Populations
Populations and communities
- a population = one species, in the same area, at the same time.
- a community = all the populations of different species in one place.
- an ecosystem = the community plus its environment, all interacting.
Practice
A population is:
A population is one species in one area at one time; all the species together make a community.
The S-shaped growth curve
- Growth depends on food supply, competition, predation and disease. In a limited space it follows four phases:
| Phase | What happens |
|---|---|
| lag | slow start, numbers still small |
| exponential | very fast growth, plenty of food and space |
| stationary | growth stops; births ≈ deaths (a limiting factor holds it back) |
| death | numbers fall — food runs out, waste builds up, disease spreads |
Practice
Put the four phases of population growth in order.
Growth goes lag → exponential → stationary → death.
Practice
Growth stops at the stationary phase because:
A limiting factor such as food shortage balances births and deaths, so the population levels off.
You've got it
Key idea
- population = one species, one area, one time; community = all species there; ecosystem = community + environment
- the S-curve has four phases: lag → exponential → stationary → death
- a limiting factor (e.g. food) ends the fast growth at the stationary phase