Level 4 — ApplicationPopulation & Community Ecology

Population & Community Ecology

⏱ 60 minutes50 marksprintable — key stays hidden on paper

Level: 4 (Application — novel problems, no hints) Time limit: 60 minutes Total marks: 50

Question 1 — Growth models (12 marks)

An invasive freshwater snail is introduced into an isolated lake with abundant resources. Field ecologists model the initial spread using the logistic equation:

$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$

with intrinsic growth rate $r = 0.8\ \text{yr}^{-1}$ and carrying capacity $K = 5000$ snails.

(a) Calculate the population growth rate $\frac{dN}{dt}$ when $N = 500$ and again when $N = 4500$ . (4)

(b) At what population size $N$ does the population grow fastest? Show your reasoning and calculate the maximum growth rate. (4)

(c) Early in the invasion the ecologists observed growth that closely matched an exponential model. Explain why, and state one condition under which the logistic and exponential predictions diverge sharply. (4)

Question 2 — Density factors & carrying capacity (10 marks)

A deer herd on an island shows the following data across four years:

Year	Population	Winter deaths from starvation (%)	Deaths from a severe blizzard (%)
1	200	5	12
2	400	11	13
3	800	23	11
4	950	41	12

(a) Classify each of the two mortality causes as density-dependent or density-independent, justifying your answer with the data. (4)

(b) From the trend, estimate the approximate carrying capacity of the island for this herd and explain your reasoning. (3)

Question 3 — Life history & survivorship (10 marks)

Two species colonise a newly formed volcanic island:

Species A: matures in 3 weeks, produces 2000 offspring per season, gives no parental care, small body size.
Species B: matures in 4 years, produces 3 offspring per season, gives extensive parental care, large body size.

(a) Classify each species as r-selected or K-selected, citing two traits each. (4)

(b) Sketch (describe in words) the survivorship curve type (I, II, or III) you would expect for each species, and justify. (4)

Question 4 — Species interactions (10 marks)

Two barnacle species live on a rocky shore. In a manipulation experiment, researchers remove Species X from the lower shore. Species Y, previously restricted to the upper shore, then spreads downward and thrives. When Species Y is removed instead, Species X shows no change in its distribution.

(a) Identify the type of interaction between X and Y and state which species is competitively dominant. (3)

(b) Name and define the ecological concept that explains why Y is normally restricted to the upper shore despite being able to survive lower. (3)

(c) A predatory whelk is then introduced that preferentially eats Species X. Predict the effect on Y's distribution and identify whether the whelk is acting as a keystone species, justifying your answer. (4)

Question 5 — Symbiosis, biodiversity & keystone (8 marks)

A coral reef contains: cleaner fish that remove parasites from larger fish; a species of algae living inside coral tissue exchanging sugars for shelter; remoras that attach to sharks and feed on scraps without affecting the shark.

(a) Classify each of the three relationships (mutualism, commensalism, or parasitism), justifying one of your choices. (4)

(b) The reef-building corals are described as a keystone species. Explain what would happen to reef biodiversity if the corals were lost, using the keystone concept. (4)

Answer keyMark scheme & solutions

Question 1 (12 marks)

(a) Using $\frac{dN}{dt} = rN\frac{K-N}{K}$ :

At $N=500$ : $= 0.8 \times 500 \times \frac{5000-500}{5000} = 0.8 \times 500 \times 0.9 = 360$ snails/yr. (2)

At $N=4500$ : $= 0.8 \times 4500 \times \frac{5000-4500}{5000} = 0.8 \times 4500 \times 0.1 = 360$ snails/yr. (2)

(Note the symmetry — both give 360, since 500 and 4500 are equidistant from K/2.)

