3.6.9 THE EVOLUTION OF SPECIES
The divergence and extinction of species through natural selection.
3.6.10 A NICHE
The home of an organism in all its dimensions, or how it fits into and makes a living in its ecosystem.
3.6.11 A HABITAT
The viable Ecosystem that is required to sustain a species.
3.6.12 CONSERVATION
The wise use of resources and the management of the environment so that all species, including mankind, can survive in a viable Ecosystem.
3.7 PLANTS AND ANIMALS IN ECOSYSTEMS
Ecosystems can be described in terms of plants or of animals but because plants do not move, their relationship to the environment is different from that of animals.
The components of an environment which determine plant characteristics, species diversity and populations are water, temperature, mineral nutrition, and competition with other plants and animals.
Plant species create microclimates for themselves, for other plant species, and for animals. This is how life initially colonises hostile environments. A good example is the colonisation of beach ridges by Casuarina trees.
COLONISATION OF BEACH RIDGES BY CASUARINA TREES
Because of the small surface area of their leaves, Casuarina trees are well suited to growing in exposed positions. They are one of the few non-legume trees able to fix Nitrogen, and in well-drained conditions their roots are able to explore large volumes of soil enabling them to anchor themselves into poorly structured soil. Thus without competition the trees grow fast in the sandy beach ridge soils. Because the leaves have a small surface area, when they drop they are not blown away and soon form a mat under the tree. This decomposes to form a very thin but humus-rich soil, hardly more than a scab on the sandy surface. Initially other casuarina trees germinate beneath the first tree so that a clump of trees is formed. Together with the new soil, this creates a protective microclimate for the development of what J.C. Okafor calls Strand Vegetation:—
The total width of this vegetation varies from a few metres to about 100 m. Perhaps because of its small extent and simplicity Nigerian strand vegetation has been infrequently described. This also places it in great danger of being totally eliminated by oil pollution which is now pervasive along the coastline. The extent to which strand vegetation contributes to stabilising beaches against erosion in Nigeria is not known, but could be significant. (Nigeria's Threatened Environment — A National Profile, Nigerian Environmental Study/Action team, 1991.)
