Interactions between populations
Big idea: Populations do not live in isolation. Population interactions influence population size, distribution, and ecosystem stability.
Why population interactions matter
Interactions between populations affect how many organisms survive and reproduce.
- They control population size
- They affect access to resources
- They influence ecosystem balance
- Changes in one population affect others
Predation
Predation
- Predators reduce prey population size
- Prey availability limits predator numbers
- Predator and prey populations are linked
More prey → more predators. Fewer prey → fewer predators.
Herbivory
Herbivory
- Herbivores reduce plant biomass
- Plant availability limits herbivore populations
- Overgrazing can damage ecosystems
Herbivory links producers to consumers in food chains.
Parasitism
Parasitism
- Parasites gain food or shelter
- Hosts lose energy or health
- Host population size may decrease
Parasites benefit; hosts are harmed.
Mutualism
Mutualism
- Both populations gain advantages
- Can improve survival and reproduction
- Often increases ecosystem stability
Mutualism = both species win.
Disease
Disease
- Disease spreads more easily in dense populations
- Can rapidly reduce population size
- Acts as a natural population control
Higher population density = faster disease spread.
Competition
Competition
- Intraspecific competition
- Interspecific competition
- Occurs when resources are limited
- Can reduce growth or survival
Competition increases as population size increases.
Big exam takeaways
- Population interactions affect population size
- Predation and herbivory link food chains
- Parasitism harms one species
- Mutualism benefits both species
- Competition occurs when resources are limited
- Disease acts as a population control
Population dynamics
Big idea: Population dynamics Populations are not constant — they increase and decrease in response to interactions, resources, and feedback mechanisms.
Why populations change
Population size changes because births, deaths, immigration, and emigration are always happening.
- Availability of food and water
- Predation pressure
- Disease and parasites
- Competition within and between species
Predator–prey cycles
Predator–prey cycles
- Prey population increases first
- More prey allows predator population to increase
- Predators reduce prey numbers
- With less prey, predator numbers fall
Predator numbers always lag behind prey numbers.
Negative feedback
Negative feedback
Negative feedback keeps populations from growing or shrinking too much.
- Increase in prey → increase in predators
- Increase in predators → decrease in prey
- Decrease in prey → decrease in predators
Predator–prey cycles are controlled by negative feedback.
Oscillations
Oscillations
- Caused by interactions like predation
- Common in stable ecosystems
- Show populations responding to feedback
Oscillations show a population is responding to changes, not failing.
Time lag
Time lag
- Predators need time to reproduce
- Effects of food shortage are delayed
- Population responses are not instant
Time lag explains why population peaks do not happen at the same time.
Putting it all together
Population dynamics result from interactions, feedback, oscillations, and time delays working together.
Big exam takeaways
- Population size changes over time
- Predator–prey cycles cause oscillations
- Negative feedback keeps populations stable
- Predator responses show time lag
- Population dynamics explain rises and falls in numbers
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Carrying capacity and limiting factors
Big idea: Limiting factors determine how large a population can become. These factors set the carrying capacity
What are limiting factors?
Limiting factors are things in the environment that prevent a population from growing indefinitely.
- Food availability
- Water supply
- Space or shelter
- Predators
- Disease
- Climate conditions
No population can grow forever — something always limits it.
Carrying capacity
Carrying capacity is not a fixed number — it depends on resources and conditions.
- When population is below carrying capacity → numbers increase
- When population exceeds carrying capacity → numbers decrease
- Populations usually fluctuate around carrying capacity
In exam answers, always link carrying capacity to resource availability.
Density-dependent limiting factors
Density-dependent factors
- Competition for food or space
- Predation
- Disease and parasites
- Stress from overcrowding
The more crowded the population, the stronger these factors become.
Density-independent limiting factors
Density-independent factors
- Droughts
- Floods
- Fires
- Storms
- Extreme temperatures
These affect small and large populations equally.
J-curve vs S-curve population growth
Population growth patterns depend on how limiting factors act.
J-curve growth
- Rapid exponential growth
- Few limiting factors at first
- Often unsustainable
- Can lead to population crash
S-curve growth
- Growth slows over time
- Limiting factors increase
- Population stabilises
- Levels off around carrying capacity
S-curves show stable populations controlled by limiting factors.
Big exam takeaways
- Limiting factors restrict population growth
- Carrying capacity depends on available resources
- Density-dependent factors increase with crowding
- Density-independent factors affect all populations
- J-curves show unchecked growth, S-curves show stability
Measuring populations
Big idea: Ecologists cannot usually count every organism, so they use sampling to estimate population size and distribution.
Counting vs estimating
Some populations can be counted directly, but most must be estimated.
Counting
- Used for small populations
- Used for large, visible organisms
- More accurate but often impractical
Estimating
- Used for large populations
- Used for small or mobile organisms
- Faster and more realistic
Sampling methods
Sampling methods help make estimates more reliable and less biased.
Random sampling
Random sampling reduces bias.
- Uses random numbers
- Avoids choosing ‘interesting’ areas
- Gives more representative results
Systematic sampling
Systematic sampling is useful for studying changes across space.
- Samples taken at fixed distances
- Often used with transects
- Good for identifying patterns
Transects
Transect
- Used to study changes across an environmental gradient
- Often combined with quadrats
- Can be line transects or belt transects
Transects show how species distribution changes across a habitat.
Quadrat sampling
Quadrat
- Used for plants or slow-moving organisms
- Placed randomly or along a transect
- Allows estimation of population size
Quadrats are commonly used to measure:
- Population density
- Percentage cover
- Frequency
Capture–mark–release–recapture
This method is used for mobile organisms.
- Capture a sample of animals
- Mark them harmlessly
- Release them back into the population
- Recapture another sample
- Use data to estimate population size
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
You must know the Lincoln Index and its assumptions for SL.
Assumptions and limitations
All sampling methods involve assumptions.
- Population is closed (no migration)
- Marks are not lost
- Marked individuals mix evenly
- Sampling is unbiased
Because of limitations, results are estimates — not exact numbers.
Big exam takeaways
- Most populations are estimated, not counted
- Sampling must be representative
- Quadrats are used for non-mobile organisms
- Capture–mark–recapture is used for mobile organisms
- The Lincoln Index estimates population size