PopulationsCambridge IGCSE Biology: Revision notes

Section 1

Population, community and ecosystem

A population is a group of organisms of one species, living in the same area, at the same time.
A community is all of the populations of different species living in an ecosystem.
An ecosystem is a unit containing the community of organisms and their environment, interacting together.

Level	Includes
Population	One species, one area, one time
Community	All populations of all species in an area
Ecosystem	Community plus the physical, non-living environment

Key termspopulationcommunityecosystem

Section 2

The rate at which a population grows is affected by:

Food supply — more food allows a population to grow faster; limited food slows or reverses growth.
Competition — organisms compete with each other (and with other species) for resources such as food, space, light or mates; more competition slows growth.
Predation — an increase in predator numbers reduces prey population growth.
Disease — disease can spread rapidly in dense populations, increasing the death rate and slowing or reversing growth.

Key termscompetitionpredation

Exam tip

When asked what limits population growth, think 'food, fight, foe, flu' — food supply, competition, predation, disease.

Section 3

Population growth over time often follows an S-shaped (sigmoid) curve with four phases:

Lag phase — growth is slow at first because there are few individuals to reproduce.
Exponential (log) phase — the population grows rapidly because resources are abundant and there is little competition, predation or disease.
Stationary phase — growth levels off as the population reaches the environment's carrying capacity; birth rate becomes roughly equal to death rate as food, space or other resources become limiting.
Death phase — the population declines, for example as resources run out, disease spreads, or waste products accumulate.

Key termssigmoid curvecarrying capacity

Example

A small number of bacteria placed in a nutrient broth will show a lag phase, then multiply exponentially, before levelling off as nutrients run low.

Section 4

When interpreting a population growth graph:

identify the four phases by their shape — a slow start (lag), a steep rise (exponential), a flat top (stationary), and a decline (death)
link each phase to its cause: the exponential phase happens because food supply is plentiful and there is little competition, predation or disease; the stationary phase happens because these same factors become limiting as the population grows; the death phase happens when a resource runs out, or disease/predation increases sharply.

Common mistake

Don't describe the stationary phase as 'no growth at all' — births and deaths are still happening, they are just roughly balanced.

A population is one species in one area at one time; a community is all populations in an ecosystem; an ecosystem also includes the non-living environment.
Population growth is affected by food supply, competition, predation and disease.
The sigmoid curve has four phases: lag, exponential, stationary, death.
The stationary phase occurs at the carrying capacity, where birth rate approximately equals death rate.
Always link each phase of a growth graph to the factor causing it, not just describe its shape.

That's the notes covered.

Carry on to the next subtopic.