LESSON 4:
DOMINICA'S WETLANDS

What are "wetlands"?

Wetland habitats can be defined as areas inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and under normal conditions do support, a prevalence of vegetation typically adapted for life in saturated soil conditions.

Plants well adapted to living in wetland conditions are called hydrophytes (hydro =water, phyte = plant). These are plants growing in water or on a substrate that, at least periodically, is deficient in oxygen due to excessive water content. Hydrophytes have adaptations that allow them to thrive in wet soils where non-hydrophytes (upland plants) cannot.

What types of wetlands are found in Dominica?

Two important types of wetlands exist in Dominica: swamps (wetlands dominated by trees) and marshes (wetlands dominated by non-woody or herbceous plants).

DOMINICA'S FRESHWATER SWAMP HABITAT:

Water Budget: Swamps in Dominica obtain their water supply from one or more sources: directly from rainfall, surface runoff from the surrounding watershed, streams flowing into the swamp, flood water from a river. Swamps lose water through evaporation, transpiration (loss of water from plants), bottom seepage, and streams flowing out of the swamp.

Some swamps are constantly flooded (e.g. mangrove swamps), while others require seasonal flooding. The period of increasing wetness at the beginning of the rainy season usually coincides with the germination of new seeds. The level of nutrient cycling is related to periods of wet and dry because the decomposition of dead plant material is greater in damp soils (more available oxygen for bacterial decomposition) than under flooded conditions. Maximum growth also occurs during damp or dry periods, but flooding helps bring an influx of nutrients into the swamp.

Description: The freshwater swamp in Dominica is dominated by a single species, Pterocarpus officinalis, or Bois Mang. In most cases the freshwater swamp is two-layered, with a canopy layer (reaching up to 100 ft) and an understory. The soils on which freshwater swamps develop are alluvial (deposited by a stream or running water), and acidic. The grade or slope of the swamps generally range from 0 to 5 degrees, resulting in the lack of drainage and the formation of anaerobic (oxygen depleted) soil conditions.

Pterocarpus officinalis: An evergreen tree reaching up to 100 ft. (30m) in height, and 2-3 ft. in diameter above the buttresses. These buttresses (frequently four large ones to a tree, rising up to 15 ft., and 2-5 ft. at the base) often extend outward 10 ft. along the ground to provide support for the massive trees, which have a shallow root system. The bark of the trunks and buttresses is light brown and smooth, with the inner bark containing a bitter, blood-red sap. Leaves of Pterocarpus officinalis are alternate and generally pinnate, 15 to 48 cm long. The yellow flowers are produced in clusters from the end of January to November. Mature fruit pods are dark brown, flat, up to 5 cm in diameter, and buoyant to enhance transport by water. Fruiting occurs between April and November. The rapid growth rate of seedlings after germination on waterlogged sites, occurs in preparation for the high water levels they will be exposed to during the rainy season.

The distribution of Pterocarpus officinalis in Dominica is limited mainly to the northern third of the island, generally on flats with slopes of less than 5 degrees The highest elevation where the species has been found growing in Dominica is between 150-200 ft. above sea level, on the banks of a tributary to the Indian River.

Frequently, the stem and branches of Pterocarpus officinalis are colonized by a wide variety of epiphytic plants including liverworts, mosses, lichens, ferns and flowering plants. An epiphyte is a plant that uses another plant, typically a tree, for its physical support, but does not draw nourishment from it. The two most noticeable groups of epiphytes which receive physical support from the limbs and trunks of Pterocarpus officinalis are the Anthuriums and the ferns. Some Anthuriums, have leaves larger than a man's back.

Stranglers are plants that are harmful to Pterocarpus officinalis. Seeds from these plants usually germinate on the branches orother suitable parts of the trees and remain as harmless epiphytes until their roots reach the ground. Eventually the pressure placedon the stem of the host tree by the expanding root system of the strangler prevents any further expansion of the stem, and starves the tree's cambium layer of its supply of oxygen, eventually killing the tree.

DOMINICA'S RAINFOREST SWAMP HABITAT:

Occurance: Rainforest swamps form in small areas in Dominica's rainforest zones where the presence of extensive, impervious rock layers, or pans in the soil, prevent the downward movement of water. The shallow, waterlogged soils in these areas create anaerobic conditions best suited for wetland-adapted plants, reducing the plant diversity normally associated with Dominica's rainforests.

