Niger Delta Ecosystems: the ERA Handbook/The Natural Freshwater Alluvial Equatorial Monsoon (FAM) Ecozone
6 THE NATURAL FRESHWATER ALLUVIAL EQUATORIAL MONSOON ECOZONE
- The Complexity of the natural FAM Ecosystems
- Palms and Other Vegetation
- Sub-Ecozones of the FAM Ecozone
- Animals Communities of the FAM
The FAM Ecozone is often referred to as tropical rainforest, fresh-water swamp or simply swamp, but these descriptions can be misleading. Palm forest, rather than the more classic LEM rainforest, covers much of the area, and at times much of the land may be dry or only temporarily waterlogged.
However, as we have discussed, water is always a limiting factor to plant growth and the freshwater regime is the distinguishing factor of this ecozone. It shares the equatorial monsoon climate of the whole Niger Delta, and together with the alluvial nature of its soils and topography this gives it the ERA name Fresh-water Alluvial Equatorial Monsoon (FAM) or 'the freshwater ecozone'.
Within the natural fresh-water ecozone there are a number of well-defined sub-ecozones including:
#lakes and seasonal lakes
#seasonally exposed recent river alluvium
There are also many ecotones, the most important being those between the swamps and drier land, and between the mangroves and the fresh-water ecosystems.
6.2 THE COMPLEXITY OF THE NATURAL FAM ECOSYSTEMS
The Fresh-water ecozone is a complex mosaic of dry, seasonally wet and wet ecosystems, and ecotones that reflect the equally complex soil patterns and Catena topography upon which they are based.
The predominant natural vegetation climax of most of the ecozone is tropical rainforest as considered in the previous chapter; however there is a different species mix, as the limited drainage causes waterlogging that cannot be tolerated by some of the plants common to the natural LEM rainforest.
The Fresh-water ecozone is significant in another way: together with the Brackishwater ecozone it is the last terrestrial sink for the drainage of much of Nigeria and a very large chunk of West Africa. Such systems generally reflect the ecosystems that they drain, and concentrate all the problems of those ecosystems. Just as measuring the sediment load of a river gives an indication of erosion upstream, so assessing the chemical compounds carried in a river indicates pollution inputs. The Niger Delta does not exist in isolation from the rest of the river ecosystem; in fact it suffers the most from any upstream misuse of the ecosystem.
Thus, some of the preconceptions about lowland tropical rainforest (see 5.3) must be reconsidered for the case of the freshwater ecozone.
- Firstly, extensive parts of the palm swamp forest are naturally impenetrable, where waterlogging limits the growth of tall, thickly crowned trees so that light is available for densely growing palms. The subsequent thickets of raffia and oil palm can be impossible to navigate.
- Secondly, seasonal swamp soils may be very rich in nutrients, again because of limited drainage (a high water table means lower flows of water through the system and therefore less leaching).
Figure 4. describes the freshwater ecozone and its sub-ecozones in diagrammatic form. Note the following points:
- The two levees are the two sides of a white-water river valley. Over time, the river has moved from right to left and is now eroding the left bank and depositing sand and silt on the right bank.
- The levees remain above water all year; in terms of topography, their soils are eluvial and colluvial (see 4.5.4); these are well-drained but shallow and may be described as 'young shallow oxisols' (see 4.5.5).
- The flood plain is under shallow water during the peak flow of the river. At these times, the soil has Illuvial characteristics. However, during the driest part of the year (December to March) when the flood recedes, the soils experience Eluvial and Colluvial processes as water drains down through them.
- The most recent silts are building up on the left flank of the young deposits; these are only exposed during the driest months when the river is at its lowest.
- The swamp to the far left of the diagram is not uniform, but is made up of permanent and seasonal patches. Permanent swamp is below the water table all year round, while seasonal swamp is actually only submerged at the very height of the wet season but remains waterlogged for the rest of the year. A black-water river may run through the swamp.
- The swamp soils are inceptisol aquepts (see 4.5.5), but there is not a sharp cut-off from the levee oxisols; from the levee towards the swamp, as the water table comes nearer and nearer to the surface, are transitional soils.
- Between the flood plain and the right-hand levee are shown a seasonal flood-river and an earthworm ridge (see 6.4.3). These are just two of the features that may complicate a section of the real FAM ecozone.
6.3 PALMS AND OTHER VEGETATION
#Palms are different from other trees in two respects.
