5.3.5Topic 5 · On the Wild Side

The Effects of Climate Change — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 25 of 34 Unit quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision

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Lesson summary

5.3.5 The Effects of Climate Change — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe and explain the effects of rising temperature on the distribution, life cycles, populations and interactions of organisms, and on humans.Every species has a range of conditions it can tole…

Growth and reproduction
Coral reefs
Ice and sea level
Extreme events
Exam-style question 1
Exam-style question 2

Part 1 of 7Change in distribution

Every species has a range of conditions it can tolerate.

As temperature rises, the area where conditions are suitable shifts towards the poles and up mountains.

Species with the ability to disperse move; those that cannot may decline.

Examples include northward movement of many European butterflies and birds, and the movement of marine plankton and fish to cooler, deeper or higher latitude water.

Species restricted to high mountain tops or polar areas, such as the polar bear that hunts from sea ice, have nowhere further to go and are at risk of extinction.

Warmer winters also allow pests and the vectors of disease to survive in new regions. Mosquitoes carrying malaria or dengue can spread to higher altitudes and latitudes, and agricultural pests such as bark beetles can increase.

Part 2 of 7Change in timing (phenology)

Phenology is the timing of seasonal life-cycle events: flowering, leaf emergence, egg laying, migration and hibernation.

Warmer springs mean many plants flower and leaves open earlier, insects emerge earlier and some birds lay eggs sooner.

Problems arise when species respond to different cues and the timing of interacting species becomes out of step.

For example, if caterpillars peak earlier but migrating birds still arrive at the old date, chicks hatch when the food peak has passed, so fewer survive.

Mismatch

Such a mismatch disrupts food chains.

Part 3 of 7Effects on populations and ecosystems

Growth and reproduction

Growth and reproduction warmer conditions and more CO₂ may increase plant growth in cold regions, but drought and heat stress reduce it elsewhere. Warmer temperatures speed development in insects, so some have more generations per year.

Coral reefs

Coral reefs heat stress makes corals expel their symbiotic algae (bleaching), and warming oceans absorb more CO₂, becoming more acidic, which makes it harder to build calcium carbonate skeletons.

Ice and sea level

Ice and sea level melting ice and thermal expansion raise sea level, flooding coastal habitats and farmland.

Extreme events

Extreme events more frequent droughts, heatwaves, fires and storms damage ecosystems and crops.

Part 4 of 7Effects on humans

Crop yields in some regions fall because of drought or heat, while growing seasons lengthen in others. Water supplies from glaciers decline. Tropical diseases spread. Coastal populations face flooding and displacement. Wildfires and heat stress affect health. Climate change interacts with habitat loss and overexploitation, putting added pressure on biodiversity.

Part 5 of 7Presenting evidence

Questions often give data such as the date of first flowering over 50 years or the northern limit of a species.

Presenting evidence

Describe the trend with figures, link it to rising temperature, but be careful: a correlation does not prove climate change is the only cause, so other variables such as land use should be considered.

Where possible, support a link using a mechanism, such as temperature acting as the cue for bud burst.

Part 6 of 7Adaptation of populations and conservation responses

Organisms can respond to change by moving, by phenotypic plasticity (altering behaviour or physiology within one lifetime) or by evolution through natural selection.

Adaptation of populations and conservation responses

Evolution requires heritable variation and enough time.

Species with short generation times and large populations, such as insects and microorganisms, can evolve quickly; long-lived species with small populations, such as large mammals, cannot easily keep pace with rapid change.

Some warm-adapted alleles are already becoming more common in certain populations, but the speed of change means many species are likely to decline.

Conservation planning now considers climate.

Adaptation of populations and conservation responses

Wildlife corridors link fragmented habitats so species can move; protected areas are designed to include a range of altitudes; seed banks and captive breeding preserve species at risk; and some projects assist the movement of species to new suitable areas.

These options are costly and uncertain, which explains why reducing the rate of climate change is also an aim.

Marine effects deserve attention too.

Warm water holds less dissolved oxygen, and ocean stratification reduces mixing of nutrients, which can lower the productivity of phytoplankton at the base of food webs.

Fish stocks move poleward, changing fisheries.

Acidification, caused by dissolved CO₂ forming carbonic acid, harms organisms with calcium carbonate shells such as molluscs and some plankton.

Remember to distinguish the effects of warming from those of acidification.

Part 7 of 7Exam-style questions

Explain how climate change could cause the population of a migratory bird to decline. [4]4 marks
The northern limit of a butterfly moved 120 km north over 30 years. Calculate the mean rate of movement and suggest one reason why some species are unable to move. [3]3 marks
Describe how rising sea temperature leads to coral bleaching. [3]3 marks

Key terms

Term	Meaning
Distribution	Where a species is found.
Phenology	The timing of seasonal events in the life cycle of organisms.
Mismatch	Loss of synchrony between the timing of interacting species.
Coral bleaching	Loss of symbiotic algae from corals under stress.
Vector	An organism that transmits a pathogen from one host to another.
Thermal expansion	Increase in the volume of water as it warms.
Dispersal	Movement of organisms or their offspring away from the parent area.

Avoid these mistakes

Saying organisms adapt by deciding to change. Populations change through natural selection over generations; individuals move or die.

Describing a species moving 'because it wants to'. Distribution changes because survival and breeding succeed in new areas and fail in old ones.

Overlooking mismatch. Marks are for explaining that interacting species respond to different cues.

Claiming all effects are harmful. Some species, and some crop regions, benefit; the overall balance is negative.

Last updated: 18 October 2026