The natural ecosystem of the levees (now extremely rare) corresponds most closely to the natural lowland tropical rainforest of the Ogoni Plain, but with a lower species diversity because of its relative youth. There are several tree layers, the tallest reaching 30m with large buttresses to support them in the shallow soil. The trees are tall, straight and unbranched almost up to the canopy, and so dark is the understory, that undergrowth is confined to young trees growing in the slash of light created by the fall of a dead tree. Beside the wider rivers, lighter conditions encourage palms, smaller shrubby trees and woody climbers.
The natural ecosystem of the flood plains is alluvial flood-plain tropical rainforest. Here, forest cover is less even than on the levees, depending on the susceptibility to flooding and the height of the water-table. Generally there is more undergrowth, plus palms, and shorter trees with buttresses, stilt roots and pneumorrhizae such as peg-roots and knee roots. Typical trees are again Mitragyna stipulosa with its knee roots and Alstonia boonei with its fluted base and adventitious surface roots.
The natural ecosystem of the back-swamps is alluvial swamp tropical rainforest. This carries the lowest species diversity of the fresh-water ecosystems, with large patches of palm forest, interspersed with broad-leaved forest where rattans grow up trees, frequently stilt rooted.
20.4.3 THE BRACKISH-WATER, FRESH-WATER ECOTONE
The ecotone between the natural brackish and fresh-water ecozones is narrow. As the mangroves create land above the high tide mark, the fresh water system takes over and the mangrove species are competed out. Thus typically, mangroves give way suddenly to freshwater plant species which tolerate only very low salt concentrations; plants such as pandanus palms, raffia palms and bamboo are common. Mangrove trees are very poor competitors with the fresh-water plants and very rarely does one see a mixture of mangrove and fresh-water plant species. Similar conditions are found beside rivers in the predominantly fresh-water ecozone where the water becomes brackish during the driest months of the year.
The natural fauna of the whole of this area would have been more diverse than the comparatively young flora might suggest, because of the land corridor extending from Okoroba to Ogbia Town.
20.5 NATURAL AND VIABLE SOCIETY
Natural society will have been part of the ecosystems of the Okoroba-Nembe district for thousands of years. But as at Botem-Tai and Akassa it was with the appearance of viable society that the natural ecosystem would have begun to be substantially altered: the very accessibility of the Niger Delta rivers and the abundance of fish in them would have made them especially attractive to human settlement. Early fishing, hunting and gathering communities would have travelled through the mangroves in canoes to the more accessible parts of the fresh-water zone. Here they would have made tracks though the levee and flood forests, provoking seed distribution of plants that were useful to them, and, where possible, taking out trees for canoes. Later, isolated exploiting settlements may have encouraged the local predominance of useful species such as the raffia and oil palms.
However the overall impact of early mankind would have been small and easily absorbed by the ecosystem. The inhospitable nature of the mangroves may have
