(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...

(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).