(a) (i) Define the term population. (ii) List two biotic and abiotic factors each that affect population growth. (b) What are the effects of food shortage o...

Answer Details

(a)(i) Definition of population

A population is a group of organisms of the same species living together in the same habitat at a particular time, and which are able to interbreed to produce fertile offspring.

(a)(ii) Factors affecting population growth

Biotic factors (living factors):

• Availability of food - abundant food supports growth while scarcity limits it.
• Predation, competition, parasitism and disease - these reduce numbers by increasing the death rate.

Abiotic factors (non-living factors):

• Temperature - extreme heat or cold slows reproduction and raises mortality.
• Availability of water, light and space - shortage of any of these restricts survival and reproduction.

(b) Effects of food shortage on a population

• Increased competition among members for the limited food, both intraspecific and interspecific.
• Increased death rate (higher mortality) as the weaker individuals starve.
• Reduced reproductive rate (lower natality) because underfed organisms produce fewer offspring.
• Migration or emigration of some members in search of food, reducing the local population.
• Stunted growth and poor health, making individuals more prone to disease.
• Overall the population size falls until it matches the available food (the carrying capacity).

(c) Methods and their principles

(i) Improving crop yield

• Application of fertilizers (manuring) - Principle: fertilizers add essential mineral nutrients such as nitrogen, phosphorus and potassium to the soil, which the crops need for the manufacture of proteins and healthy growth, thereby raising yield.
• Use of improved (high-yielding) varieties through selective breeding - Principle: plants with desirable genes (fast growth, disease resistance, large produce) are selected and bred so that these favourable characteristics are passed on, giving heavier harvests.

Other acceptable methods: irrigation (supplies water for photosynthesis and transport of nutrients) and pest and weed control (removes organisms that compete with or destroy the crop).

(ii) Preserving and storing food

• Refrigeration or freezing (low temperature) - Principle: low temperatures slow down the activity of enzymes and inhibit the growth and multiplication of decay-causing micro-organisms (bacteria and fungi), so the food does not spoil quickly.
• Drying (dehydration) - Principle: removing moisture from the food deprives micro-organisms of the water they need to grow and reproduce, thereby preventing decay.

Other acceptable methods: salting or sugaring (high salt/sugar concentration draws water out of microbial cells by osmosis, killing or inhibiting them) and canning/pasteurisation (heat kills micro-organisms and sealing prevents re-entry of air and microbes).