Plant Species: The main tree species found in these swamps are Symphonia globulifera (Mangle Blanc) and Amanoa caribaea (Carapite), both well adapted to wet soils. Prop roots, pneumatophores, and adventitious roots that extend above water level are adaptations which help these plants exchange gases between the roots and stem in anaerobic soils. Roots below ground absorb minerals and water required for normal plant functions. Because of their shallow root systems, these tree species are susceptible to being uprooted by strong winds.

DOMINICA'S MARSH HABITAT:

Description: The largest tracts of predominantly herbaceous plants comprising marsh wetlands in Dominica are located on the Cabrits Peninsula and near the Indian River. The plant species that form the larger part of these marsh communities are the fern Acrostichum spp., which grows in large tussocks (raised clumps) reaching 3.7 m in height, and the sedges Cyperus sp. and Eleocharis mutata, known locally as "jon". These plants have spongy tissue called aerenchyma which traps air and aids in the exchange of gases between stems and their roots growing in anaerobic, waterlogged soils.

Wildlife: The wetlands area and the dry shrub forest on the Cabrits Peninsula supports a wide variety of wildlife types. The area is inhabited or visited by species that represent most of the major groups of wildlife on Dominica:

Mammals: Five species of bats (bats are the only native mammal group on Dominica of which there are 13 species) have been recorded in the area. Three of the bats species are fruit-eating, two species are insectivores. Other mammals on the peninsula were introduced (rats, mice, Agouti, oppossum).

Birds: 60 recorded species, including at least 25 breeding species

Reptiles: 6 species of lizards and 3 species of snakes . Two species of sea turtles, the Green Turtle (Chelonia mydas) and the Hawksbill Turtle (Eretmochelis imbricta) are known to nest on the two beaches situated to the southeast (Prince Rupert's Bay) and northeast (Douglas Bay) of the peninsula. A third species, the Leatherback (Dermochelys coriaceae) the largest of all sea turtles, nests on sandy beaches elsewhere on Dominica.

Amphibians: 1 species frog

Crabs: 5 species

What mangrove species are found in Dominica?

Although rare, three mangrove species have been found in Dominica, the Black Mangrove (Avicennia germinans), the White Mangrove (Laguncularia racemosa) and the Button Mangrove (Conocarpus erectus). True mangrove species are halophytes, and are salt tolerant, often living at the mouths of rivers and along coasts where there is tidal flow. Mangroves are able to survive in muddy, tidal conditions by using special breathing roots called pneumatophores, which project up from shallow lateral roots to grow above high water level where they can absorb oxygen.

How do coastal wetland habitats influence coastal marine habitats?

Most wetlands have high rates of biological activity. Plant and algae growth can be extensive (primary productivity), as can the corresponding breakdown of this material by decomposition. The high rates of production and decay in wetlands creates a source of nutrients, that when transported out of the coastal wetlands (by tides or river flow), enhances the productivity of nearby coastal marine waters. Many wetlands, particularly estuaries, may also serve as nursery areas for coastal marine organisms such as shrimp, crabs and fish.

QUESTIONS TO LESSON 4

After reviewing the information in Lesson 4, write a brief, but exact explanation for the following questions. See how your ideas match the answers that will be provided Friday Jan. 21.

Question 1: What are the physical conditions that create a "wetland" and why do these conditions favor the growth of hydrophytes ?

Question 2: What vegetation-types differentiate a swamp wetland from a marsh wetland?

Question 3: Dominica's freshwater swamp habitat is dominated by a single species, Pterocarpus officinalis, also known as Bois Mang or the freshwater mangrove. The tree, which can reach 100 ft., in height, often comprises 90% of the vegetation. Why do you think such dominance occurs?

Question 4: What are some adaptations of the main tree species found in Dominica's rainforest swamp habitat, and how do these adaptations allow these trees to survive in rainforest swamp conditions?

Question 5: After viewing the aerial shot and the topographic map section of the Cabrits Peninsula: Explain what physical factors have placed the largest tract of wetlands in Dominica between the high hills of west Cabrits and the mainland.

Question 6: Why are Dominica's wetlands valuable and important to protect?
All Dominica lessons and photography © 1999 Lance Leonhardt