(b) Maximum growth rate occurs at $N = K/2$ (vertex of the parabola $\frac{dN}{dt}$ vs $N$ ). (2) $N = 5000/2 = 2500$ snails. $\frac{dN}{dt} = 0.8 \times 2500 \times \frac{2500}{5000} = 0.8 \times 2500 \times 0.5 = 1000$ snails/yr. (2)

(c) Early on $N \ll K$ , so the term $\frac{K-N}{K} \approx 1$ ; the logistic reduces to $\frac{dN}{dt} \approx rN$ , i.e. exponential growth, because resources are effectively unlimited (1 mark idea, 2 marks explanation). (3) Divergence becomes sharp as $N$ approaches $K$ — resource limitation slows logistic growth while exponential keeps accelerating. (1)

Question 2 (10 marks)

(a) Starvation = density-dependent: mortality % rises with population size (5→11→23→41%), showing effect intensifies with crowding/resource competition. (2) Blizzard = density-independent: mortality % (~11–13%) stays roughly constant regardless of population size, characteristic of abiotic/weather events. (2)

(b) Carrying capacity ≈ 900–1000 deer. (1) Reasoning: starvation mortality escalates dramatically near 950 (41%), indicating resources cannot sustain many more individuals; population is nearing/at the limit set by resources. (2)

(c) In Year 5 the population is at/above $K$ , so density-dependent starvation will cause net decline (deaths exceed births); the herd should fall back toward $K$ . (2) This is negative feedback — high density → resource scarcity → increased mortality → population correction. (1)

Question 3 (10 marks)

(a) Species A = r-selected: rapid maturation, many offspring, no parental care, small size (any two traits). (2) Species B = K-selected: slow maturation, few offspring, high parental care, large size (any two traits). (2)

(b) Species A → Type III curve: high early mortality among the many, low-investment offspring; few survive to adulthood. (2) Species B → Type I (or II) curve: high parental care and few offspring → high juvenile survival, most mortality late in life. (2)

(c) Species A dominates numerically early on. (1) Its fast reproduction and short generation time exploit the empty, resource-rich habitat rapidly (colonising strategy). (1)

Question 4 (10 marks)

(a) Interspecific competition (both need lower-shore space). (2) Species X is competitively dominant — it excludes Y from the lower shore. (1)

(b) Fundamental vs realised niche. Y's fundamental niche includes the lower shore (it can survive there), but competition from X restricts Y to a smaller realised niche (upper shore only). (3)

(c) The whelk removes X from the lower shore, releasing Y from competition; Y expands downward and its realised niche enlarges. (2) The whelk is acting as a keystone species: by preferentially predating the competitive dominant, it prevents competitive exclusion and increases overall species coexistence/diversity — a disproportionately large effect relative to its abundance. (2)

Question 5 (8 marks)

(a)

Cleaner fish & large fish = mutualism (cleaner gets food, host loses parasites — both benefit). (1 + justification)
Algae in coral = mutualism (algae get shelter/CO₂, coral gets sugars — both benefit). (1)
Remora & shark = commensalism (remora benefits, shark unaffected). (1) Justification of any one (e.g. remora = +/0 → commensalism). (1)

(b) Loss of corals → loss of the physical reef structure that provides habitat, shelter and food substrate for countless species. (2) As a keystone species, corals have an effect on community structure far exceeding their biomass; their removal triggers collapse/loss of dependent species, drastically reducing biodiversity (habitat destruction → cascade). (2)

[
  {"claim":"dN/dt at N=500 equals 360","code":"r=0.8; K=5000; N=500; val=r*N*(K-N)/K; result = (val==360)"},
  {"claim":"dN/dt at N=4500 equals 360","code":"r=0.8; K=5000; N=4500; val=r*N*(K-N)/K; result = (val==360)"},
  {"claim":"Max growth at N=K/2 gives 1000","code":"r=0.8; K=5000; N=K/2; val=r*N*(K-N)/K; result = (val==1000)"},
  {"claim":"N=K/2 maximises dN/dt","code":"from sympy import symbols, diff, solve; N=symbols('N'); r=Rational(8,10); K=5000; f=r*N*(K-N)/K; crit=solve(diff(f,N),N); result = (crit==[2500])"}
]