Big idea: Populations are like teams in a big game — they don’t live alone! Population interactions decide who wins, who loses, and how the ecosystem stays balanced.
Why population interactions matter
How populations interact decides who survives, who grows, and how balanced the ecosystem is.
- Control population size (wolves keep deer numbers in check)
- Affect access to resources (plants compete for sunlight)
- Keep the ecosystem balanced (bees pollinate flowers, helping both)
- A change in one population can ripple through the whole system
Predation
- Predators (like lions) eat prey (like zebras)
- More prey = more predators; fewer prey = fewer predators
- Predator and prey numbers rise and fall together
Example: If rabbits increase, foxes have more to eat and their numbers go up. If rabbits decrease, foxes go hungry and their numbers drop.
More prey → more predators. Fewer prey → fewer predators.
Herbivory
- Herbivores (like cows or caterpillars) eat plants
- Too many herbivores can damage plant populations (Overgrazing)
- If plant food is scarce, fewer herbivores can survive and reproduce
Example: If there are lots of grasshoppers, they can eat all the grass in a field, leaving little for other animals.
Herbivory links producers (plants) to consumers (animals) in food chains.
Parasitism
- Parasites (like tapeworms or fleas) get food or shelter from hosts
- Hosts lose energy or health
- Too many parasites can weaken or kill hosts
Example: Ticks feed on deer blood. Too many ticks can make deer sick.
Parasites benefit; hosts are harmed.
Mutualism
- Both species gain something (win-win!)
- Helps both survive and reproduce
- Often makes the ecosystem more stable
Example: Bees get nectar from flowers (food), and flowers get pollinated by bees (help making seeds).
Mutualism = both species win.
Disease
- Spreads faster in crowded populations
- Can quickly reduce population size
- Acts as a natural population control
Example: A virus spreads through a dense rabbit population, causing numbers to drop.
Higher population density = faster disease spread.
Competition
- Intraspecific competition (e.g. two oak trees for sunlight)
- Interspecific competition (e.g. squirrels and birds for nuts)
- Happens when resources are limited
- Can reduce growth, survival, or reproduction
Example: If two species of birds nest in the same tree, they compete for space. If food is scarce, only the best competitors survive.
Competition increases as population size increases.
Big exam takeaways
- Population interactions affect population size and balance
- Predation and herbivory link food chains
- Parasitism harms one species, mutualism benefits both
- Competition happens when resources are limited
- Disease can control population size
Population dynamics
Big idea: Population dynamics Populations are always changing — they grow and shrink like waves, depending on food, predators, disease, and other factors.
Why populations change
Population size changes because of births, deaths, immigration (moving in), and emigration (moving out).
- More food and water = more babies survive (like a buffet for animals!)
- More predators = fewer prey survive
- Disease and parasites can wipe out large numbers
- Competition for resources (like a race for the last slice of pizza!)
Predator–prey cycles
- Prey numbers go up first (e.g. lots of rabbits)
- More prey = more food for predators (e.g. foxes), so predator numbers go up
- Predators eat more prey, so prey numbers go down
- With less food, predator numbers drop too
Example: If rabbits increase, foxes have more to eat and their numbers rise. When rabbits run out, foxes go hungry and their numbers fall.
Predator numbers always lag behind prey numbers.
Negative feedback
Negative feedback acts like a thermostat — it keeps populations from getting too big or too small.
- More prey → more predators (which eat more prey)
- More predators → fewer prey (so predator numbers drop)
- Fewer prey → fewer predators (so prey can recover)
Predator–prey cycles are classic examples of negative feedback.
Oscillations
- Caused by interactions like predation and food supply
- Common in healthy, stable ecosystems
- Show that populations are responding to changes, not failing
Example: Snowshoe hare and lynx populations in Canada rise and fall in regular cycles — a classic oscillation!
Oscillations = natural ups and downs, not a sign of disaster.
Time lag
- Predators need time to reproduce after prey numbers rise
- Effects of food shortage or disease may take months or years to show
- Population responses are not instant — there’s always a delay
Example: If rabbit numbers go up this year, fox numbers might not rise until next year — that’s a time lag.
Time lag explains why population peaks don’t happen at the same time.
Putting it all together
Population dynamics are like a dance — births, deaths, food, predators, and time lags all work together to create the ups and downs we see in nature.
Big exam takeaways
- Population size changes over time (never truly constant!)
- Predator–prey cycles cause regular oscillations
- Negative feedback keeps populations stable
- Predator responses show time lag
- Population dynamics explain why numbers rise and fall
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Carrying capacity and limiting factors
Big idea: Limiting factors act like brakes — they stop populations from growing forever. Together, they determine the carrying capacity.
The key exam rule: the limiting factor is the one in shortest supply
Liebig’s Law (barrel idea): A population is limited by the factor in shortest supply, even if all other resources are plentiful. In the barrel model, the shortest stave sets the maximum water level — just like the limiting factor sets population size.
