Conservation & Human Impact

Question 1 — The Silver Estuary (12)

(a) Biomagnification factor = concentration at level n+1 ÷ concentration at level n. (1 for method)

• Zoo/Phyto = 0.10/0.02 = 5.0
• Small fish/Zoo = 0.55/0.10 = 5.5
• Large fish/Small = 3.30/0.55 = 6.0
• Osprey/Large = 18.5/3.30 = 5.6 (≈5.61) (2 for correct values.) Comment: factors are roughly constant (~5–6×) per trophic level, showing consistent step-wise magnification. (1)

(b)

• Bioaccumulation: an individual organism absorbs mercury (esp. lipid-soluble methylmercury) faster than it can excrete/metabolise it, so concentration rises within that organism over its lifetime. (2)
• Biomagnification: as a predator eats many contaminated prey, it takes in all their accumulated mercury; concentration therefore increases up each trophic level. Persistent, non-degradable and stored in tissue → passed on rather than lost. (2)

(c) Introducing fast-growing algae would not reduce osprey mercury and could worsen it. (1)

• Mercury total mass in the system is unchanged; more algal biomass may briefly dilute concentration per gram of algae. (1)
• But the mercury is still cycled up the food chain; biomagnification restores high concentration at the top. (1)
• Extra organic matter could also increase methylation of mercury by anaerobic bacteria (if it drives eutrophication/anoxia), potentially increasing bioavailable Hg. (1)

Question 2 — The Fragmented Forest (12)

(a) Only the 15 km² fragment ≥ 12 km² can sustain the beetle. (1) Fragments unable to support it: 10 + 8 + 5 = 23 km². (1) (Note original 100 km² is now fragmented; only 15 km² qualifies.) (1)

(b) Any two, each explained (2 each):

• Microclimate change at edges: increased light, wind, temperature and reduced humidity penetrate from the edge, making a strip of the fragment unsuitable interior habitat.
• Increased predation/invasion: edge-dwelling predators, weeds or invasive species penetrate the fragment, so the true "core" habitat is smaller than the mapped area. (Accept: higher tree mortality/desiccation at edges.)

(c)

• Corridors allow individuals to move between fragments → gene flow, reducing inbreeding and maintaining genetic diversity. (2)
• Demographically, they allow recolonisation of a fragment after local extinction and rescue of small declining populations. (2)
• Limitation (1): corridors can also spread disease, fire, or invasive species; or may be too narrow/be avoided by the target species.

Question 3 — The Lake That Turned Green (12)

(a) Named process: eutrophication. (1) Sequence (1 each intermediate step, up to 5):

1. Development → sewage/fertiliser/detergent runoff adds nitrates & phosphates.
2. Nutrient enrichment removes the limiting factor for algal growth.
3. Algal bloom (lake turns green).
4. Bloom blocks light → submerged plants and lower algae die.
5. Decomposer bacteria multiply, decomposing dead organic matter.
6. Aerobic decomposition consumes dissolved O₂ → hypoxia/anoxia → fish suffocate (fish kill).

(b)

• Surface water: in contact with air (O₂ diffusion) and receives O₂ from photosynthesis in the lit zone. (1.5)
• Bottom water: dead matter sinks and is decomposed there, consuming O₂; little light for photosynthesis and (if stratified) poor mixing prevents reoxygenation → anoxic. (1.5)

(c)

• Source control: reduce nutrient input (buffer strips, tertiary sewage treatment, ban phosphate detergents, control fertiliser runoff). (1)
• Symptom control: aeration/oxygenation of the lake, or removing algae/biomass. (1)
• Source control preferred because it prevents recurrence and is more sustainable/cheaper long-term; symptom control only treats effects and must be repeated. (1)

Question 4 — Ozone vs Greenhouse (12)

(a) 2 marks per correctly contrasted criterion (1 per phenomenon):

(b) Claim is largely false but has a grain of truth. (1)

• Ozone depletion and climate change are distinct problems with different mechanisms/gases. (1)
• Banning CFCs mainly protects the ozone layer; it does not address CO₂/CH₄, the main greenhouse gases. (1)
• Grain of truth: CFCs are themselves potent greenhouse gases, so their removal gives a small climate benefit — but not a solution. (1)

Question 5 — Designing for the Future (12)

(a)

• Rising effort with falling total catch indicates the fishery is being exploited beyond maximum sustainable yield (MSY). (2)
• MSY is the largest catch removable without reducing the future stock; past this point the breeding population declines, so each unit of effort returns less — a sign of overfishing/stock collapse. (2)

(b) Any three, each with justification (up to 5):

• Catch quotas/limits set below MSY → allows stock to recover and reproduce.
• Minimum mesh/net size or minimum catch size → lets juveniles breed before capture, sustaining recruitment.
• Closed seasons/no-take zones (MPAs) → protect breeding grounds, act as source of recruits.
• Effort limits (fewer licences) → reduces pressure. (3 measures = 3 marks; up to 2 further marks for quality of justification.)

(c) Any two (1.5 each):

• Island native species evolved with no defences/co-evolutionary history against the snail → high predation impact.
• Snail likely lacks natural predators/parasites on the island → unchecked population growth.
• Island communities are small/isolated → low functional redundancy, so loss of species cascades easily.

[
  {"claim":"Biomagnification factors are ~5-6 each step", "code":"vals=[0.10/0.02,0.55/0.10,3.30/0.55,18.5/3.30]; result=all(5.0<=v<=6.1 for v in vals)"},
  {"claim":"Osprey/large-fish factor rounds to 5.6", "code":"result=round(18.5/3.30,1)==5.6"},
  {"claim":"Only 15 km2 fragment meets 12 km2 threshold; 23 km2 fails", "code":"frag=[15,10,8,5]; ok=[f for f in frag if f>=12]; fail=sum(f for f in frag if f<12); result=(ok==[15] and fail==23)"},
  {"claim":"Total fragment area equals 38 km2 (sum of four fragments)", "code":"result=sum([15,10,8,5])==38"}
]