understory, depending on how dense the canopy is. The more light that gets through, the more vegetation beneath. This understory may include red bay, sweet bay, ash, maple, and some pines. There is also always lots of Spanish moss (which is not actually moss but is an epiphyte related to pineapples-it hangs on the trees but does not take anything from them). If the water is constant in these, you also get floating mats of algae.

There is a high diversity and high biomass oƒ invertebrates which depend on the abundant detritus available. Once again, this is the major source of nutrients for the food web.

Reptiles and amphibians are quite diverse because they can adapt to changing flood levels. These habitats are frequently deep enough and stable enough to support alligators. If they are that deep and stable, there is probably also a diverse fish population.

7. Riparian Wetlands

The forest you get here depends on how wet it gets. Typically in the south you find red maples, willow, willow oaks, and sweet gum closest to the water. As you move back from the water, these would give way to other oaks, hickories, and pines.

There is high animal diversity due to the edge effect.

a. predominance of woody plants

b. surface water

c. diversity of habitat

d. corridors for dispersal and migration

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