IB Biology · Theme C: Interaction and interdependence · SL and HL
C4.1 Populations and communities
A one-page summary of C4.1 Populations and communities, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How do interactions between organisms regulate sizes of populations in a community?
What interactions within a community make its populations interdependent?
What C4.1 covers
Measuring populations
- C4.1.1One species, one breeding group
- C4.1.2Why sample instead of count
- C4.1.3A frame, randomly placed
- C4.1.3 · SkillsWhat the spread is telling you
- C4.1.3 · Data bookletThe booklet’s normal curve shows what SD means
- C4.1.4 · SkillsMarking a sample, twice
- C4.1.4 · SkillsWhen the assumptions break
Population growth
- C4.1.5The ceiling an environment sets
- C4.1.6Pulled back toward the ceiling
- C4.1.7Growth with nothing in the way
- C4.1.7 · SkillsTurning a curve into a line
- C4.1.8 · SkillsGrowth that curves over
Communities and interactions
- C4.1.9Same species, two outcomes
- C4.1.10Every population, together
- C4.1.11Consumer relationships, three ways
- C4.1.11Sharing, and taking slowly
- C4.1.12When both sides gain
- C4.1.13More food from the same trees
- C4.1.14 · NOSSuggestive is not proof
- C4.1.15 · SkillsTesting whether two species avoid each other
- C4.1.16A cycle that lags behind
- C4.1.17Control from above, or from below
- C4.1.18Chemical warfare, two forms
C4.1 Populations and communities: summary
Defining a population
- Population: same species, interbreeding; separated by reproductive isolation, not just distance.
- Sample rather than census — sampling must be random to avoid bias; some sampling error is unavoidable.
Measuring populations
- Quadrats (sessile organisms) — small SD = evenly spread; large SD = patchy.
- Lincoln index: population = (M × N) ÷ R, where N = total recaptured. Trap-shy → R low → overestimate; trap-happy → R high → underestimate.
Growth curves
- Exponential (J-shaped, unlimited resources) tested by a straight line on a log-scale plot.
- Sigmoid (S-shaped) plateaus at carrying capacity (K) — no lag phase in the idealised model.
Regulation & competition
- Density-dependent factors (competition, predation, disease) push populations back to K by negative feedback.
- Intraspecific relationships: competition or cooperation, within one species.
Interspecific relationships
- Herbivory, predation, parasitism, pathogenicity (+/−: one gains, the other is harmed); interspecific competition (−/−).
- Mutualism (both benefit): root nodules, mycorrhizae, zooxanthellae. Invasive species gain from "enemy release."
Testing & community control
- Competition tested by lab experiment, field observation, or field manipulation; chi-squared tests species association.
- Predator-prey cycles (lynx-hare): the predator peak lags the prey peak; communities show top-down or bottom-up control; allelopathy/antibiotics deter competitors chemically.
Sample exam questions
Three of the 54 multiple-choice questions for C4.1. Try each one before opening the answer.
Question 1. Which term describes all the populations of different species living and interacting in a pond?
- A habitat
- A community
- An ecosystem
- A population
Show the answer
Answer: B. A community is all the interacting organisms of different species in an area. An ecosystem also includes the abiotic environment.
Question 2. Which correctly identifies what each partner gains in the mutualism between a legume and Rhizobium?
- Legume: carbon compounds; Rhizobium: ammonia
- Legume: ammonia; Rhizobium: carbon compounds
- Legume: nitrates; Rhizobium: oxygen
- Legume: water; Rhizobium: nitrogen gas
Show the answer
Answer: B. Rhizobium in the root nodules fixes nitrogen gas into ammonia for the plant, and receives carbon compounds made by photosynthesis.
Question 3. Which observation would give the best evidence that frogs on the two sides of a mountain ridge form separate populations?
- They have the same colour pattern.
- They feed on the same insect prey.
- Marked frogs never cross the ridge.
- They breed in the same month.
Show the answer
Answer: C. If marked frogs never cross the ridge, the two groups cannot interbreed, so they are reproductively isolated and form separate populations.
Linking questions
Questions that connect C4.1 to other parts of the course, the kind that come up in Paper 2.
- Energy for food chains originates from photosynthesis (C1.3). Explain why the efficiency of photosynthesis limits the total energy available to support higher trophic levels. (see C1.3)
- Predator-prey cycles demonstrate negative feedback regulation in ecosystems. Compare this with negative feedback in blood glucose regulation (D3.3). (see D3.3)
- Climate change alters the distribution of species. Using the niche concept (B4.2), explain why some species can shift their range while others face extinction. (see B4.2)
- Carbon stored in fossil fuels was originally fixed by photosynthesis millions of years ago. Explain why burning fossil fuels represents a transfer of carbon from the long-term geological cycle to the short-term atmospheric cycle. (see C4.2)
Practise C4.1
Study notes, every question and full markschemes for C4.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.