Level 2 — RecallPopulation & Community Ecology

Population & Community Ecology

⏱ 30 minutes40 marksprintable — key stays hidden on paper

Level 2: Recall & Standard Problems

Time Limit: 30 minutes | Total Marks: 40

Answer all questions. Show working where calculations are required. Use $...$ notation for equations.

Q1. Define population density and briefly state the difference between it and population distribution (dispersion). (4 marks)

Q2. Write the equations for exponential growth and logistic growth, and state the key difference between them. (4 marks)

Q3. A population of 200 bacteria grows exponentially with an intrinsic growth rate $r = 0.5$ per hour. (a) Write the value of the instantaneous growth rate $\dfrac{dN}{dt}$ at this instant. (2 marks) (b) State what happens to $\dfrac{dN}{dt}$ as $N$ increases under logistic growth as $N \to K$ . (2 marks)

Q4. Define carrying capacity ( $K$ ). For a logistic population with $K = 1000$ and $r = 0.4$ , calculate $\dfrac{dN}{dt}$ when $N = 400$ . (4 marks)

Q5. Distinguish between density-dependent and density-independent factors, giving one example of each. (4 marks)

Q6. Compare r-selected and K-selected species. Give two contrasting characteristics for each. (4 marks)

Q7. Name the three types of survivorship curve (Type I, II, III) and describe the survival pattern of each with one example organism. (6 marks)

Q8. Define the following symbiotic relationships and indicate the effect (+/–/0) on each partner: (a) Mutualism (b) Commensalism (c) Parasitism (6 marks)

Q9. (a) Define a keystone species. (2 marks) (b) Define biodiversity and state one reason why it is important to ecosystems. (2 marks)

End of Paper

Answer keyMark scheme & solutions

Q1. (4 marks)

Population density = the number of individuals of a species per unit area or volume. (2)
Distribution/dispersion describes the spatial pattern (clumped, uniform, or random) of individuals within the area, whereas density is a count per unit space. (2) Why: density is a "how many," dispersion is a "how arranged."

Q2. (4 marks)

Exponential: $\dfrac{dN}{dt} = rN$ (1.5)
Logistic: $\dfrac{dN}{dt} = rN\left(\dfrac{K-N}{K}\right)$ (1.5)
Difference: exponential growth is unlimited (J-shaped); logistic includes the limiting term $(K-N)/K$ so growth slows and levels off at carrying capacity (S-shaped). (1)

Q3. (4 marks) (a) $\dfrac{dN}{dt} = rN = 0.5 \times 200 = 100$ individuals per hour. (2) (b) As $N \to K$ , the term $(K-N)/K \to 0$ , so $\dfrac{dN}{dt} \to 0$ ; growth slows and the population stabilises. (2)

Q4. (4 marks)

Definition: carrying capacity is the maximum population size an environment can sustain indefinitely given available resources. (2)
Calculation: $\dfrac{dN}{dt} = rN\dfrac{(K-N)}{K} = 0.4 \times 400 \times \dfrac{(1000-400)}{1000}$ $= 0.4 \times 400 \times 0.6 = 96$ individuals per unit time. (2)

Q5. (4 marks)

Density-dependent factors: effect intensifies as population density increases (e.g. competition, predation, disease). (2, incl. example)
Density-independent factors: affect the population regardless of its density (e.g. floods, drought, temperature extremes, fire). (2, incl. example)

Q6. (4 marks) — 1 mark per valid contrasting pair (max 4):

r-selected	K-selected
Small body size	Large body size
Many offspring, little parental care	Few offspring, high parental care
Short lifespan, early maturity	Long lifespan, late maturity
Populations near/below K, unstable	Populations near K, stable
Examples: r — insects, weeds; K — elephants, humans.

Q7. (6 marks) — 2 marks each:

Type I: low mortality early/mid-life, high mortality in old age (convex). Example: humans, large mammals.
Type II: constant mortality rate throughout life (straight line). Example: birds, some rodents, hydra.
Type III: very high early mortality, survivors live long (concave). Example: oysters, fish, many plants/insects.

Q8. (6 marks) — 2 marks each (definition + signs): (a) Mutualism: both partners benefit → (+ / +). (b) Commensalism: one benefits, the other unaffected → (+ / 0). (c) Parasitism: parasite benefits, host is harmed → (+ / –).

Q9. (4 marks) (a) Keystone species: a species whose presence has a disproportionately large effect on community structure/biodiversity relative to its abundance; its removal causes major ecosystem change. (2) (b) Biodiversity = the variety of life (genes, species, ecosystems) in an area. (1) Importance: increases ecosystem stability/resilience, provides ecosystem services, food, medicine, etc. (any one). (1)

[
  {"claim":"Q3a exponential rate rN = 100","code":"r=0.5; N=200; result = (r*N == 100)"},
  {"claim":"Q4 logistic dN/dt = 96","code":"r=Rational(4,10); N=400; K=1000; val=r*N*(K-N)/K; result = (val == 96)"},
  {"claim":"Logistic term vanishes at N=K","code":"K=1000; N=K; result = ((K-N)/K == 0)"}
]