Sustainable harvesting removes biological resources no faster than populations can replace them, preserving biodiversity, food-web structure, and long-term ecosystem productivity.
A renewable-resource harvest is sustainable only when long-term removal remains below replacement and leaves a viable reproducing population.
| Resource | Evidence used to assess sustainability |
|---|---|
| Scots pine (Pinus sylvestris) in managed Finnish forest | Compare timber volume removed with regrowth/replanting; survey logged and unlogged forest structure and soil disturbance |
| Atlantic cod (Gadus morhua) | Use stock size, age structure, reproductive rate, juvenile recruitment and a precautionary estimate of maximum sustainable yield |
Replacement rates vary with age structure, habitat and climate, so monitoring must update quotas or harvest methods rather than treating one limit as permanent.
A renewable species is not automatically harvested sustainably; incomplete stock data and illegal or unreported removal increase uncertainty.
This objective is assessed through essay response, commonly using Discuss.
Discuss
Build the answer around this relationship: Harvesting is sustainable only when removal stays at or below the population replacement rate.
Representative question
Discuss the impact of overfishing in Lake Kariba and how sustainable harvesting of resources can be assessed.
Impact: [2 max]
a. overfishing can disrupt food chains/webs/ecosystems;
b. named example from the diagram;
c. decrease biodiversity / OWTTE;
Assessing: [2 max]
d. monitoring/measuring that the rate of harvesting of species is lower than the rate of replacement of species/OWTTE;
e. monitoring/enforcing application of regulations;
f. monitoring population size (over time)
OR
monitoring biodiversity (over time);
e. Accept monitoring of regulations, e.g., size/age of fish, number in a catch,
mesh size of nets, quotas, etc., but not setting of regulations.
4
Marking guidance:
max
Core D4.2 is secure when students can judge whether a system is being stabilized or pushed toward change. The route is: identify the stability support or disturbance, explain the mechanism, and state the ecosystem consequence using evidence.
Core transfer questions ask students to explain why an ecosystem remains stable or why a disturbance pushes it toward change. Strong answers do not list threats; they explain mechanisms such as lost rainfall recycling, trophic cascade, overharvest, nutrient enrichment, toxin biomagnification, plastic movement, or restoration through rewilding.