Niger Delta Ecosystems: the ERA Handbook/The Natural Lowland Equatorial Monsoon (LEM) Ecozone
5 THE NATURAL LOWLAND EQUATORIAL MONSOON ECOZONE
- Tropical Rainforest: Definition and Classification
- Correcting Some Misconceptions About the Rainforest
- Distribution of the Natural Tropical Rainforest Biome
- Micro-Climate of the LEM Rainforest
- Physical Characteristics of the LEM Rainforest
- Diversity of Tree Forms in the LEM Rainforest
- Biodiversity of the LEM Rainforest
- Plant Species of the LEM Rainforest
- Animal Species of the LEM Rainforest
Rainforest is the natural climax vegetation of the whole Niger Delta, because it lies within a lowland equatorial climate and is subject to monsoonal rains.
However, in many areas this forest has largely disappeared and is unlikely to return within the life-span of humankind. To describe particular areas as lying within a West African Lowland Tropical Rainforest ecozone would therefore be misleading. Instead we use the terms given at the end of the preceding chapter: West African Lowland Equatorial Monsoon ('LEM'), West African Freshwater Alluvial Equatorial Monsoon ('FAM'), West African Brackish-water Alluvial Equatorial Monsoon ('BAM'), and the Sand-barrier Islands.
Although it would be misleading to describe the present-day Human ecozones of the Delta as being Rainforest, chapters 5 to 8 will nonetheless describe the natural ecozones as they would exist without the activities of modern society. This will then help in understanding the current situation as discussed in later chapters.
The LEM natural rainforest ecosystem serves as a good introduction to the natural rainforest of the Niger Delta in general, because the LEM ecozone covers the greater part of the West African Tropical Rainforest biome. The LEM ecozone is different from the BAM and FAM ecozones because the soils are comparatively well drained and relatively uniform; yet the principles of LEM rainforests hold true for other ecozones of the Delta. The particular features of their natural rainforests will be more easily explained in succeeding chapters once we have considered the more general case of the LEM.
5.2 TROPICAL RAINFOREST: DEFINITION AND CLASSIFICATION
Tropical rainforest is the climatic vegetation in the tropics wherever rainfall and low temperatures are not limiting factors. But any definition of tropical rainforest begs the question: 'what is a forest?' As we will show, it is far more than a collection of trees; all forests contain non-tree plant communities such as "tree" ferns and bamboo, and the forest ecosystem is comprised of plants, animals, microbes and soils and their interrelationships.
Rainfall as a 'limiting factor' is also complex. Research in Amazon rainforests has shown that they reach their maximum height, not in the wettest zones, but in zones adjacent to forest savannah (where limited rainfall reduces tree population to a degree that grasses dominate ground cover). Excess rainfall seems to limit the growth of rainforest trees especially where drainage is limited, as seen in the river flood plains and swamps throughout the Niger Delta: rainforest plants, like most others, prefer dry feet.
There are essentially three major ecozones of the tropical rainforest biome: Lowland, Alluvial and Montane. Lowland and Alluvial rainforest would be the general case in the Delta, but there is no land of sufficient height for Montane tropical rainforest.
Tropical rainforests are not easy to study, which is why so much less is known about their ecosystems than about many others. Three reasons for this express the complexity of the rainforest.
Firstly, the sheer dimensions of tropical rainforest are a problem. A meaningful sample area must cover square kilometres; vertically, access must be gained to intermediate levels and to the canopy which may be over 30m high, then root systems must be followed as far as 5m down through a soil that is hard and crammed with vegetable life.
Secondly, the life span of the dominant life forms (mainly trees) makes for observation periods that exceed human life spans.
And thirdly, the high diversity of plants and animals is daunting; a square kilometre of natural Nigerian rainforest will contain thousands of plant species and countless more animal species.
5.3 CORRECTING SOME MISCONCEPTIONS ABOUT THE RAINFOREST
These points will help in understanding the descriptions and ideas that follow.
5.3.1 THE TROPICAL RAINFOREST IS NOT AN IMPENETRABLE JUNGLE
Rarely is the natural tropical rainforest a nightmare 'jungle' of vines and thick undergrowth. More often the dense tree canopy above ensures that light intensity on the forest floor is limited, so that there is little undergrowth and passage is easy. (The isolated and relatively undisturbed forest on the low mountains of the Nigeria-Cameroon border, North of Calabar, is a good example.)
