Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/219

Human Ecosystems: Sangana in Akassa soils are not extensive on Akassa and are described in more detail in the essay on Okoroba-Nembe.

Finally there are the strand soils on the ridge of sand that has most recently been thrown above the high water mark (seen on Brass beach, not on Akassa). These soils are shallow and (to a high water table) free draining with a pioneer humus surface horizon that may be only a few centimetres deep. They are classified as Inceptisols.

19.4 THE NATURAL ECOSYSTEM

The natural ecosystems of Akassa are alluvial tropical rainforests, youthful in relation to the lowland tropical rainforest beyond the Delta. The Sand Barrier Islands are an ecozone in their own right containing three major sub-ecozones: the brackish-water (mangrove) sub-ecozone, the fresh-water forest sub-ecozone, and the coastal strand sub-ecozone; (and the ecotones between them).

The biodiversity of the Akassa natural ecosystem is lower than the natural ecosystems beyond them and in some parts of the Niger Delta, not only because of its relative youth, but also because of the limitations on plant diversity caused by the high water tables. Low plant diversity limits associated terrestrial animal diversity (arthropods and the lower orders), however, surrounded by tropical waters, both fresh and saline, of high biological productivity, the aquatic biodiversity is high

The ecosystem of Akassa is no longer natural and to understand the natural ecosystem of a Sand Barrier Island we must take an imaginary walk two or three thousand years ago, starting on the beach and walking towards the mangrove swamps. We walk up the beach, over a newly forming sand ridge (like the one at Okpoama on Brass Island but without the casuarina trees) with patches of the Convolvulus, Ipomea pes-carpea and the shrub, Hibiscus tiliaceus, both with their thick waxy leaves to resist the salty Atlantic winds. Then across a small lagoon formed by a stream trying to find its way to the sea, and to the narrow band of strand vegetation (which may, as at Akassa today, directly confront the beach).

The strand vegetation is low littoral scrub on the sea-side getting taller and more tree-like inland (also with tough waxy leaves tolerant of the salty winds) merging into and forming a protective barrier to the catina alluvial tropical rainforest beyond. Then on through another but better developed swamp so thick with palm trees that it takes us an hour to hack our way through the 10 or 20 metres to where stilt rooted trees become more apparent. This gives way to an area dominated by trees with various roots evolved to breath in soil seasonally inundated or subject to high water tables (buttresses, pegroots and knee-roots) such as Mitragyna stipulosa with its knee roots and Alstonia boonei with its fluted base and adventitious surface roots.

Finally at the top of the sand ridge we reach a thin belt of taller trees such as Terminalia superba, Lophira procera (Ironwood), Symphonia globulifera and perhaps Chlorophora excelsa (Iroko). Undergrowth is least here so that we are tempted to turn East or West where the walking is easy.

We move on to swamp forest again and see the pattern repeating itself, but with variations: the further North, the older the forest and therefore the more diverse. In some places the swamps are wide enough and deep enough to form a lake or swamp, and in others a salt-water creek might intrude with mangrove forest. The sand ridge is so wide that there is a substantial forest with a range of high trees, whilst in others it is so narrow that it is barely distinguishable from the neighbouring swamp.

Quite suddenly, after a swamp dominated by raffia palms, we are in mangrove forest which has formed on the brackish waterlogged alluvial deposits, between the high 217