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. A community in ecology is defined as:
- All the organisms of a single species in a given area
- All the populations of different species living and interacting in a given area at a given time
- All the living organisms plus the abiotic environment
- The physical location where a species lives
Show the answer
Answer: B. Community = all populations of different species in an area. Population = all individuals of one species in an area. Ecosystem = community + abiotic environment. Habitat = physical location.
Question 2. Invasive species succeed because:
- Specially evolved for the new habitat
- Bring their own food supply
- Always superior competitors to natives
- Released from natural enemies (predators, parasites, competitors) in the new environment
Show the answer
Answer: D. Enemy release hypothesis: leaving co-evolved enemies behind → reduced mortality → competitive advantage over natives still controlled by their enemies. Examples: cane toads (Australia), zebra mussels (Great Lakes), kudzu (southeastern US).
Question 3. A population of 200 rabbits is introduced to an island with a carrying capacity of 1000. The growth follows the logistic model. At which population size is the growth rate fastest?
- 100 — growth is fastest at low density due to minimal competition
- 1000 — growth rate increases continuously until carrying capacity
- 200 — growth is fastest immediately after introduction
- 500 — the growth rate is maximum at K/2 (half the carrying capacity) in the logistic model
Show the answer
Answer: D. In the logistic growth model (dN/dt = rN(1−N/K)), the per-capita growth rate decreases linearly as N approaches K, and the absolute growth rate (dN/dt) is a parabola peaking at N = K/2. At N = 500, there are enough individuals to reproduce but not enough to cause severe resource limitation — maximum population growth rate.
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.