Where light does enter the forest, the high intensity of the tropical sun causes an explosion of growth: in the interior, regeneration of many plant species only occurs with the collapse of a dead mature tree, cutting a slash of light through the canopy to the forest floor. This is a natural ecotone.
The high light intensities on the edges of the forest—naturally alongside rivers, lakes and swamps, and man-made beside roads—do present a tangle of vines, shrubs and young small pioneer trees that may indeed seem impenetrable until the deep forest is reached. However, this is defined as 'disturbed forest'. Vines, shrubs and pioneer trees similarly fill the forest where it has been disturbed by logging or by the artillery of war.
5.3.2 THE TROPICAL RAINFOREST DOES NOT NECESSARILY HAVE AN EVEN CLIMATE
Rainforest on the savannah fringe experience the seasonal variations of temperature and rainfall one would expect. Similarly, areas closer to the equator which are subject to continental influences show no dramatic rainfall or temperature variations throughout the year.
However, large areas of the tropical rainforest do not fall into these simple subecozones. The upland tropical rainforests in the Northern Okwangwo Hills of Nigeria, for instance, experience a well-defined dry season often exceeding three months. Even the coastal monsoon rainforests of the Southern Niger Delta experience very low rainfalls in January and December (less than 10mm per month in Brass).
Temperature varies, too. Although Akassa (near Brass) has a mean annual temperature variation of 24 to 27°C, the temperature can drop to 22°C on during Harmattan days when the dry, dusty wind blows down from the Sahara. Anyone who has lived in the tropical rainforest knows the discomfort of sudden cold during the rains other animals, and plants feel this stress also. Yet the temperature may rise to 30°C on clear sunny days.
Temperature also rises with hours of sunlight, and within the Niger Delta (lying as it does between the latitudes 4 and 6 degrees N) the days are up to an hour longer during June than in December. And climate also varies over longer periods. Figures collected between 1948 and 1980 in Port Harcourt show annual rainfall rising steadily from 2075mm in 1950 to 2800mm in 1963 and then falling to 2000mm in 1974 before rising again. Over the same period, mean annual temperatures ranged from 25°C to 27°C with a general upward trend since 1952.
5.3.3 TROPICAL RAINFOREST SOILS ARE NOT UNIVERSALLY POOR
Tropical rainforests cover whole landscapes, from mountain plateaux, through valleys of every conceivable gradient, to coastal flood plains, which contain the whole range of humid tropical climates: correspondingly there is a range of topographical and drainage conditions and therefore of soils, rich and poor.
Because of the high rainfall there is a strong tendency for nutrients to leach out, resulting in poor sandy soils. This is balanced to some extent by the high rate of biomass addition to the soil, as vegetation dies and decays, rapidly re-cycling nutrients. But within this tendency may be specific areas of richer soil, especially on flood plains, where sandiness is off-set by high clay and silt contents, and where leaching is limited by high water tables.
Furthermore, in relation to the Niger Delta in particular, not all deposits are the nutrient poor products (quartz, kaolinite clay and iron compounds) from the erosion of an old landscape. Via the Benue River, some very rich deposits originate from the young, volcanic Adamawa massif.
5.3.4 TROPICAL FOREST SOILS MAY BE SUBJECT TO EROSION
It is often assumed that there is no soil erosion beneath tropical rainforest. However, under certain conditions where there is very little plant ground cover, soil protection is dependant on leaf litter and other fallen vegetation. Very heavy rains can disturb this litter, allowing secondary raindrops to severely damage the soil surface and cause localised erosion. Potentially more damaging than primary raindrops, secondary raindrops are those that collect on vegetation and then fall to the ground. They are usually larger, and fall with a higher velocity.
Effects of this secondary rain-drop sheet erosion can be seen in the relatively natural rainforest of the Oban Hills in Southeast Nigeria, where roots that would not normally be above level ground are exposed on slopes. This is natural erosion and part of the geological cycle.
