AHL: Climate change impacts

IB Biology Higher Level· Theme C: Interaction and Interdependence· 20 min read

1. Geographic Range Shifts★★☆☆☆⏱ 5 min

As global temperatures increase, many species shift their geographic ranges to remain within their preferred temperature niche. Terrestrial species typically shift towards the poles or to higher elevations, while aquatic species shift to deeper, cooler water.

📘 Definition

Ecological Niche

The range of abiotic conditions (e.g. temperature, pH) and biotic interactions required for a species to survive, grow, and reproduce.

📐 Worked Example

A 20-year study of alpine plants in the European Alps found 80% of species moved upslope at an average rate of 29 meters per decade. Explain why this shift occurs and what it means for mountain summit species.

1
Rising temperatures at lower elevations push alpine species' temperature niche outside of their historic lower-altitude range.
2
Plants disperse seeds upslope to colonize areas that were previously too cold to support their growth.
3
Species already living at mountain summits have no higher elevation to shift to, as they already occupy the highest available habitat.
4
As temperatures rise, these species lose all suitable habitat, leading to local or global extinction.

Exam tip:

Always link range shifts to the niche concept — examiners consistently award marks for this connection instead of just describing the shift.

2. Changes in Seasonal Phenology★★☆☆☆⏱ 5 min

Climate change alters the timing of seasonal biological events (phenology), creating mismatches between interacting species that rely on synchronized timing for survival. Common changes include earlier flowering in plants, earlier breeding in birds, and earlier emergence of insects.

📘 Definition

Phenological Mismatch

A disruption in the timing of interactions between two or more species, caused by differing rates of phenology change in response to warming.

Example:

Mismatch between plant flowering and pollinator emergence

📐 Worked Example

In the UK, great tit birds breed to coincide with peak caterpillar emergence, the main food source for their chicks. Caterpillar emergence advances 2 days per decade with warming, while great tit breeding advances only 1 day per decade. Explain the likely impact on great tit populations.

1
Peak food demand for great tit chicks no longer aligns with peak caterpillar availability.
2
When chicks hatch, the caterpillar population has already peaked and declined, so less food is available to support chick growth.
3
Lower chick survival rates follow, leading to an overall decline in the great tit population.
4
Over time, this mismatch can lead to local extinction if great tits cannot adapt their breeding timing fast enough.

3. Impacts on Coral Reef Ecosystems★★★☆☆⏱ 6 min

Coral reefs are one of the most vulnerable ecosystems to climate change, facing two major overlapping stressors: elevated ocean temperatures and ocean acidification from excess atmospheric $C O_{2}$ absorption.

📘 Definition

Coral Bleaching

The process where elevated water temperature causes corals to expel their symbiotic photosynthetic zooxanthellae, leading to loss of color and loss of the coral's main food supply.

📐 Worked Example

A mass bleaching event on the Great Barrier Reef saw sea surface temperatures 1-2°C above the long-term average for several weeks. Explain why this led to high coral mortality.

1
Zooxanthellae provide corals with organic nutrients from photosynthesis, in exchange for shelter and inorganic nutrients from the coral.
2
Elevated temperatures damage the zooxanthellae's photosynthetic machinery, leading to production of toxic reactive oxygen species.
3
To avoid poisoning, the coral host expels the zooxanthellae, losing their primary food source.
4
If elevated temperatures persist for more than a few weeks, the coral cannot re-establish the symbiosis and dies of starvation.

Exam tip:

Always mention the symbiotic relationship between corals and zooxanthellae when answering coral bleaching questions — this is almost always a required marking point.

4. Biodiversity and Food Web Outcomes★★★☆☆⏱ 4 min

Cumulative climate change impacts alter food web structure and reduce global biodiversity, with cascading effects on ecosystem services that humans rely on for food, water, and climate regulation.

Changes to primary productivity alter the base of all food webs
Keystone species loss triggers cascading secondary extinctions
More frequent extreme weather events destroy critical habitat
Invasive species often benefit from warmer temperatures, outcompeting native species

📐 Worked Example

Arctic sea ice decline reduces habitat for polar bears, which hunt seals on sea ice. Explain how this causes cascading effects on the Arctic marine food web.

1
Polar bear populations decline as less sea ice reduces their ability to hunt seals successfully.
2
Seal populations increase as their main top predator declines, increasing predation pressure on the fish and invertebrates that seals eat.
3
Populations of fish and invertebrates decline, reducing food availability for other predators like Arctic foxes and seabirds.
4
The entire food web structure shifts, reducing overall biodiversity and altering ecosystem function.

5. Common Pitfalls

Wrong move:

Assuming all species can shift their range to cope with warming

Why:

Many species are limited by habitat fragmentation, slow dispersal, or lack of available habitat (e.g. mountain summit species)

Correct move:

Discuss limitations to range shifts when explaining impacts on biodiversity

Wrong move:

Confusing coral bleaching with permanent coral death

Why:

Coral bleaching is a stress response; corals can recover if temperatures return to normal quickly

Correct move:

Distinguish between bleaching (expulsion of zooxanthellae) and mortality (if bleaching is prolonged)

Wrong move:

Claiming all phenology changes create mismatches

Why:

Some interacting species shift phenology at the same rate, so no mismatch occurs; some species even benefit

Correct move:

Explain that mismatches only occur when interacting species shift at different rates

Wrong move:

Only mentioning temperature impacts on oceans

Why:

Climate change also causes ocean acidification, which has separate impacts on calcifying organisms

Correct move:

Include both temperature-driven bleaching and acidification when discussing ocean impacts

6. Quick Reference Cheatsheet

Impact Type	Driver	Key Outcome
Range Shift	Rising temperatures	Shift poleward/upslope; extinction if no habitat
Phenology Change	Earlier seasonal warming	Mismatch if species shift at different rates
Coral Bleaching	Elevated sea temperature	Expulsion of zooxanthellae; death if prolonged
Ocean Acidification	Increased $C O_{2}$ absorption	Reduced calcification in corals/shellfish
Cascading Extinction	Keystone species loss	Food web disruption, reduced biodiversity

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 2
Explain coral bleaching causes and impacts
2023 · 2
Discuss range shifts in Arctic species
2021 · 2
Describe phenology mismatch impacts

Going deeper

What's Next

Understanding climate change impacts is foundational to assessing global biodiversity loss and conservation strategies in IB Biology. This AHL sub-topic connects abiotic global change to measurable biotic responses across terrestrial and aquatic ecosystems, which is a core focus of Theme C: Interaction and Interdependence. Many IB exam questions require you to apply these concepts to real-world case studies, so linking observed impacts to underlying ecological principles like the niche concept is key for full marks. These impacts also underpin modern conservation efforts, so connecting this content to mitigation and adaptation strategies will prepare you for extended response questions.