Short notes on land as an important resource and on energy flow in an ecosystem

The question asks for any two notes; notes (a) and (b) are answered.

(a) Land as an important resource

Land is the surface on which almost every human activity rests. People need it to build homes, grow food, keep pastures for domestic animals, set up industries and build the towns and cities that support them; equally, wilderness in forests, grasslands, wetlands, mountains and coasts must be kept to protect biodiversity. Landforms such as hills, valleys, plains, river basins and wetlands are each resource-generating areas on which the people living there depend.

Why it is special:

• It is finite. The land on earth is as limited as any other natural resource, and people cannot live comfortably on polar ice, under the sea or in space.
• It is renewable only if used carefully. Tree and grass roots bind the soil; if forests are cut or grasslands overgrazed, the land becomes unproductive wasteland, and dumping toxic industrial or nuclear waste can make it a non-renewable resource.
• It is under growing pressure. A growing population's hunger for land for food, together with pollution by industrial waste and sewage, is degrading it, while wetlands are drained and semi-arid land is over-irrigated.

Hence a rational land-use policy is needed, deciding how much land goes to each use and where; at least 10 per cent of each ecosystem's land and water should be left as wilderness. A familiar case is the farmland on a city's edge that disappears under housing and highways within a decade.

(b) The energy flow in an ecosystem

Source. Except in deep-sea hydrothermal ecosystems, the sun is the only source of energy for ecosystems on earth. Less than half of the incoming sunlight is photosynthetically active radiation (PAR), and plants capture only 2 to 10 per cent of that PAR, yet this small amount sustains all life.

One-way flow. Producers (green plants, algae, phytoplankton) fix the sun's energy as food; consumers obtain it by eating producers or other animals. Energy therefore flows in one direction, from the sun to producers to consumers, and is not recycled; ecosystems need a constant supply of it. At each step much of it is lost as heat to the environment.

Trophic levels and food chains. Producers form the first trophic level, herbivores (primary consumers) the second and carnivores (secondary consumers) the third. Energy moves along the grazing food chain (grass → goat → man) and along the detritus food chain, which begins with dead organic matter broken down by fungi and bacteria; on land, much more energy flows through the detritus chain than through the grazing chain, while in water the grazing chain is the main route. Interlinked chains form a food web.

The ten per cent law

Only about 10 per cent of the energy at one trophic level is passed on to the next; the rest is used or lost. The amount of energy therefore falls at each successive level, which limits the number of levels in a grazing food chain to a few. For example, if the plants of a field fix 10,000 units of energy, the grasshoppers feeding on them get about 1,000 units, the frogs about 100 and a snake about 10.