Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/46

What is the Environment? just thinking about the relationship of soils and vegetation and animals, and of grass and antelopes and lions, indicates the complexity and dynamics of Ecosystems in general.

Energy is vital to an Ecosystem being the only component which comes from outside the biosphere: all others are recycled. Looking at the forest edge again, the herbage grows because it receives energy from the sun, but also because it gets water from the rain and nutrients from the soil. The nutrients arise from inorganic sources released in solution in the water, and from the decay of organic matter that is either the wastes of animals (such as urine) or matter that was once alive and has died (Detritus).

Detritus is broken down by the decomposers (e.g. termites and vultures) which eat it, and by the primary decomposers (the micro-organisms, such as bacteria) which mineralise it, so that it become nutrients for plants again, which can be eaten by animals.

The Ecosystem of a tree is a more contained example: at its simplest, leaves fall to ground and decay with the help of the primary decomposers; the minerals so formed dissolve in the soil water to be taken up by the tree's roots and to become leaves again. If larger animals eat the leaves the cycle is enlarged and nutrients do not reach the tree again until they have been passed out as animal waste and/or the animal dies.

This is the nutrient cycle, one of the many ecological cycles, into which the dynamics of whole Ecosystems can be broken down. Within the nutrient cycle, the movement of carbon or nitrogen can then be described in their own cycles. Energy from the sun may be stored and transferred but some is lost at every stage and must be replaced from outside the ecosystem.

It is difficult to analyse natural Ecosystems as total and self-contained entities, in order to understand the dynamics of the natural relationships that are the subject matter of ecology. In reality Ecosystems consist of many inter-related cycles that link one with another, so that in the end there is only one Ecosystem, and that is the Biosphere. However the description of a hypothetical island serves to illustrate ecological dynamics: thus, Eco Island.

LIFE ON ECO ISLAND

Eco Island is isolated in a tropical ocean thousands of miles away from any other land. The Eco Island Aphid Eater is a small bird that lives on the island. It is a poor flyer, spending all its life on the island. Its diet consists exclusively of the aphids that suck the sap out of the young green stems of the only type of tree which grows on the island: the Eco Tree.

A fungus grows on the leaves of the Eco Trees because of the honeydew that drips onto them from the aphids while they are sucking the sap from the stems. The aphids are cared for by ants which milk them for the honeydew, and carrying it to their nests at the foot of the trees.

The Eco Tree can only grow in very special conditions which include: the climate of Eco Island, the aeration at its roots created by the ants' nests, and the high levels of phosphate in the soil that comes from the guano or faeces of the Aphid Eater. However if the population of aphids gets too large the Eco Tree is weakened and likely to die because too much sap is sucked from it and because the fungus on the leaves grows so thick that they cannot photosynthesise. Moreover, the aphids are unable to move much on their own and depend on the ants to move them 44