5.4 DISTRIBUTION OF THE NATURAL TROPICAL RAINFOREST BIOME
The natural tropical rainforest biomes generally correspond to those areas of the tropics with annual rainfalls exceeding 1200mm. This includes, in South America, the Amazon Basin, the Columbian and Ecuadorian Andes and the East coast of Brazil between Salvador and Rio de Janeiro; the whole of Central America including the Southern Caribbean; the Western Ghats of India; Southeast Asia (with the notable exception of the Vietnamese Highlands); the Indonesian/Melanesian and Philippine archipelagos; Hawaii; and the Northeast coast of Australia.
In Africa there are two main areas of the tropical rainforest biome (see Map 2). The Zaire Basin extends North-west to the Dahomey Gap; a second area stretches from the other side of the gap, as far west as Freetown. Other, smaller areas in Africa include areas of high local rainfall such as the East coast of Madagascar (see Map 1); riverine forest, as along the Zambezi and Ruvumu rivers, both in Mozambique; mangrove forest in small protected areas of the East African coast from Northern Kenya to Southern Mozambique; islands of montane forest in such places as the Ruwenzori Mountains, Mount Uhuru (Kilimanjaro) and Mount Kenya; and gallery forest where narrow river valleys maintain high humidity, such as in the Shire Hills of Malawi and the Adamawa massif in Cameroon/Nigeria.
However, with the exception of the Amazon and Zaire basins, a few very isolated areas (such as the highlands of the West half of New Guinea and Southern Guyana), and some national parks, the tropical rainforests in all the areas described are either gone, severely depleted or under extreme threat of destruction. It is only the size of the Amazon and Zaire basins, the largest areas of tropical rainforest on Earth, that has saved them so far.
Nigeria's main tropical rainforest biome runs in a belt from the coast to between 150 and 200 kms. inland. (Ibadan and Onitsha lie just within the belt, Enugu just outside, and Benin City in the centre.) It contains three ecozones in roughly parallel bands. Working inland from the coast, these are first the Brackish and then the Freshwater Alluvial Equatorial Monsoon ecozones (the BAM and FAM), and then the Lowland Equatorial Monsoon or LEM. Where the Adamawa massif juts into Nigeria, and in the south-facing valleys of the Yoruba uplands, there are islands of seasonal tropical rainforest, where there is a marked dry season. The northernmost fringe of the Nigerian tropical rainforest biome is a forest-savannah mosaic.
Another source of variation in the West African rainforests is age. Between 15 and 18 thousand years ago, an ice age in higher latitudes caused a squeezing of the climatic bands towards the equator. The West African rainforest shrank back until it was restricted to refuge areas that correspond roughly to the present-day areas of very high rainfall shown in Map 1. These areas, including the Niger Delta and rainforest to the East, are therefore substantially older than those to the West—perhaps by hundreds of thousands of years.
5.5 MICRO-CLIMATE OF THE LEM RAINFOREST
Interactions between tropical rainforests and their environment are far more complex than those of temperate forests, and this also complicates their study. The most important interactions are between roots and soil, and between leaves and the atmosphere. These are relatively straightforward in temperate forest, where the former occurs below ground and the latter above ground. In rainforest, however, it is not so simple.
For example, the nature and location of soil itself is difficult to define because of the spatial confusion of the various soil and semi-soil components: litter, detritus, humus, inorganic particles, organic compounds, and the soil-water and soil-atmosphere interfaces. Apart from the parasitic plants, all plants need soil of some sort and since rainforest trees give physical support all sorts of other plants—in branch forks, in holes made by animals, on stems and branches, and in any nook or cranny—there is soil in places other than on the ground.
Similarly, in the rainforest we find bodies of water other than in the ground, often high in the crown of trees. Whole aquatic ecosystems will live in these water tanks: some, such as tree frogs, never coming into contact with the ground in their entire life cycle.
The microclimate within a rainforest has evolved with the plants themselves; upon it they and their associated animals depend for survival. There are four prime aspects of the tropical rainforest microclimate.
5.5.1 MICRO-CLIMATE: LIGHT
Light intensity can be defined as bright, dim and dark. Photosynthesis, and therefore bioactivity, is at its maximum in the areas of bright light.
Two main vegetative layers can also be defined within tropical rainforest according to the light intensity they receive:
#The Euphobic Layer,
Which receives light, made up of light-demanding or 'Heliophilic' plants. This layer includes the tree canopy with its associated animal life, Heliophilic epiphytes such as orchids, and lichens. The Euphobic layer receives between 25 and 100% of the sunlight falling on the forest, so that bio-activity is prolific and there is a dense pack of leaves, flowers and seeds.
