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
