Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/96

The Brackish Ecozone 7.6.2 CRUSTACEANS

Land crabs burrow into the mangrove soil itself, and during high tides take refuge in Rhizophora stilt roots, stems and branches, or on adjacent dry land. Shrimps and prawns spend part of their juvenile phase feeding on mangrove detritus.

7.6.3 VERTEBRATES

There are few vertebrates associated with the ecozone.

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Being either fresh-water species that can tolerate low levels of salinity, or salt-water species that can move freely between salt and fresh-water (see Chapter 11 for a discussion of fish as a resource). However the mudskipper, Periophthalmus spp., is endemic to the BAM ecozone.

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Are absent in the ecozone because they cannot tolerate brackish or salt water.

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Although there is little evidence, reptiles are said to survive in the mangroves. Reptiles from adjacent ecosystems presumably visit the mangrove forest for short periods; these may include monitor lizards, the dwarf crocodile, marine turtles and snakes.

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Similarly, few birds spend their entire life in the ecozone. Some nest in the BAM, such as the Hammerkop; many others visit the ecozone to scavenge and to feed on insects or fish. Some of the most common are heron, egret, kingfishers, sunbirds, weaverbirds, swallows, African Grey Parrots and Fishing Owls.

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There are no fully resident mammals, although visitors include the Mona monkey (Cercopithecus mona), Squirrels (Epixerus spp.) and Bats. There is some evidence of Otter (Aonyx spp.) and the Manatee Trichechus senegalensis).

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As mammals, humankind is also only a temporary resident, building small hunting, fishing and gathering island camps using chicoco soil, river sand and shells.

7.7 THE BRACKISH WATER/FRESHWATER ECOTONE

As explained above, mangrove forest serves as a purely transitory community colonising new ground, stabilising it and then being replaced by other more permanent vegetation. While mangrove species are uniquely able to tolerate brackish-water regimes, in freshwater they do not compete well with other plants and are quickly overtaken.

This means that the ecotone between brackish and freshwater systems is very narrow. It tends to be dominated by shallow rooting freshwater plants, or the deeperrooting plants that can tolerate very low salinity (such as the raffia and pandanus palms, and the paspalum grass Paspalum vaginatum). Mangrove trees themselves are rare in the ecotone, although Avicennia is sometimes seen.

Animal species of the ecotone are dominated by the adjacent freshwater communities (see Chapter 5). However, burrowing crabs are common. 94