The impact of unsustainable exploitation — AQA A-Level Environmental Science

Unsustainable exploitation refers to the extraction of wild plant and animal species at a rate faster than their biological replenishment through reproduction and recruitment. Within AQA A-Level Environmental Science, this topic explores the mechanisms and consequences of overharvesting across terrestrial and aquatic ecosystems, including commercial fishing, timber logging, hunting, and the illegal wildlife trade.

Understanding these processes requires integrating ecological population dynamics—specifically logistic growth models and carrying capacity—with human socio-economic drivers. It underpins subsequent study of biodiversity conservation, species survival strategies, and statutory management tools like fishing quotas and CITES regulations.

• →Maximum Sustainable Yield (MSY): The maximum biomass or number of individuals harvestable per unit time without long-term population decline, occurring at the point of maximum population growth rate (approx. K/2).
• →Commercial Extinction: The depletion of a population to a level where harvesting is no longer economically viable, typically occurring before biological extinction.
• →Demographic Structuring: The disproportionate removal of larger, older individuals, which reduces overall population fecundity, genetic diversity, and reproductive resilience.
• →Bycatch and Habitat Destruction: Collateral impacts of non-selective harvesting methods, such as bottom trawling, which eliminate non-target species and obliterate benthic habitats.

• Identifies a named human activity and the specific hydrological process it changes, such as infiltration, interception or evapotranspiration.
• Explains the mechanism linking the activity to the change in the process rather than merely asserting that change occurs.
• States the consequence for catchment stores or flows, for example reduced recharge or a shorter lag time and higher flood peak.
• Uses several distinct activities, such as urbanisation, deforestation, abstraction, irrigation or dam construction, to show breadth.
• Keeps each causal chain explicit so the activity, the process and the outcome are clearly connected.

• 💡Use because to force an explicit mechanism in every causal chain.
• 💡Structure the answer by activity, giving each one a process and a consequence.
• 💡Include a range of activities to demonstrate breadth across the hydrological cycle.
• 💡Distinguish clearly between overfishing (harvest rate exceeds replacement rate) and overfished (stock status is currently below target biomass thresholds).
• 💡Use quantitative terms and refer to demographic metrics such as recruitment, fecundity, and survival curves whenever analysing population exploitation scenarios.
• 💡When evaluating exploitation management methods (e.g. mesh size, quotas, closed seasons), always weigh ecological benefits against enforcement difficulties and compliance challenges.

• Describes an activity without naming the process it changes; state the process and explain the mechanism.
• Asserts that runoff increases without explaining why; give the causal link from impermeable surface to reduced infiltration.
• Treats all human activities as having identical effects; distinguish the specific process each one alters.
• Assuming MSY is completely safe: Students frequently believe harvesting at MSY guarantees sustainability, ignoring stochastic environmental events, data collection errors, and age-sex biases that easily tip populations into decline.
• Equating commercial extinction with biological extinction: Students often state that overfished stocks are totally extinct, whereas commercial extinction means numbers are too low to justify harvesting costs, though remaining individuals are vulnerable to Allee effects.
• Believing tree harvesting only reduces standing biomass: Students overlook that clear-felling removes nutrient pools, disrupts soil mycorrhizal networks, causes severe soil erosion, and alters microclimates.

1. 1Day 1-2: Master population curves, carrying capacity, and calculate Maximum Sustainable Yield (MSY) from logistic models.
2. 2Day 3-5: Review case studies of unsustainable terrestrial exploitation, including clear-fell logging and bushmeat hunting, focusing on ecological consequences.
3. 3Day 6-8: Study marine exploitation methods (drift netting, longlining, bottom trawling) and their direct consequences (bycatch, benthic damage, trophic cascades).
4. 4Day 9-10: Practice 6-mark and 9-mark evaluation questions focusing on management approaches versus biological limits of overexploited species.

• 📋Calculation questions: Determining MSY from growth curves, calculating percentage population changes, or computing bycatch proportions.
• 📋Data interpretation questions: Analysing fish landing data, catch-per-unit-effort (CPUE) graphs, or population age pyramids.
• 📋Extended response / Evaluate questions: 6- to 9-mark questions assessing the ecological impacts of harvesting techniques or the efficacy of conservation controls.