Ecology — IB Biology HL HL Study Guide

For: IB Biology HL candidates sitting IB Biology HL.

Covers: all core and HL Ecology syllabus content including energy flow through trophic levels, carbon and nitrogen biogeochemical cycles, population dynamics drivers, biodiversity conservation strategies, and biological impacts of anthropogenic climate change.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology HL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Ecology?

Ecology is the scientific study of interactions between living organisms and their biotic (other living organisms) and abiotic (non-living physical, e.g. temperature, soil pH, water availability) environments, and the patterns and consequences of these interactions at scales from individual organisms to the entire biosphere. In the IB DP Biology HL syllabus, this topic builds on IGCSE ecosystem fundamentals, includes HL-only content such as nitrogen fixation pathways and population growth modelling, and accounts for ~15% of your total HL exam marks across Papers 1, 2 and 3. Common synonyms include ecosystem biology and environmental biology, though ecology focuses specifically on interaction dynamics rather than just descriptive environmental science.

2. Energy flow and trophic levels

All energy in terrestrial and shallow marine ecosystems originates from solar radiation, captured by autotrophs (producers, e.g. plants, cyanobacteria) via photosynthesis, with ~1% of incoming solar energy converted to chemical energy stored in biomass. Trophic levels are the feeding positions of organisms in a food chain, with four standard levels: Level 1 = Producers, Level 2 = Primary consumers (herbivores), Level 3 = Secondary consumers (carnivores that eat herbivores), Level 4 = Tertiary consumers (top carnivores with no natural predators). The 10% energy transfer rule applies between all consecutive trophic levels: only ~10% of energy stored in biomass at one level is transferred to the next, while the remaining 90% is lost as heat from cellular respiration, uneaten biomass, or indigestible waste products. This energy loss limits most food chains to a maximum of 4-5 trophic levels, as insufficient energy remains to support higher-level consumers.

Worked Example

If savanna grass (producers) stores 20,000 kJ m⁻² yr⁻¹ of energy, calculate the energy available to tertiary consumers in the same ecosystem.

1. Primary consumers receive 10% of producer energy: $0.1\times 20,000=2,000$ kJ m⁻² yr⁻¹
2. Secondary consumers receive 10% of primary consumer energy: $0.1\times 2,000=200$ kJ m⁻² yr⁻¹
3. Tertiary consumers receive 10% of secondary consumer energy: $0.1\times 200=20$ kJ m⁻² yr⁻¹ Exam note: Examiners frequently ask to distinguish between pyramid types: pyramids of energy are always upright, while pyramids of numbers or biomass can be inverted (e.g. a single oak tree supporting thousands of insects creates an inverted pyramid of numbers).

3. Carbon and nitrogen cycles

Biogeochemical cycles describe the movement of essential elements between the biotic (living) and abiotic (non-living) components of ecosystems.

Carbon Cycle

Key processes driving carbon movement:

1. Photosynthesis: Autotrophs fix atmospheric CO₂ into organic carbon compounds: $$6C{O}_2+6H_{2}O\stackrel{\text{light, chlorophyll}}{\to }{C}_6{H}_{12}{O}_6+6O_2$$
2. Respiration: All organisms break down organic carbon for energy, releasing CO₂ back to the atmosphere or ocean.
3. Fossilization: Dead organic matter buried under anaerobic conditions forms coal, oil, and natural gas over millions of years, storing carbon long-term.
4. Combustion: Burning fossil fuels or biomass releases stored carbon as CO₂.
5. Ocean sequestration: CO₂ dissolves in seawater to form carbonate ions, used by marine organisms to build calcium carbonate shells, which form limestone over geologic time.

Nitrogen Cycle (HL-only core content)

78% of the atmosphere is gaseous N₂, but most organisms cannot use this form directly. Key processes:

1. Nitrogen fixation: Nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules, free-living Azotobacter in soil) convert N₂ to ammonia (NH₃), which forms ammonium ions (NH₄⁺) in soil, usable by plants.
2. Nitrification: Nitrifying bacteria convert NH₄⁺ first to nitrite (NO₂⁻) then to nitrate (NO₃⁻), the most readily absorbed nitrogen form for plants.
3. Assimilation: Plants absorb NH₄⁺ or NO₃⁻ to synthesize amino acids and nucleic acids; consumers obtain nitrogen by eating other organisms.
4. Ammonification: Decomposers break down dead organic matter and waste, releasing NH₄⁺ back to soil.
5. Denitrification: Denitrifying bacteria in waterlogged, anaerobic soil convert NO₃⁻ back to N₂, releasing it to the atmosphere.

Worked Example

A farmer plants clover (a legume) in a field after harvesting corn, instead of leaving it fallow. Explain how this increases soil nitrogen content for the next crop. Answer: Clover hosts Rhizobium bacteria in root nodules that fix atmospheric N₂ into ammonium ions, which are added to the soil when clover roots decompose, increasing available nitrogen for the next crop without synthetic fertilizers.

4. Population dynamics — birth, death, immigration, emigration

A population is a group of organisms of the same species living in the same area at the same time, capable of interbreeding. The net change in population size over a given period is calculated with the formula: $$\Delta N=(B+I)-(D+E)$$ Where $N$ = population size, $B$ = number of births, $I$ = number of immigrations (individuals moving into the area), $D$ = number of deaths, $E$ = number of emigrations (individuals moving out of the area). The sigmoid (S-shaped) population growth curve describes growth in environments with limiting factors, with three distinct phases:

1. Exponential phase: No limiting factors, birth rate >> death rate, population grows at its maximum intrinsic rate.
2. Transitional phase: Limiting factors (e.g. food shortage, predation, disease, limited space) start to act, birth rate falls, death rate rises, growth slows.
3. Plateau phase: Birth rate = death rate, population size stabilizes at the carrying capacity ($K$), the maximum population size an environment can support indefinitely.