- Firstly they are monocotyledons, plants that emerge from their seed with a single leaf. (Broad-leaved trees are dicotyledons—plants whose seedlings begin with a pair of leaves).
- Secondly, palms have only a single stem which does not bear branches.
Palms are one of the few groups of monocotyledonous plants which grow tall enough to be called trees. Monocotyledons are normally shorter, because they do not develop the bark and cork layers that give dicotyledons the physical strength to support a tall plant body. In evolutionary terms, palms are less advanced and need higher light intensities than the broad-leaved trees; they cannot compete with them and are not found in the taller-canopied climax rainforest.
#Palms grow well in swamps for three reasons
These are all related to their fibrous root systems:
- First, they have adventitious roots at the base of the trunk, parts of which can grow above ground, in some species acting as breathing roots;
- Second, the root system is a thick, raft-like mat of surface roots that support the palm on a sort of raft, keeping the adventitious roots near the surface of the muddy soils; and
- Third, this mat of roots, being near the surface, may retain bubbles of oxygen during inundation of floodwater allowing the palm to avoid 'drowning'.
#The reason for the dominance of palms in the swamps is two-fold.
- First, there is little competition as few other species are well adapted to swamp conditions.
- Second, palm fronds can grow almost vertically allowing individual plants to crowd together without unduly obstructing each other's light, while restricting the development of other plants.
#The palms of the FAM ecozone
The main palm species of the FAM are the oil and raffia palms. For a start, the ecozone is the endemic home of the Oil Palm (Elaeis guineensis), which prefers the drier and lighter ground of swamp margins.
The Raffia Palms (mainly Raphia gigantia, R. hookei and R. vinifera) can tolerate longer periods of water-logged, reduced conditions and even short periods of salinity, if it is low; some Raffia species have breathing peg-roots (Pneumataphores) which makes them even more able to withstand reduced soil conditions.
Rattan palms are found where there are tall broad-leaved trees for them to climb with their sharp hooks. Calamus deerratus is most common to the Delta FAM, with Eremospatha laurentii and E. wendlandiana also being found. (Morakinyo)
Oil and Rattan palms do not grow on the freshwater/brackish water ecotone, but Raffia palms are characteristic. However the stilt-rooted Pandanus palm, Pandanus candalabrum, tends to predominate on the very brackish edge of this ecotone. (Although because of their relative shortness they cannot normally compete with the Raffia palms for light, they are more tolerant of salinity.)
6.4 SUB-ECOZONES OF THE FAM ECOZONE
There are seven sub-ecozones in the Freshwater ecozone.
6.4.1 THE LEVEE TROPICAL RAINFOREST SUB-ECOZONE
#This is similar to the LEM rainforest described but with three characteristic variations:
- it tends to be long and narrow in shape, so that there is less 'deep forest' and a greater proportion is forest margins;
- on these forest margins, palm trees are more apparent than in the LEM because of their predominance in the adjacent sub-ecozones; and
- species requiring soils deeper than 1 metre, such as the Silk Cotton tree, are less common.
#Eleven trees typical of the levee tropical rainforest after okafor
|Irvingia grandifolia||('Bitter Bush Mango')|
|Irvingia smithii||('Sweet Bush Mango')|
6.4.2 THE SWAMP SUB-ECOZONE
Fresh-water swamp ecosystems may be permanent or seasonal, but both may be subject to tidal influences. Very little is known about either because access is so difficult.
The soils form illuvial Catenas that are entirely hydromorphic; that is, water is the prime factor in their formation. They are basically all Inceptisol Aquept Gley soils (see 4.5.5), but vary between seasonal and permanent swamp areas.
Obviously, reduction is greater in the permanent swamps, although seasonal rising and falling of water levels still allows some oxidation. The soils are likely to be peaty, organic matter building up in these reduced conditions.
The seasonal swamp soils have a higher clay content than the levee soils, with a thin peaty surface horizon; they are also relatively reduced.
Reduced soil conditions exclude many lowland tropical rainforest plants. However most palms, although not dependent on waterlogged conditions, are more tolerant of them than are the classic rainforest tree species. Palms cannot compete successfully for light in the LEM ecozones, but together with the bamboo they predominate in the swamps.
However, the trade-off is one sided, because the number of plant species tolerating reduced conditions are limited. Swamp plant communities have low biodiversity.