Which means literally 'without light'. Here the prolific nature of the euphotic layer means that the underlying Oligophotic layer is becomes increasingly dark towards the ground. There is great competition for the light that does reach the Oligophotic layer, and shade-tolerant plants, called the Sciophiles, grow here. As little as 1% of the available sunlight may reach the forest floor, which is therefore much less bioactive.
- Within the Oligophotic layer a further three subdivisions can be defined:
A Dim Phase, where there is uniform shade, and which is the common condition for natural tropical rainforests;
A Light Phase, where light can reach the lower forest through small gaps made by fallen trees or big branches and where new tree seedlings may germinate; and
A Dark Phase beneath tangles of dead branches and living vines that have fallen with them, where there is no light.
This is another source of light within the Oligophotic layer. Flecks of sunlight may reach through the canopy directly, or be reflected down by the vegetation itself. The movement of the sun changes sunfleck position and intensity, but over short periods it may follow a daily pattern. Many plants in the Oligophotic layer depend on sunflecks to set off flowering and seed formation.
5.5.2 MICRO-CLIMATE: TEMPERATURE
We have discussed the seasonal and diurnal (daily) temperature cycles within the macroclimate of the tropical rainforest biome as a whole. However, micro-climatic temperature changes also occur.
The surface temperature of any living thing affects its activity. In the direct midday sunlight of the tropics, surfaces of human skin, naked soil and leaves can reach temperatures in the high thirties Celsius. When this happens, we humans can move into the shade; leaves may close their stomata and/or the whole leaf structure, wilt or turn away from the direct sunlight.
If naked soil reaches these temperatures, its microbiological life is killed. However, the forest floor is protected from such fierce heat, as we notice when we enter its shade during the day. In fact the temperature can be as much as 4 degrees lower than at the canopy, and oligophotic plants have adapted accordingly. However, this does mean that when forest is damaged or disturbed it is less well able to withstand the temperature stress than the euphotic plants. Lianes and other climbing euphotic plants tend to take over the lower forest stories when this happens, and create a real 'jungle'—thereby restricting the development of more economically useful tree species.
5.5.3 MICRO-CLIMATE: WATER
Water plays an important role in the microclimate of the tropical rainforest. While forests in the brackish and freshwater alluvial ecozones (the BAM and FAM) also receive surface and groundwater from neighbouring ecosystems, rainforests of the lowland equatorial monsoon (the LEM) depend upon rainfall and are more subject to rainfall fluctuations.
However the net amount of rainfall actually available may be reduced by evaporation before it enters the biological regime.
Moreover, rainfall not lost by evaporation may be lost by leaching through the soil and by run-off. Nonetheless a healthy rainforest ecosystem acts like a sponge: the soil system (including litter, detritus and humus) soaks up the water, subsequently releasing it slowly to plants and to rivers by percolation.
Evapotranspiration is greatest in a clearing in the tropical rainforest, where a lot of vegetation are disturbed and exposed to the heat of the sun; it is least (as one would expect) in the shaded undergrowth. Also, as one would expect, for any given season, daily evapotranspiration potential is at its maximum at noon.
Temperature varies during the day but also within the vertical space of the forest. These inequalities create localised rainfall, when transpired vapour from one area condenses back to water in a cooler area or on a cooler surface. On a still day evapotranspiration may be limited by the build-up of a saturated layer of vapour above individual leaves and the forest canopy as a whole; on windy days evapotranspiration increases because the saturated layer is removed.
Rainforests effectively regulate their own humidity. When it is very high, less water can be transpired because the air is already so moist. So in turn the plants draw up less water from the soil. When humidity is low, water demand and transpiration both increase, but this serves to increase the localised humidity again.
5.5.4 MICRO-CLIMATE: NUTRIENTS
Most nutrients in the tropical rainforest are bound up in the active biomass. The most nutritious level of the soil lies in the very narrow surface horizons that contain the detritus and humus, and it is here that plants have the best chance of taking up soil nutrients before they leach any further down.
Thus the plants of the lowland tropical rainforest have evolved to concentrate shallow feeder roots near the soil surface. They are so efficient at doing this that the natural rain forest ecosystem as a whole loses very small amounts of nutrients. Where deeper taproots are put out, they serve mainly as physical anchors and to access water from lower soil horizons.
