Habitat destruction — the four named causes
Habitat destruction is the loss or damage of the natural environment organisms live in, caused by human activity. Four named causes: increased area for farming (crops, grazing); increased area for housing and industry (homes, roads, factories); extraction of natural resources (mining, quarrying, drilling for fossil fuels); and marine pollution (oil spills, plastics, run-off). All four follow from a growing population needing more food, space and resources.
Food-web disruption — trace the causal chain
Habitat destruction removes the producers or shelter a species depends on, so its population declines — or it becomes locally extinct if it cannot migrate. Organisms that fed on it lose a food source and decline too, and their predators follow. Because most organisms sit in several food chains at once, the effect spreads across the whole web: one lost producer can cut several consumer populations, while competitors of the declining species may increase.
(Extended) Deforestation — four consequences
Deforestation is large-scale clearing of forest. Soil erosion: roots no longer bind the soil, so rain washes it away and fertility falls. Flooding: trees no longer intercept rainfall or take up water, so bare ground sheds rain into rivers faster. Increased atmospheric : less photosynthetic uptake, plus stored carbon released by burning felled trees. Loss of biodiversity: forest-dependent species that cannot survive elsewhere become extinct.
Drawn from real examiner reports.
"Oxygen levels change" ≠ the CO₂ mechanism
Asked how deforestation affects the atmosphere, candidates answer "oxygen levels change" or "there is less air". The mark scheme wants the biology: fewer trees means less photosynthesis, so less carbon dioxide is removed — and burning the felled trees releases stored carbon as extra . Naming oxygen gains nothing.
Flagged s22 P31 Q8c; P32 Q4c
Vague "gas levels" instead of named effects
"Deforestation changes the oxygen levels in the air" is not a whole answer. Creditable effects are specific and named: loss of biodiversity (forest-dependent species become extinct), loss of habitat (organisms lose home and food source), and disruption of food chains and webs (removing producers hits other trophic levels). Name them; don't gesture at the atmosphere.
Flagged s22 P43 Q3b; w22 P43 Q3b
"Disrupts the food chain" with no mechanism
A one-line assertion — "the food chain is disrupted", "it affects the ecosystem" — scores nothing. Trace the chain instead: habitat destroyed → named producer or prey loses its habitat/food source → its population falls (or it goes locally extinct) → organisms at the next trophic level have less food → they decline too. Name the organism at each step.
General ecology pattern in the digest (Sections 19 and 21) — unexplained one-line assertions lose marks against a stated causal mechanism.
Causes ≠ consequences
A rise in global average temperature is not one of the named causes of habitat destruction — it is a consequence of the enhanced greenhouse effect, which deforestation feeds by raising atmospheric . The four causes are farming, housing/industry, resource extraction and marine pollution. Offered as an MCQ option here, temperature rise is a deliberate distractor.
Habitat destruction ≠ deforestation
Habitat destruction is the general Core idea — any habitat, any of the four named causes. Deforestation is one specific example, and its four consequences (soil erosion, flooding, raised , biodiversity loss) are Extended/Supplement content. Offering that consequence list to a Core question on causes of habitat loss misses the question asked.
Local extinction ≠ global extinction
A species wiped out of one destroyed habitat is locally extinct — it may still survive elsewhere, so it is not necessarily globally extinct. Its loss from that ecosystem is nonetheless a fall in biodiversity (the variety of species) in that area. Write "locally extinct" unless the question gives evidence the species lived nowhere else.
Direct effect ≠ indirect food-web effect
The direct effect of habitat destruction is the physical loss of habitat, shelter or food source for one species. The indirect effect is the knock-on population change in other species, at other trophic levels, that depended on it. Full-mark "describe the effect" answers usually need both — many candidates state the direct loss and stop there.
Cause → direct effect → food-web effect
Scaffold every "describe the effect" answer in three moves: name the human activity from the syllabus list; state what is physically lost (habitat, shelter or food source for a named organism); then trace the knock-on decline through the food web to the next trophic level.
Split deforestation into four points
(Extended) Give soil erosion, flooding, increased atmospheric and loss of biodiversity as four distinct points, each with its mechanism — never merged into one vague sentence. A four-mark deforestation question almost always awards one mark per named consequence.
Match the command word
"State" wants the reasons named briefly. "Describe" wants the mechanism or sequence of events. "Explain" wants the reason WHY each consequence follows — why less photosynthesis raises atmospheric , not merely that it does.
Percentage lost uses the area LOST
In habitat-area data questions the numerator is the area lost, not the area remaining, and the denominator is the original area. Falling from 800 ha to 200 ha is a 75% loss, not 25%. For a percentage change in a measured value, always divide by the initial value.
Habitat destruction is the loss or damage of the natural environment in which organisms live, caused by human activity.
Four named reasons for habitat destruction (Core):
Effect on food chains and food webs: habitat destruction removes a food source or shelter that a species depends on, so that species' population may decline (or become locally extinct). Because food webs interconnect many species, this effect can spread to organisms at other trophic levels that depended (directly or indirectly) on the affected species.
State FOUR named reasons for habitat destruction.
(a) State THREE reasons for habitat destruction. [3]
(b) State how habitat destruction can affect a food web. [2]