Biomass in the swamps is also limited, partly because tree height is limited. Palms never grow to a great height, and other trees cannot develop sufficiently deep taproots in the waterlogged soil to support massive growth. On the other hand, bioactivity is high; above the actual water table, water is never a limiting factor.
6.4.3 THE FLOODPLAIN TROPICAL RAINFOREST SUB-ECOZONE
#The natural freshwater flood-plain tropical rainforest has the following Characteristics:
- An uneven canopy compared to the levee forest, as a result of the shorter life-span of the emergent species
- Trees with root systems that can tolerate high water-tables
- Soils that do not provide a very solid medium for growth
- Woody and herbaceous climbers
- The absence of ground vegetation except for mosses.
#Roots of the high water-table trees
Of special interest in the floodplain sub-ecozone is the variety of forms of tree roots. The high water table restricts their underground development, but the trees must still be anchored, and must still obtain oxygen. Various strategies have developed and include four main types:
- a 'floating mat' of palm tree roots
- buttresses, often very high, extending up to 5m
- stilt roots which may be as high as 3m high and act like divided stems
- Pneumatophore roots, which take in Oxygen above the surface and are of four types; lateral and serial knee-roots, root-knees, and peg-roots.
The ground of the flood plains is criss-crossed with small flood rivers, and rivulets that work their way around and sometimes underneath the tree roots.
#Some commons trees of the flood-plain ecozone
Some trees of the FAM that show the root adaptations described above include:
- Alstonia boonei, which has lateral primary roots above ground, carrying secondary breather roots;
- Anthocleista nobilis, a pioneer species with peg roots;
- Ceiba pendantra, the silk cotton-tree, which has high buttresses;
- Mitragyna ciliat ( or M. stipulosa), which develops knee-roots like branches from the horizontal 'mother' root below the surface;
- Piptadena africana, with flat wavy buttresses;
- Symphonia gabonensis, an example of knee-roots;
- Uapaca guineensis, which puts out aerial roots;
- Nauclea pobeguinii;
- Nauclea diderrichii;
- Chlorophora excelsa, called Iroko and highly prized as joinery and building timber;
- Cleistopholis patens, used for making canoes;
- Garcinia spp. used for chewing sticks;
- Lophira procera, Ironwood;
- Terminalia superba (Afara); and
- Pterocarpus santaniloides.
#Other vegetation of the floodplain rainforest
Because of the uneven or broken canopy, the floodplain forest is home to many climbing plants that use existing trees for support in order to reach direct light. These include the lianas and other epiphytes. In fact the freshwater ecozone has one of the most diverse ranges of orchid species in the world.
An interesting phenomenon of the Freshwater ecozone, especially evident in the Floodplain sub-ecozone, is the Earthworm Ridges. These can be nearly a metre high and appear to made by the usual activity of Earthworms that work up the soil in order to escape annual flooding (much more research is required). Thus they rise above the high flood mark, and are not only refuges for worms during the flood season, but also for other animals (including rodents and ants). They carry plants that are sometimes more typical of the Lowland tropical rainforest. Like the termite mounds of the savannah ecozones they carry a wide variety of life forms and make fascinating ecosystems in their own right.
6.4.4 THE LAKES SUB-ECOZONE
Permanent and seasonal lakes occur throughout the natural Fresh-water ecozones, and are sometimes known as aquatic grasslands and herbaceous swamps. They form in any condition where water collects either permanently or temporarily, which is either too deep for swamp forest to develop (such as an ox-bow lake), or too young to have been colonised by Freshwater plants. Fresh-water lagoons come into this category.
#The Vegetation of Fam lakes
Permanent lakes are more likely to contain floating plants which obtain oxygen through the stomata in their leaves, and distribute it to the plant body via air canals permeating the stems and roots. Examples are the water-cabbage (Salvina spp.), the water lilies (such as Nymphaea lotus and Nymphaea micranthus) and the floating grass (Vossia cuspidata).
On the edges of the permanent lakes and in seasonal lakes are sedges (Cyperus spp), ferns (e.g. Nephrolepis biserrata), grasses such as Paspalum (Paspalum) and Bamboo. (These species are also found on river margins.)
#Animal communities of the FAM lakes
The lake ecosystems are important in animal ecology because they provide tranquil and euphotic (light) conditions for some animals to breed. The larvae of certain insects which develop in water, such as mosquitoes and dragonflies, need such conditions, as does the reproduction of frogs and toads.