The major nutritional elements are Nitrogen, Phosphorus and Potassium (the chemical symbols for these elements are N, P and K respectively). In tropical rainforests, 50 to 60% of N and P is found in the leaves, stems and roots of plants, and nearly 90% of the K. By the time the C horizon of the soil is reached, no N is found, only about 11% of the P and only minute amounts of K. Nitrogen is not found in mineral deposits, but must enter the system by being 'fixed' from the air into organic molecules.
Trees of the Legume family Papilionaceae, Casalpiniaceae and Mimosaceae) form a major part of the lowland tropical rainforest canopy, and provide the ecosystem with much of its nitrogen. However not all legume trees always carry nodules, and nitrogen also comes from bacteria, blue-green oligophotic algae and certain fungi living on the detritus.
5.6 PHYSICAL CHARACTERISTICS OF THE LEM RAINFOREST
Again remembering that the West African LEM no longer exists as a natural ecosystem, we may still define some of its physical characteristics. It was crossed by slow moving black-water rivers, creating narrow sub-ecozones of flood plain and swamp forest, and was dotted with small permanent and seasonal lakes as a result of isolated impervious sub-soil clay deposits.
We know that it grew largely on free-draining, deep Oxisols and that the forest was only likely to become waterlogged between May and September (when an average of 2300mm of rain falls on the area).
These factors would be reflected in the physical dimensions of the forest.
5.6.1 VERTICAL DIMENSIONS
Excessively high rainfall prevents trees reaching their maximum height, but the deep Oxisols will nonetheless have favoured tall, deep-rooting tree species. With the exception of forest bordering on lakes and rivers, a vertical section of the forest would show recognisable layers as follows:
#Upper Tree Layer
Above 25 metres, composed of emergent trees, woody climbers and epiphytes.
#Middle Tree Layer
From 10 to 25 metres, made up of large trees and woody climbers.
#Lower Tree Layer
Between 5 and 10 metres, made up of small trees and woody saplings
#Shrub Layer and tree seedlings
Smaller tree seedlings and ferns covering no more than 10% of the ground (absent in some areas).
#Upper Root Layer
From the surface down to about 5 cms deep and made up of a compact root-mass in the topsoil.
#Middle Root Layer
From 5 to 50 cms deep, having a less abundant root-mass in the subsoil
#Lower Root Layer
Scattered roots below 50 cms, with occasional deep taproots down to 5m.
Although not exclusive to a given layer, different animal communities are associated with different levels.
5.6.2 HORIZONTAL DIMENSIONS
Looking at an overhead plan of the forest, different phases can also be seen.
The most extensive, within which the other phases exist. Stable, and dim under an unbroken canopy, it is actually quite easy to navigate through mature rainforest.
Where fallen trees allow sunlight to penetrate the lower levels of the forest, luxuriant growth of climbers and herbs is stimulated. Dormant seeds germinate and existing tree seedlings and saplings are encouraged to grow faster.
Were a gap phase is beginning to be filled in by new growth and darken again.
Along substantial rivers and bordering on lakes, there may be a dense barrier of euphotic plants.
#Riverine Sub-Ecozones and Ecotones
Along narrower rivers or river sections, seasonal and permanent swamp communities of plantspalms, bamboo and stilt-rooted broad-leaved trees may develop. (These are described in more detail in Chapter 6.)
5.7 DIVERSITY OF TREE FORMS IN THE LEM RAINFOREST
A wide diversity of tree forms have evolved to suit the varying climatic, topographical and drainage conditions within the LEM rainforest, as distinct from those of the FAM and BAM ecozones. They can be considered in terms of the following parameters:
#all Tree roots fall into four main categories:
- Tap roots, grow vertically down into the soil. Tap root systems are less common in the tropical rainforest than in the temperate forest, partly because nutrients are concentrated in the surface layers of soil and partly because deep roots are also unnecessary as far as seeking for water is concerned. The physical work of anchoring the plant that is another function of deep taproots is often carried out by aerial or buttress roots to compensate.
- Lateral roots, which grow horizontally.
- Fibrous roots which form thick interwoven mats as in palm trees.
- Aerial roots, which grow in the air.
