(a) Write short notes on each of the following: (i) ecological niche (ii) population density (iii) climax continually (b) Describe briefly the interactions ...

(a) Short notes

• (i) Ecological niche - An ecological niche is the particular role or function of an organism in its habitat, that is, the total way of life of the organism including where it lives, what it feeds on, how it obtains its food, its relationships with other organisms, and the conditions it needs to survive and reproduce. No two species can occupy exactly the same niche in the same habitat for long.
• (ii) Population density - Population density is the number of individuals of a species per unit area (or per unit volume) of a habitat. It is calculated as \[ \text{Population density} = \frac{\text{Total number of individuals}}{\text{Total area (or volume) of the habitat}} \]
• (iii) Climax community - A climax community is the final, stable and self-perpetuating community of organisms that is established at the end of a natural succession, and which remains more or less unchanged as long as the environmental conditions (especially climate) remain constant.

(b) Interactions between the abiotic and biotic components of an ecosystem

An ecosystem is made up of biotic components (living things: producers, consumers and decomposers) and abiotic components (non-living factors such as sunlight, temperature, water, air, soil and mineral salts). These two interact continuously.

• Green plants (producers) use abiotic factors such as sunlight, carbon dioxide, water and mineral salts to manufacture food by photosynthesis; the food and oxygen produced support the animals.
• Consumers (animals) feed on plants and on one another, obtaining energy and nutrients; they release carbon dioxide into the air by respiration, which plants re-use.
• Decomposers break down dead organisms and waste, returning mineral nutrients such as nitrates and carbon dioxide to the soil and air (nutrient recycling) for re-use by plants.
• Abiotic factors such as temperature, rainfall and light determine the type, number and distribution of organisms that can live in the habitat, while the organisms in turn modify the abiotic environment (for example plants provide shade and add humus to the soil).

In this way there is a continuous flow of energy and cycling of materials between the living and non-living parts of the ecosystem.