This problem is less severe on the levees, because the high water table stops nutrients being leached away so rapidly. However it is severe on deep soils such as those found on the Ogoni Plain.
4.6 NATURAL VEGETATION
Soils and vegetation are develop together. For instance, the stilt-rooted or Rhizophora mangroves colonise newly deposited mud; eventually they create a humus-rich soil made up largely of a thick mat of their own decomposing secondary roots.
The 'natural vegetation' of the Niger Delta, the climax vegetation, if unaffected by modern human activity, and is described in later chapters. However, we must be aware of two ecological principles.
4.6.1 SPECIES DIVERSITY IS HIGHER IN THE TROPICS
For any given altitude and rainfall level, there will be a greater diversity of species in the tropics than in the temperate zones. There are four reasons for this:
- Climatic Stability - the tropics have not experienced recent devastating glaciations;
- Faster Evolution - at higher temperatures, generation times are shorter;
- Longer Growing Seasons - plants in the tropics are not required to spend large amounts of energy simply surviving the adverse seasons, but can 'afford' to expend more energy on new ways to compete;
- Greater Variability of Habitats - this naturally encourages the evolution of a greater variety of plant species.
4.6.2 CLIMAX VEGETATION VARIES WITH RAINFALL AND DRAINAGE
For any given conditions of latitude, altitude and maturity, the climax vegetation and its bioactivity both vary with rainfall levels and are constrained by the level of drainage.
The West African coast itself is a good example of the effect of rainfall levels; Southern Ghana is at the same latitude and altitude as Southern Nigeria, yet the climax vegetation is savannah. This is because it lies in the 'Dahomey Gap' where far less rain falls.
Drainage is just as important, however. Total bioactivity generally increases towards the equator, towards sea level, and with increasing rainfall; the highest bioactivities on earth are found in the tropical rainforests. The Niger Delta lies in such a biome, but the restricted drainage means that bioactivity here is somewhat lower than in other West African rainforests. (The area of highest bioactivity on earth is actually just to the West of the Delta, on the Cross River State border with Cameroon.)
4.7 NATURAL ANIMAL COMMUNITIES
In any given location, the bioactivity of the vegetation in turn determines local bioactivity of animals. In the case of the Niger Delta, for example, there are 134 freshwater and brackish water fish species, as compared with 192 for the entire continent of Europe.
The fact that animals can move complicates the assessment of their population levels and of species diversity in a given area. For example, they may feed in one