#The Roots of the Typical Rainforest Tree
With the exception of the palms, which have exclusively fibrous root, a typical tropical rainforest tree tends to have a combination of root types, following one of four patterns:
- Thick horizontal surface roots, frequently merging into large spurs or buttresses, with no taproots and only weak vertical 'sinkers'.
- Thick horizontal surface roots, and well-developed taproots and sinkers.
- Weak surface roots, but a rich system of many oblique roots and a prominent taproot.
- Numerous sizeable aerial roots, plus a network of weaker underground roots.
#roots of emergent trees
Emergent trees are those that rise to become part of the canopy and which therefore dominate the tropical rainforest. These trees all tend to have thick horizontal surface roots which frequently merging into large spurs or buttresses. However they may or may not have taproots, while the vertical 'sinkers' can be weak or well developed.
#Roots of Oligotrophic Conditions
Many of the smaller pioneer trees and trees which grow in Oligotrophic (nutrient-poor) conditions trees are more likely to have many oblique roots and a prominent tap-root.
Aerial roots are a major feature of tropical rainforest trees, especially when subsurface rooting is impeded by hard soil pans or high water tables (waterlogging). Aerial roots take advantage of above-ground moisture and nutrients as well as giving added physical support to the tree.
Aerial roots of the natural LEM ecosystem will have included buttresses, knee roots (where portions of the root, either the main lateral root or a branch of it, have left the soil, travelled above it and returned to it) and root-knees, which are bumps of intense bark growth (see figure 1.).
Other Aerial roots, common only in the waterlogged sub-ecozones of the LEM tropical rainforest are stilt roots and hanging roots. Both are common in the mangrove tropical rainforests.
The upward growing breather roots or Pneumatophores of some mangrove species are discussed later, under alluvial rainforest (7.3).
Fibrous roots are most common in waterlogged conditions, and are covered in the discussion on palms, under alluvial rainforest (6.3).
These are common to many parasites.
A host tree may be smothered by clinging roots or live to an old age, growing happily together with its parasite species for years. There is a good example in the ancient Ficus preserved as a sacred tree in the village square of Botam-Tai.
The architecture of a tree is defined as much by its roots as by other body parts. However the more immediately visible characteristics of architectural variation are the shapes and branching habits of trunks and crowns.
There are two main types of architecture: branched and unbranched. Branched trees may have branches of equal status where no single branch dominates, or a hierarchical system of branches with varying status.
The great Silk Cotton tree, Ceiba pendantra, which dominates the canopy of the tropical rainforest on the margins of streams and lakes, shows an unbranched habit in its youth (with the typically thorny stem) and a branched habit in its maturity.
Although the trunks of some tropical rainforest trees have special characteristics such as fluting, they tend to have very similar smooth, light-coloured cylindrical shaped trunks. As the crown and leaves of a mature emergent tree are well out of sight, species can often only be distinguished by making a slash through the outer and inner bark of the trunk with a machete. The colours, texture, odours, taste and the presence or nature of exudates can then be used to identify the tree.
For instance, L.G. Cooper describes Terminalia superba (White Afara) as having a light yellow slash, and Lophira procera (Iron Wood) as having a hard, red slash, often showing a yellow powder under the outer bark.
5.7.4 LEAF, FLOWER AND FRUIT FORMS
The trees of the tropical rainforest show an incredibly wide variety of leaf, flower and fruit forms, far too many to be listed here. However an interesting feature of tropical rainforest trees (and rarely found elsewhere) is Cauliflory, whereby flowers and fruits occur on leafless woody trunks and branches (such as cocoa), and sometimes even on aerial roots.
It is also worth noting that flower and fruit forms may be elaborately adapted for pollination or seed dispersal by animals. The mass of vegetation makes pollination or dispersal by wind an ineffective strategy, especially in the middle and lower layers where wind rates are negligible, thus bats, of all the animals are essential in this respect.
5.8 BIODIVERSITY OF THE LEM RAINFOREST
The dynamics of the natural tropical rainforests exhibit the essence of ecosystems on the grandest scale: finite supplies of nutrients are efficiently recycled in a highly energetic system. These rainforest ecosystems are the most bioactive and biomassive on earth, and this is the key to their biodiversity.
