Ecosystem and biodiversity — the two definitions
Ecosystem — a unit containing the community of organisms and their environment, interacting together. Three ideas, all needed: the community (every species present), the environment (the non-living surroundings — soil, water, climate), and the two actually interacting rather than merely coexisting. Biodiversity — the number of different species that live in an area. A wheat field holds millions of individuals but only one species, so its biodiversity is very low.
Habitat destruction — three named causes
Habitat destruction is the permanent loss or damage of a natural habitat. Three named causes: (a) increased area for housing, crop plant production and livestock production — land cleared for towns, crops and grazing; (b) extraction of natural resources — mining, quarrying, logging and access roads; (c) freshwater and marine pollution — sewage, farm chemicals, industrial waste, oil and plastic entering rivers, lakes and seas. Eutrophication detail is not required.
Deforestation — the five undesirable effects
Deforestation, the large-scale clearance of forest, is the syllabus's named example of habitat destruction. Its five undesirable effects: reducing biodiversity; extinction; loss of soil (no roots to bind it, no canopy to break the rain); flooding (no roots or leaves to absorb rainwater); and an increase of carbon dioxide in the atmosphere (less photosynthesis). Core candidates state these five; (Extended) candidates explain the mechanism behind each.
Drawn from real examiner reports.
Ecosystem: three ideas, not one
The mark scheme wants the community of organisms, their environment, and the two interacting together. "A group of living things in an area" gives the community only. "Living and non-living things in a habitat" names both components but never says they interact. Listing the parts without the interaction typically costs a mark.
Flagged Nov 2023 P41 Q10b — "environment"/"interacting" often omitted
The missing photosynthesis link
An "explain" question needs the whole chain: fewer trees, so less photosynthesis, so less carbon dioxide removed from the air, so its atmospheric concentration rises. The common answer just restates the fact — "deforestation increases carbon dioxide" — without saying why trees normally lowered it. This is the most frequently missing idea on this sub-topic.
Flagged Jun 2023 P31 Q11bii · Nov 2023 P41 Q10ai, P33 Q4a
Rising carbon dioxide ≠ ozone depletion
Asked what a rise in atmospheric carbon dioxide leads to, candidates often answer "the ozone layer is damaged". Ozone depletion is caused by different chemicals in the upper atmosphere and is not part of this sub-topic. Carbon dioxide is a greenhouse gas, so the correct consequence is global warming / climate change.
Flagged Jun 2023 P31 Q11bii
Biodiversity ≠ population size
Biodiversity counts the number of different species in an area; population size counts the individuals of one species. A single-crop wheat field can hold millions of individual plants and still have very low biodiversity, because only one species is present. Defining biodiversity as "the total number of organisms" scores zero.
Habitat destruction ≠ deforestation
Habitat destruction is the general term for any loss of a natural habitat, from any of the three named causes. Deforestation is one specific example, used by the syllabus to illustrate the undesirable effects in detail. Marine pollution wrecking a coral reef is habitat destruction but not deforestation — the two terms are not interchangeable.
Extinct ≠ endangered
An extinct species has died out completely — no living individuals remain anywhere. An endangered species still has survivors but is at serious risk of extinction. Deforestation reduces biodiversity first, and causes extinction only where a species was restricted to that forest. Conservation of endangered species is B19.2, a separate sub-topic.
(Extended) Build the four-link chain
Say what trees normally do (roots bind soil; canopy breaks the rainfall; leaves photosynthesise), state that felling removes it, give the direct physical consequence, then name the syllabus effect. Jumping from "trees are cut down" straight to the effect loses the linking mark.
State vs explain — read the command word
A Core "state" question wants only the named effect: reducing biodiversity, extinction, loss of soil, flooding, increase of carbon dioxide in the atmosphere. An (Extended) "explain" question wants the causal mechanism as well. Omitting it there loses marks.
Pick three genuinely different effects
Asked to explain three of the five effects, avoid choosing loss of soil and flooding together — they share almost the same mechanism. Spreading the choice across biodiversity, soil or water, and carbon dioxide gives three distinct mark points and shows breadth.
Quote the scenario as your evidence
When a scenario asks which of the three named reasons apply, match each reason to the specific detail that evidences it, and say explicitly when one is absent. "Pollution happens" without pointing to the sewage discharge described earns no credit.
Ecosystem -- a unit containing the community of organisms and their environment, interacting together.
Biodiversity -- the number of different species that live in an area.
Habitat destruction -- the permanent loss or serious damage of the natural habitat in which organisms live. It has three named causes:
| Reason for habitat destruction | What it involves |
|---|---|
| (a) Increased area for housing, crop plant production and livestock production | Natural habitat (forest, grassland, wetland) is cleared and converted into towns, farmland for growing crops, or grazing/rearing space for livestock, as the human population and its food needs grow |
| (b) Extraction of natural resources | Habitats are destroyed directly by mining, quarrying and logging for timber, or indirectly by the roads and other infrastructure built to support extraction |
| (c) Freshwater and marine pollution | Rivers, lakes and coastal waters are damaged when pollutants (sewage, agricultural chemicals, industrial waste, oil, plastic) enter the water (a detailed description of eutrophication is not required) |
Deforestation -- the large-scale clearance of forest, usually to convert the land to another use (e.g. farmland). It is the syllabus's named example of habitat destruction, with five undesirable effects:
| Effect (Core: state) | Explanation (Extended: explain) |
|---|---|
| Reducing biodiversity | Fewer species can survive once their forest habitat (and the food/shelter it provided) is removed |
| Extinction | Species restricted to that forest, found nowhere else, may die out completely once their only habitat is destroyed |
| Loss of soil | Without tree roots to bind it, and without a leaf canopy to break the force of rainfall hitting the ground, topsoil is washed or blown away (soil erosion) |
| Flooding | Without tree roots and leaves to absorb and intercept rainwater, more rain reaches the ground and runs off the surface quickly into rivers, instead of being taken up and released slowly |
| Increase of carbon dioxide () in the atmosphere | Fewer trees means less photosynthesis is occurring, so less is removed from the atmosphere than before |
Core candidates need only state these five effects; Extended (Supplement) candidates must be able to explain each one with its causal mechanism, as shown in the right-hand column above.
Define biodiversity.
(a) Define the term "ecosystem". (2 marks)
(b) Define the term "biodiversity". (1 mark)
