Conservation & Human Impact

Question 1 (12)

(a) (4)

• Bioaccumulation = build-up of a persistent substance within one organism over its lifetime, because uptake exceeds excretion/breakdown (1). The pesticide is fat-soluble and not metabolised, so it is stored in tissues (1).
• Biomagnification = increase in concentration between trophic levels (1), because a predator eats many prey, each already carrying an accumulated dose, and retains the toxin while biomass (energy) is lost as heat up the chain — so toxin/biomass ratio rises (1).

(b) (4)

• Phytoplankton: $0.003 \times 500 = 1.5\ \text{ppm}$ (1)
• Zooplankton: $1.5 \times 10 = 15\ \text{ppm}$ (1)
• Small fish: $15 \times 10 = 150\ \text{ppm}$ (1)
• Pike: $150 \times 10 = 1500\ \text{ppm}$ (1)

(c) (4) Any two, 2 marks each (property + why):

• Fat-soluble / lipophilic (1) — so it dissolves in and is stored in fatty tissue rather than being excreted in urine (1).
• Chemically stable / non-biodegradable / persistent (1) — so it is not broken down metabolically and accumulates over time and up the chain (1).
• (Also acceptable: not readily excreted.)

Question 2 (12)

(a) (7) — 1 mark per correct ordered link:

1. Fertiliser (nitrates/phosphates) runs off into water = nutrient enrichment.
2. Nutrients act as limiting factor removed → rapid algal growth / algal bloom.
3. Algae cover surface, blocking light to submerged plants.
4. Submerged plants (and later algae) die from lack of light.
5. Decomposer (saprotrophic bacteria) populations increase, feeding on dead matter.
6. Bacteria carry out aerobic respiration, consuming dissolved $O_2$ → BOD rises / deoxygenation.
7. Fish and aerobic organisms die of oxygen deprivation (asphyxiation).

(b) (3)

• Delay because algal growth, then die-off, then decomposition each take time — the oxygen crash lags the nutrient input (1).
• Worse at night because algae/plants stop photosynthesising (no light) so no $O_2$ is produced (1), yet respiration by all organisms continues consuming $O_2$, so dissolved $O_2$ falls to its minimum before dawn (1).

(c) (2)

• Reduce fertiliser application / use buffer strips of vegetation / restore wetlands to intercept runoff (1) — this stops the nutrient reaching water, addressing the cause; treating symptoms (e.g. aerating water) does not prevent recurrence (1).

Question 3 (10)

(a) (6)

1. Short-wavelength solar radiation (incl. UV/visible) passes through atmosphere (1).
2. It is absorbed by Earth's surface, warming it (1).
3. Earth re-emits energy as longer-wavelength infrared radiation (1).
4. Greenhouse gas molecules ($CO_2$, $CH_4$, water vapour) absorb this outgoing IR (1) because their molecular bonds vibrate at IR frequencies (1).
5. They re-radiate IR in all directions, including back to the surface, trapping heat and raising surface temperature (1).

(b) (2) $\frac{420-280}{280}\times100 = \frac{140}{280}\times100 = 50\%$

• Correct method (1), answer $50\%$ (1).

(c) (2)

• Greenhouse effect is the natural warming that keeps Earth ~33 °C warmer, making it habitable (1).
• Global warming is the enhanced effect from human-added extra greenhouse gases causing harmful climate change — a problem of degree/rate, not the mechanism itself (1).

Question 4 (10)

(a) (6) — 2 marks each for three consequences (state + explain):

• Reduced population size per patch → smaller gene pool, inbreeding, loss of genetic diversity, higher extinction risk (2).
• Barriers to movement → prevents migration, dispersal and finding mates; roads cause direct mortality (2).
• Loss of large-territory / interior species that need continuous habitat larger than a single patch (2).
• (Also acceptable: disrupted ecological interactions, increased edge effects.)

(b) (4)

• Perimeter-to-area ratio increases as patches shrink; four small squares have greater total perimeter than one square of the same total area (2) — hence more edge exposed to wind, sunlight, invasive species and predators (1).
• Interior-dependent species (deep-forest specialists) are disadvantaged because core habitat shrinks disproportionately (1).

Question 5 (8)

(a) (5)

1. CFCs are stable and drift up to the stratosphere (1).
2. UV radiation breaks them, releasing chlorine free radicals ($Cl\cdot$) (1).
3. $Cl\cdot + O_3 \rightarrow ClO + O_2$ — chlorine destroys ozone (1).
4. $ClO + O \rightarrow Cl\cdot + O_2$ — chlorine is regenerated (1).
5. The $Cl\cdot$ is a catalyst, so one atom destroys many thousands of ozone molecules in a repeating cycle (1).

(b) (3)

• Ozone depletion is a stratospheric problem letting harmful UV reach the surface; it does not primarily trap heat (1).
• Global warming is caused by IR-absorbing greenhouse gases in the lower atmosphere (1). The two involve different gases, different atmospheric layers and different mechanisms — thinning ozone does not cause the warming (1).

Question 6 (8)

(a) (5)

• Maximum sustainable yield (MSY) = the largest catch that can be taken repeatedly without reducing the stock long term (1).
• Populations follow logistic growth: slow at low numbers, fast at intermediate density, plateauing at carrying capacity $K$ (1).
• Growth rate is maximum at about half the carrying capacity ($K/2$) (1).
• Harvesting at this point removes only the "surplus" that the population regenerates fastest (1), keeping the stock at the level where recruitment replaces the catch, so the resource is not depleted (1).

(b) (3) — any valid reason, e.g.:

• Difficulty accurately measuring population size / $K$, so MSY is over-estimated (1)… leading to overfishing and stock collapse (1)… plus environmental variability, illegal fishing, or delayed data make real stocks fall below the model's assumptions (1).

[
  {"claim":"Pike DDT concentration = 1500 ppm from water 0.003 ppm x500 then x10 three times","code":"conc = 0.003*500*10*10*10\nresult = (conc == 1500)"},
  {"claim":"CO2 percentage increase 280 to 420 is 50%","code":"pct = (420-280)/280*100\nresult = (pct == 50)"},
  {"claim":"Phytoplankton level is 1.5 ppm","code":"result = (0.003*500 == 1.5)"},
  {"claim":"Small fish level is 150 ppm","code":"result = (0.003*500*10*10 == 150)"}
]