Species evolution into more diverse forms is like a fast breeder nuclear reactor: the more species there are in an ecosystem, the more interaction there will be both within and between species, resulting in the evolution of yet more species that in turn feed the process. The bigger the gene pool, the greater the number of possible gene combinations, and the greater the chances of a useful genetic mutation.
Competition for survival tends to limit the number of species in a given area, as the rate of evolution of new species is eventually balanced by extinction of the unsuccessful.
However the high rate of bioactivity and biomass in tropical rainforests means that this balance, or equilibrium, is reached with a wider diversity of forms than in a temperate forest.
For example, a natural oak forest in a temperate zone has lower biomass and bioactivity than the Ogoni forest of the West African LEM, and a correspondingly low biodiversity. In the oak forest are two species of songbirds; the song-thrush and the blackbird (another variety of thrush). The natural Ogoni forest would hold 18 sub-species of the Bulbul alone, and many other species of song-bird. This is against a background of very diverse vegetation; while the range of tree species in a European oak forest is usually two or three (the oaks themselves, plus holly and/or birch), in the Ogoni Forest it would have been about seventy trees alone, in addition to many palms.
5.9 PLANT SPECIES OF THE LEM RAINFOREST
As discussed in Chapter 2, natural rainforests of the Niger Delta LEM would exhibit an even higher diversity of vegetation than neighbouring forests. This is for two reasons. Firstly, rainforests West of the Niger river (such as on the Ogoni plain) survived the ice ages that shrank other African rainforests to a few small islands; they therefore did not suffer the heavy extinctions this pressure put on existing species. Secondly, the oxisol soils of the LEM in the region are sufficiently well drained to support a wider range of plant species than the adjacent freshwater and brackish water alluvial ecozones, where waterlogging limits diversity.
There may at one time have been around 5,000 plant species in the LEM ecozone, many of them large trees. Today a limited range survive; nonetheless many are highly valued for their social and economic uses.
- The following examples are all taken from the Botam-Tai area of the Ogoni plain, and are as also described by Cooper:
Ceiba pendantra, the Silk Cotton tree, used locally to determine the beginning of the planting season (when the leaves fall) and recognisable by its red flowers in November. This heavily buttressed tree would have been the tallest in the forest, needing especially fertile soils but tolerant of high water tables. It would have been found beside the lakes and swamps, but not in them. It is a good pioneer tree.
Lophira procera. The Iron-Wood, with its characteristically red young leaves and red timber. It is locally known as Gwulee Gwure and is used to making gongs, and also the pestles and mortars for pounding yam. Commercially, the tree is famous for making railway sleepers and is still exported to France for this purpose. The tree has survived because until the arrival of the modern chain saw it was extremely difficult to cut.
Piptadenia africana. This is locally known as Gboo and is characterised by large wavy buttresses. It is a very good saw wood, and survives for the time being around Botam-Tai as a number of sacred trees on the sites of early settlements.
Terminalia superba. This is known locally as Gara. It is a tall, spreading tree with a small crown of whorled branches which often suddenly terminate the trunk, and a light yellow slash. This tree is good for general carpentry and grows easily and fast from seed.
Cynometra hankei is tall, with a fan shaped crown and high buttresses. It may be an allied species of Brachystegia but it is common in Southern Cameroon, to which the Ogoni Forest was related.
Albizzia zygia is a light-loving leguminous tree of the secondary forest. It has a short trunk and an orange slash, which gives out a gummy resin. The timber is not strong but is used for light furniture. A good pioneer species, in the natural forest this tree may have colonised riverine margins because of the available light, but only along the higher wider river banks (it does not like wet feet).
Anthocleista vogelli is also a tree of the secondary forest, having a short trunk and needing light. Now the most common tree around Botem-Tai, because it is resistant to fallow burning, it is also the second most common pioneer tree in Southern Nigeria (after the Umbrella Tree, Musanga cecropioides).
