Big idea: A population is the basic unit used to study how organisms survive, grow, and interact with their environment.
What is a population?
A population includes only one species, but many individuals.
- Same species
- Same place
- Same time
If organisms are different species, they are NOT a population.
Population vs species
Students often confuse species with population — but they are not the same.
- Species = all members worldwide
- Population = one local group
All populations belong to a species, but a species contains many populations.
Examples (make it concrete)
- All wolves on Earth = species
- Wolves in one forest = population
- All oak trees worldwide = species
- Oak trees in one park = population
Population size can change over time.
- Births increase population size
- Deaths decrease population size
- Immigration increases population size
- Emigration decreases population size
Population size is dynamic — it is always changing.
Population distribution is not always even.
- Clumped distribution – individuals group together (most common)
- Uniform distribution – individuals evenly spaced
- Random distribution – no clear pattern
Most natural populations show clumped distribution due to resources and social behaviour.
Population abundance describes how many individuals are present relative to space.
- High abundance = many individuals
- Low abundance = few individuals
- Abundance can change with seasons and conditions
Abundance is often estimated using sampling, not exact counts.
Why populations change
Population size, distribution, and abundance are influenced by abiotic factors and biotic factors.
Populations do not exist in isolation — they respond to their environment.
Big exam takeaways
- Population = same species, same area, same time
- Species ≠ population
- Population size changes through births, deaths, immigration, and emigration
- Distribution describes where individuals live
- Abundance describes how many individuals are present
Abiotic and biotic factors
Big idea: Where organisms live and how well they survive depends on abiotic factors and biotic factors.
What are abiotic factors?
Abiotic factors are the physical and chemical parts of the environment.
- They are non-living
- They vary between ecosystems
- They strongly affect where organisms can live
Key abiotic factors you must know
- Temperature – affects enzyme activity and metabolism
- Light intensity – limits photosynthesis in producers
- Water availability – essential for all living processes
- pH – affects enzyme function
- Salinity – limits freshwater vs marine organisms
In exams, always link abiotic factors to distribution or survival.
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What are biotic factors?
Biotic factors are caused by interactions between living organisms.
- They involve living organisms
- They depend on population interactions
- They often change over time
Main biotic roles in ecosystems
- Producers – plants, algae
- Consumers – herbivores, carnivores, omnivores
- Decomposers – bacteria and fungi
Without decomposers, nutrients would not be recycled.
How factors limit distribution
A limiting factor is any abiotic or biotic factor that prevents a population from increasing.
- Too little water limits plant growth
- Low light limits photosynthesis
- High salinity limits freshwater species
- Predation and competition reduce population size
The most limiting factor has the greatest effect on population size.
Abiotic vs biotic factors (quick contrast)
- Abiotic = non-living (temperature, water, pH)
- Biotic = living (predators, competitors, decomposers)
- Both control where organisms live
- Both can act as limiting factors
Big exam takeaways
- Abiotic factors are non-living environmental conditions
- Biotic factors are living interactions
- Both affect population size and distribution
- Limiting factors restrict growth
- Always link factors to survival, growth, or distribution