Big idea: A population is like a team in a sports league: all the players (organisms) are on the same team (species), playing in the same stadium (area) at the same time.
What is a population?
A population means only one species is counted, but there can be many individuals.
- Same species (like all lions, not lions + zebras)
- Same place (like one savanna, not all of Africa)
- Same time (right now, not over many years)
If you mix different species (like lions and zebras together), that's NOT a population!
Population vs species
Students often mix up species and population — but they are different!
- Species = all members worldwide (e.g. all tigers on Earth)
- Population = one local group (e.g. tigers in one forest)
Think of a species as the whole 'fan club', and a population as one group of fans at a single concert.
Examples (make it concrete)
- All wolves on Earth = species
- Wolves in Yellowstone Park = population
- All oak trees worldwide = species
- Oak trees in your local park = population
In IB exams, always specify the location and time when describing a population!
Population size is like counting all the students in your school — the number can go up or down.
- Births add to the population (like new students enrolling)
- Deaths reduce the population (students leaving)
- Immigration = new individuals arrive (students transferring in)
- Emigration = individuals leave (students transferring out)
Population size is dynamic — it changes all the time, just like your school’s student list.
Population distribution is about where everyone sits — is the classroom crowded in one corner, or spread out?
- Clumped distribution – like students sitting in groups with friends (most common in nature)
- Uniform distribution – like students sitting one per desk, evenly spaced
- Random distribution – like students sitting wherever they want, no pattern
Most wild populations show clumped distribution — think of animals gathering where food or water is found.
Population abundance is like asking: is your classroom packed or nearly empty?
- High abundance = lots of individuals (crowded classroom)
- Low abundance = few individuals (nearly empty classroom)
- Abundance can change with seasons (like more students during exam time!)
Scientists often estimate abundance using sampling — like counting students in a few classrooms to guess the whole school’s size.
Why populations change
Population size, distribution, and abundance are affected by abiotic factors and biotic factors.
Populations are always reacting to their environment — just like students respond to changes in school rules or weather.
Big exam takeaways
- Population = same species, same area, same time
- Species ≠ population
- Population size changes through births, deaths, immigration, and emigration
- Distribution = where individuals are
- Abundance = how many individuals there are
Practice: Can you describe the population of squirrels in your local park? What factors might change their size or distribution?
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
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Tolerance curves (range of tolerance)
Big idea: A species can only survive and reproduce within a certain range of tolerance. Outside this range, it becomes stressed or dies. This helps explain where species can live (distribution).
What is a tolerance curve?
A tolerance curve is usually bell-shaped. The x-axis is the abiotic factor (like temperature), and the y-axis shows performance (like growth rate).
Key parts you must be able to identify
- Optimum
- Zone of tolerance
- Zone of stress
- Lower lethal limit
- Upper lethal limit
On a graph: optimum = highest point, zone of tolerance = the full range where the curve is above zero, lethal limits = where the curve drops to zero.
How this links to limiting factors & distribution
If the environment goes outside the tolerance range (e.g., water gets too hot), that abiotic factor becomes a limiting factor and the species will not be found there (its distribution shrinks).
- Within tolerance → species can live there
- Near limits → stress, low reproduction, population may decline
- Beyond limits → death, species absent from that area
Example (temperature): A fish species may grow best at ~22°C (optimum). It might survive from ~10°C to ~35°C (tolerance range). Below 10°C or above 35°C it cannot survive (lethal limits).
How to answer the common exam question
- Step 1: Read the optimum from the peak x-value
- Step 2: Read the zone of tolerance as the full range between the lower and upper limits (where organisms survive)
- Step 3: If shown, state upper/lower lethal limits where the curve meets zero
💡 Exam Tip: Remember: The tolerance curve is always bell-shaped. The peak = optimum, the tails = stress zones, and beyond = lethal!