The insects and amphibia, young and adult, in turn attract birds.
Such a lake may also serve as a drinking hole and be visited by larger mammals; it may be permanent home for a wide range of other vertebrates (see 3.7) including fish, reptiles such as crocodiles, and even mammals such as hippopotamus.
6.4.5 THE SEASONALLY EXPOSED ALLUVIUM SUB-ECOZONE
Seasonally exposed alluvium is made up of the fresh deposits of silt that appear on the outside of curves of rivers when the water level drops in the dry season. (See figure 4.) These areas always appear opposite areas of riverbank erosion, which is often quite severe (as at Anyama), and represent the formation of new land. This rapid and visible erosion and deposition is evidence of the dynamic and young geological nature of the Delta as a whole.
As farmers know, these sub-ecozones are very fertile; they are intensively cultivated.
In the natural ecosystem, however, they tend to carry annual species (species whose entire life cycle can occur within a year) similar to those found in the seasonal swamps, and they are also an important feeding ground for migrant birds.
6.4.6 THE RIVERS SUB-ECOZONE
Although we have included rivers as part of sub-ecozones already considered, they may also be seen as sub-ecozones in their own right. This can be a useful distinction, as they are the main link between the ecosystems of the freshwater and estuary/offshore ecozones, also coming into contact with the brackish-water mangrove and sand-barrier island ecozones on the way.
River ecosystems are best defined as permanent flowing water, that is sufficiently deep to discourage swamp forest development and sufficiently swift to discourage lake type vegetation. By this definition, river boundaries shift according to the seasons and a river may be said to have marginal seasonal ecotones on either side. A river may spill over into a lake during flooding, or take on the characteristics of a lake when water is at its lowest and barely moving. So it is not easy to define simply in terms of vegetation. However animals such as fish, particularly those that need flowing water, define a river more clearly.
Plant and animal life in rivers are largely determined by the nature of the river water.
As already explained (see 4.4), the rivers in the Niger Delta Fresh-water ecozone are either black or white: black-water is clear and arises from the slow release of local water from the forest 'sponge', while white-water carries a sediment load of erosion products (the Nun and Forcardos are the prime examples).
The black rivers are generally slow moving and lighter (more euphotic) than the white rivers.
The white rivers move faster and have more dissolved oxygen but are more cloudy and are thus oligophotic or 'light-poor'. (A classic white-water animal is the 'electric eel', which provides its own light.) White-water rivers often also contain blackwater, which will form a greater proportion of the volume during the dry season when less white-water is being carried down.
6.4.7 THE MANGROVE/FRESHWATER ECOTONE SUB-ECOZONES
FAM ecozones sometimes abut onto mangroves of the BAM, so that there are freshwater/mangrove ecotones. These are considered in the next chapter.
6.5 ANIMAL COMMUNITIES OF THE FAM
As discussed in 3.7, the fact that most animals can and do move means that their distribution is not necessarily restricted to particular sub-ecozones. Many animals of the natural freshwater ecozone are found in all its sub-ecozones and also in neighbouring areas. This is especially true of the insects and birds. The harrier hawk, for instance, nests in the levee forest and feeds on oil-palm fruit in the palm swamp.
The mammals are less mobile, and their movements may be restricted by water. Nonetheless the Chimpanzee, which cannot swim, will nest in the levee forest but feed in the flood plain, and also in the seasonal swamp forest during the dry season. Other Primates, such as some of the Guenons (tree-living monkeys), mammals such as genet and squirrels, and many reptiles such as snakes and lizards, all cross the boundaries between levee, swamp and flood-plain forest regardless of ground conditions.
However it is the animals associated with watery environments which are more exclusive to the freshwater ecozone. These are primarily the fish and amphibians, and reptiles such as crocodiles and fresh-water turtles. However there are certain endemic mammals such as hippopotamus (including the 'Pigmy Hippopotamus') and the Manatee (the Freshwater variety of the marine Dugong).
Bird life is more prolific in the Fresh-water ecozone than in the natural LEM ecozone, because of the food and space provided by the water. The variety of subecozones and ecotones has produced a very high species diversity. An illustration of this diversity is the following list of 78 individual species sighted by the ornithologist Phil Hall during a single 36-hour period in January 1994:
BIRDS OF THE NIGER DELTA FRESHWATER ECOZONE – PHIL HALL