Population ControlA population comprises all the members of a species living in an area. E.g. population of humans on earth.

A number of factors reduce the population when numbers are high and increase the population when numbers are low.

Factors that control populations:

1.Competition

Competition occurs when organisms actively struggle for a resource that is in short supply.

Two types:a)Intra-specific competition

takes place between members of the same species. E.g. dandlions compete with each other for space.

b)Inter-specific competition occurs between members of different species. E.g. red and grey squirrel competing for food

Types of competition:Two main types: Contest Competition is where there is an active physical contest between two individual organisms.

e.g. Deer displaying territorial behaviour only the fittest animals can reproduce

Scramble competition this is where all of the competing individuals get some of the resource.

E.g. overcrowded seedlings in a flower bed. Unless some of the seedlings are removed none of them will get sufficient light, water etc. to grow properly.

2. PredationPredation is the catching, killing and eating of another organism.

A predator is an organism that catches, kills and eats another organism.

The prey is the organism that is eaten by the predator.

Examples of Predator and Prey:Ladybird and aphidBlackbird and earthworm

Adaptations of predators and preya)Predators:i) Hawks and other birds of prey have excellent

sightii)Ladybirds have strong mouthparts to chew

aphids.

b. Prey:i) Frogs are well camouflages so they

cannot be seen.ii) Ladybirds contain large amounts of

formic acid so they do not taste nice.

3. ParasitismParasitism occurs when two organisms of different species live in close association and one organism (the parasite) obtains its food from and to the disadvantage of the second organism (the host)

Parasites live either in or on a host and normally cause it harm.

Two types:Exoparasites live on the outside of the host.e.g. fleas

Endoparasites live inside the host.e.g. tapeworm or liver fluke

Parasites differ from predators because:They are smaller than the hostThey often attack from within the hostThey do only a small amount of damage to the host so

not to harm their food source.

Some parasites e.g. potato blight significantly reduce the population of potato plants.

4. SymbiosisSymbiosis occurs when two organism of different species live in close association and at least one of them benefits.

e.g. parasitism where the parasite gets food from the host but the host is harmed.

Mutualism this is a form of symbiosis where both organisms benefit.

e.g Lichens (composed of algae and fungus). The algae gets protection, minerals and support while the fungus gets food.

Population DynamicsPopulation dynamics refers to factors that cause population numbers to change

Predator-prey relationships• The number of predator and prey are inter-

related.• As the number of prey increases the number of

predators will rise.• The predators will kill the prey and the number

of prey will then start to fall. As a result the number of predators will also start to fall.

• Eventually the number of prey will begin to rise again starting the cycle once more.

• Availability of food: As the prey are killed off there is less food and predator numbers decrease.

• Concealment: which means that some prey survive by hiding from the predators. E.g. using camouflage

• Movement of predators: which means that predators move to new areas when prey numbers are low. This allows prey in the old location to increase in number.

Human population growth has continued to rise rapidly since the 1900s because of falling death rates.

World population is increasing by about 85 million people per year

Factors affecting human population numbers include:

1. War – war reduces population numbers due to death.

2. Famine – A lack of food leads to malnutrition and death due to disease and starvation.

3. Contraception – Increase availability of contraception since the 1960s has help reduce population numbers.

4. Disease control – Ability to control and cure some diseases has lead to a reduction in death rate. E.g smallpox was declared to have been eradicated world wide by 1980.