What is a population?
A population is a group of the same species living in the same place at the same time. Example: all the frogs in one pond.
- Community = all the populations of different species in that place (frogs, fish, reeds, insects).
- Ecosystem = the community plus the non-living things (water, soil, sunlight, air).
Population size changes in four ways: births and immigration (moving in) add; deaths and emigration (moving out) take away.
Resources control growth
Every living thing needs resources: food (or sunlight for plants), water, space, shelter and mates.
- When resources are plentiful, most young survive. Births beat deaths, so numbers rise quickly, faster and faster.
- When resources become scarce, animals grow slower, have fewer young and more die. Growth slows down.
So the amount of resources decides how big a population can get. More food → more animals; less food → fewer animals.
Carrying capacity
The carrying capacity is the largest population an area can support for a long time with its resources.
On a graph, numbers first rise slowly, then fast, then flatten into an S shape near the carrying capacity. After that, the number goes up and down a little around the line.
If the population goes above the carrying capacity, food runs out and many die, so it drops back. If the habitat changes (drought, new farmland, a new water source), the carrying capacity itself changes.
Limiting factors: competition, predators and more
A limiting factor is anything that keeps a population from growing bigger.
- Living (biotic): predators, disease, competition with others for the same food or space.
- Non-living (abiotic): water, temperature, sunlight, space, floods, fire.
Competition happens when two organisms need the same limited resource. It can be within one species (two rabbits, one patch of grass) or between species (rabbits and deer both eating grass).
Predator and prey numbers are linked: more rabbits → more food for foxes → foxes increase → rabbits fall → foxes then fall too.
Key formulas and definitions
- Population change = (births + immigration) − (deaths + emigration)
- Population density = number of organisms ÷ area
- Carrying capacity: the maximum population the habitat can support long-term
- Limiting factor: any resource or condition that stops a population growing larger
Worked examples
1. A pond has 200 frogs. In a year 80 are born, 50 die, 10 move in and 20 move out. What is the new population?
Change = (80 + 10) − (50 + 20) = 90 − 70 = +20. New population = 200 + 20 = 220 frogs.
2. A 4 hectare field has 120 field mice. Find the population density.
Density = 120 ÷ 4 = 30 mice per hectare.
3. Yeast in a sugar solution grows fast for 6 hours, then the number stops rising. Why?
The sugar (food) is being used up and waste builds up. The population has reached the carrying capacity of the flask.
4. After a forest fire, grass grows back thickly and the deer population rises. Which factor changed?
Food (grass) became more available, so the carrying capacity for deer went up.
Common mistakes
- Calling all the animals in a forest one population; a population is one species only.
- Thinking a population keeps growing forever; limited resources always slow it down.
- Thinking carrying capacity never changes; it changes when food, water or space change.
- Forgetting that predators also depend on prey; when prey falls, predators fall later.