Other likely tree species and their relations are as follows (with thanks to Sylvestor Orchiere, Manager of the Okomu Wildlife Sanctuary in Edo State):
|Distemonathus benthamianus||('African Satin Wood')|
|Futuma elastica||(an early source of rubbber)|
|Khaya grandifolia||(also 'Mahogany')|
|Lovoa trichiliodes||('African Walnut')|
|Pycnanthus angolensis||('African Nutmeg')|
Other trees that may have been found in the Ogoni Forest, but are more associated with the lowland tropical rainforests on the West side of the Niger, are:—
This list only covers 38 of the seventy or more tree species that would naturally have been present. Furthermore, these seventy tree species would have been associated with hundreds of other plant species, including palms, bamboo (in the wetter sub-ecozones), ferns, micro-epiphytes (mosses, algae, liverworts and lichens), macro-epiphytes (such as orchid, ferns and ficuses), climbers (such as the rattans), and many herb species (begonia, sedges, the ginger family).
5.10 ANIMAL SPECIES OF THE LEM RAINFOREST
As in all ecosystems, the animals without backbones ('invertebrates') vastly out-number the vertebrates, and of all the groups the Arthropods are most numerous (see 4.7). Populations of micro-arthropods in Nigerian tropical rainforests have been estimated to exceed 38,000 per m2 of surface soil.
One of the most conspicuous Arthropods is the ant, exploiting every level of the ecosystem of the tropical rainforest as individuals and in multitudinous drives, found at every level from deep under ground to the very top of the Silk Cotton Tree. They are extraordinarily active; it has been estimated that ants are responsible for eating 1% of all leaves of the tropical rainforest each year.
Of the vertebrate groups in the LEM natural ecosystem, the reptile and bird species would have been most numerous. The snakes alone will have included tens of species like the Black and Spitting Cobras, the Royal Python, the Puff Adder, and the Green Mamba. Fish would be next most numerous.
As discussed in chapter 2, the importance of mammals are often overstressed but they are nonetheless very meaningful to humans and their loss is significant.
Likely mammals of the LEM rainforest would include the following:
|Cercopithecus mona||('Mona monkey')|
|Cercopithecus erythrogaster||('White-throated monkey')|
|Cercopithecus nictitans||('Putty-nosed monkey')|
|Cercocebus spp.||(the 'Mangabeys')|
|Colobus spp.||(the Colobus monkeys)|
|Pan troglodytes||(or Chimpanzee)|
|Perodicticus potto||('Bosman's potto')|
|Galago demidovii||('Lesser Bush Baby')|
|Vivera civetta||(African Civet)|
|Genetta spp.||(the Genets)|
|Panthera pardus||(or Leopard)|
|Herpestes spp.||(the Large and Little mongooses)|
#Artiodactyla (the even-toed Ungulates)
|Cephalophus maxwelli||(Maxwell's duiker or ‘blue duiker’)|
|Cephalophus sylvicultor||(Yellow-backed duiker)|
|Cephalophus jentinki||(‘Jentink's duiker’)|
|Cephalophus niger||(‘Black duiker’)|
|Cephalophus rufitatus||(‘Red-flanked’ duiker)|
|Syncerus caffer||(Buffalo, or ‘Bushcow’)|
|Potamochoerus porcus||(‘Red River hog’ or ‘Bushpig’)|
#Proboscidea (the elephant order)
|Loxodonta africana||(Forest elephant)|
|Dendrohyrax dorsalis||(the Tree hyrax)|
#Pholidota (the pangolin order)
|Manis gigantea||(the Giant pangolin)|
|Manis tricuspis||(the White-bellied pangolin)|
|Manis tetradactyla||(the Black-bellied pangolin)|
#Rodentia (the rodents)
|Artherurus africanus||(the Brush-tailed porcupine)|
|Hystrix cristata||(the Crested porcupine)|
|Thryonomys swinderianus||(the Grass-cutter)|
|Cricetomys ugambianus||(the Giant rat)|
|Crossarchus obscurus||(the Cusimanse)|
|Protoxerus stangeri||(the Giant forest squirrel)|
|Anomalurus derbianus||(Derby's flying squirrel)|
|Anomalurus beecrofti||(Beecroft's flying squirrel)|
5.11 THE SUB-ECOZONES OF THE LEM ECOZONE OF THE NIGER DELTA
As we have said, within the LEM ecozone are contained smaller areas of riverine, swamp and flood plain ecosystems that may be considered as 'sub-ecozones'. These would have been dominated by broad-leafed trees suited to waterlogged conditions, palms, bamboo and sedges, and associated animals. As these ecosystems are the more general case in FAM and BAM ecozones, they are discussed more fully within the following chapters.