- If water is lowest → water is the limiting factor
- Improving the limiting factor → carrying capacity increases
- Worsening the limiting factor → carrying capacity decreases
In diagrams or bar charts, always identify the lowest bar and state it is the limiting factor because it caps population size.
What are limiting factors?
Limiting factors are environmental conditions that prevent populations from growing without limit — like a room that can only hold a certain number of people.
- Food — not enough food means starvation
- Water — drought reduces survival
- Space or shelter — crowding increases competition
- Predators — reduce prey numbers
- Disease — spreads faster in dense populations
- Climate — heat, cold, storms, drought
Every population is limited by at least one factor.
What is carrying capacity?
Carrying capacity is like the number of seats on a bus — once it’s full, no more can fit comfortably.
- Below carrying capacity → population grows
- Above carrying capacity → population declines
- Populations usually fluctuate around carrying capacity
Example: If a forest can support 100 deer but there are 120, food becomes limited and the population falls back towards 100.
Density-dependent factors
Density-dependent factors become stronger as the population gets more crowded.
- Competition for food and space
- Predation (prey easier to find)
- Disease and parasites
- Stress from overcrowding
The more crowded the population, the stronger these effects become.
Density-independent factors
Density-independent factors impact small and large populations equally.
- Droughts
- Floods
- Fires
- Storms
- Extreme temperatures
Example: A wildfire can reduce a population whether there are 50 or 5,000 individuals.
J-curve vs S-curve population growth
J-curve (exponential growth)
- Rapid growth with few limiting factors
- Often short-term
- Usually followed by a crash
- Unsustainable
S-curve (logistic growth)
- Growth slows as limiting factors increase
- Population stabilises
- Levels off around carrying capacity
- More realistic long-term
S-curves show populations controlled by limiting factors near carrying capacity.
Big exam takeaways
- Limiting factors restrict population growth
- The limiting factor is the one in shortest supply
- Carrying capacity depends on available resources
- Density-dependent factors increase with crowding
- Density-independent factors affect all populations
- J-curves often crash; S-curves stabilise
Measuring populations
Big idea: Ecologists can’t count every animal or plant, so they use sampling — like tasting a spoonful of soup to guess the flavor of the whole pot!
Counting vs estimating
Sometimes you can count every individual, but usually you have to estimate.
Counting
- Works for small populations (e.g. trees in a schoolyard)
- Works for large, slow, or easy-to-see organisms (e.g. elephants in a park)
- Very accurate, but often impossible for wild populations
Estimating
- Used for large or hidden populations (e.g. insects, fish, grass)
- Used for small or fast-moving organisms
- Faster and more practical, but less exact
Sampling methods
Sampling helps ecologists make good guesses about the whole population by looking at a small part.
Random sampling is like picking names from a hat — it avoids bias.
- Use random numbers to pick spots
- Don’t just choose ‘interesting’ areas
- Gives a fair picture of the whole area
Systematic sampling is like checking every 10th step on a path.
- Samples taken at set distances (e.g. every 5 meters)
- Often used with transects (lines across a habitat)
- Good for spotting patterns (e.g. how plants change from a riverbank to a forest)
Transect
- Used to study changes across a habitat (e.g. from a pond edge into a meadow)
- Often combined with quadrats (see below)
- Can be a simple line (line transect) or a wider strip (belt transect)
Example: Lay a rope from a forest into a field and record what plants you find every meter.
Transects show how species change across a habitat.
Quadrat
- Used for plants or slow-moving animals (e.g. snails, barnacles)
- Placed randomly or along a transect
- Lets you estimate population size, density, and coverage
Quadrats help measure:
- Population density (e.g. 10 daisies per m²)
- Percentage cover (e.g. grass covers 60% of the square)
- Frequency (e.g. moss found in 7 out of 10 quadrats)
Capture–mark–release–recapture
This method is used for mobile organisms (like mice, fish, or butterflies).
- Capture a sample of animals (e.g. 20 mice)
- Mark them in a harmless way (e.g. a dot of paint)
- Release them back into the wild
- Later, capture another sample (e.g. 20 more mice)
- Count how many are marked from before
- Use the numbers to estimate the total population
The Lincoln Index is:
N = (n₁ × n₂) / m
- N = estimated population size
- n₁ = number marked in first capture
- n₂ = number captured in second sample
- m = number of marked individuals recaptured
Example: If you mark 10 frogs, then catch 10 more and 2 are marked, the estimate is (10×10)/2 = 50 frogs in the pond.
You must know the Lincoln Index and its assumptions for IB exams.
Assumptions and limitations
All sampling methods have assumptions and possible errors.
- Population is closed (no animals move in or out)
- Marks are not lost or overlooked
- Marked animals mix evenly with others
- Sampling is random and unbiased
Because of these limits, results are always estimates — not exact numbers.
Big exam takeaways
- Most populations are estimated, not counted
- Sampling must be random and representative
- Quadrats are for non-mobile organisms
- Capture–mark–recapture is for mobile organisms
- The Lincoln Index estimates population size