Worked Example

A population of deer on an isolated island has a starting size of 120. In one year, 35 fawns are born, 18 deer die, 5 deer swim to the island from a nearby mainland, and 2 deer swim away. Calculate the new population size.

1. Total additions: $B+I=35+5=40$
2. Total losses: $D+E=18+2=20$
3. Net change: $\Delta N=40-20=+20$
4. New population size: $120+20=140$

5. Biodiversity and conservation

Biodiversity is the total variety of living organisms in a region, measured at three levels: genetic diversity (variation of alleles within a species), species diversity (number of different species and their relative abundance), and ecosystem diversity (variety of distinct ecosystems in a region).

Core reasons for conservation

1. Ecological: Keystone species maintain ecosystem structure; healthy ecosystems provide free ecosystem services (e.g. pollination, water purification, carbon sequestration, flood control).
2. Economic: 25% of modern medicines are derived from wild plants; ecotourism generates over $700 billion annually globally; crop wild relatives provide genetic material for crop improvement against pests and climate change.
3. Ethical: All species have an inherent right to exist; humans have a responsibility to protect ecosystems for future generations.

Conservation strategies

• In-situ conservation: Protecting species in their natural habitat (e.g. national parks, wildlife reserves, marine protected areas). Advantages: protects the entire ecosystem and natural species interactions, allows evolutionary processes to continue.
• Ex-situ conservation: Protecting species outside their natural habitat (e.g. zoos, seed banks, captive breeding programs). Advantages: used for species on the brink of extinction, enables controlled breeding to increase genetic diversity.

Worked Example

A conservation group is protecting an endangered orchid species found only in one small rainforest patch threatened by logging. Compare the suitability of in-situ and ex-situ conservation for this species. Answer: In-situ conservation via a protected reserve protects the orchid, its pollinators, natural soil conditions, and the wider ecosystem, but requires ongoing enforcement to stop illegal logging. Ex-situ conservation via botanical garden cultivation and seed bank storage eliminates the immediate threat of logging but does not protect its natural habitat, and reintroduction may fail if the rainforest is destroyed. A combination of both strategies is optimal.

6. Climate change impacts

Anthropogenic increases in atmospheric greenhouse gases (CO₂ from fossil fuel combustion, methane from livestock and landfills, nitrous oxide from synthetic fertilizers) trap more outgoing long-wave radiation, leading to global average temperature rise known as the enhanced greenhouse effect. Key biological impacts include:

1. Species range shifts: Many species are moving towards the poles or higher elevations to find suitable temperature conditions; sessile or range-restricted species (e.g. alpine plants, polar bears) face high extinction risk.
2. Phenology shifts: Timing of seasonal biological events (e.g. flowering, bird migration, insect emergence) is shifting, leading to trophic mismatch: for example, if caterpillars emerge earlier before migrating birds arrive to feed their chicks, bird reproductive success falls sharply.
3. Ocean acidification: Dissolved CO₂ in seawater forms carbonic acid, lowering ocean pH: $$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3\rightleftharpoons H^{+}+HC{O}_3^{-}$$ Lower pH reduces the availability of carbonate ions needed for corals, shellfish and calcifying plankton to build calcium carbonate shells and skeletons, leading to coral bleaching and collapse of marine food webs.
4. Increased extinction risk: The IPCC estimates that 20-30% of terrestrial species are at risk of extinction if global temperature rise exceeds 2°C above pre-industrial levels.

7. Common Pitfalls (and how to avoid them)

• Pitfall 1: Confusing the 1% solar energy conversion rate for producers with the 10% trophic transfer rule. Why it happens: Students mix up initial energy capture and subsequent trophic level transfers. Correct move: Remember ~1% of incoming solar energy is converted to producer biomass, while only ~10% transfers between each trophic level after that. This is a frequent multiple-choice question trap.
• Pitfall 2: Mixing up nitrifying and denitrifying bacteria in the nitrogen cycle. Why it happens: Similar names and multiple sequential steps. Correct move: Use the mnemonic: Nitrifying "makes Nitrates for plants" (converts ammonium to nitrite to nitrate), Denitrifying "Destroys Nitrates" (converts nitrate back to gaseous N₂, returning it to the atmosphere).
• Pitfall 3: Stating that birth rate equals zero at the plateau phase of population growth. Why it happens: Students confuse zero net growth with no new births. Correct move: At carrying capacity $K$, birth rate equals death rate (and immigration equals emigration in closed populations), so net growth is zero, but births and deaths still occur at equal rates.
• Pitfall 4: Claiming the greenhouse effect is entirely man-made. Why it happens: Students only learn about anthropogenic climate change. Correct move: The natural greenhouse effect is essential for life on Earth, keeping average global temperatures at ~15°C instead of ~-18°C; the enhanced greenhouse effect is the anthropogenic increase in greenhouse gases driving current climate change.
• Pitfall 5: Recommending ex-situ conservation as the preferred strategy for all endangered species. Why it happens: Students focus only on immediate species safety, ignoring ecosystem context. Correct move: In-situ conservation is the first-line preferred strategy, as it protects entire ecosystems and species interactions; ex-situ is a last resort for species at immediate risk of extinction.

8. Practice Questions (IB Biology HL Style)

Question 1 (3 marks)

A temperate grassland ecosystem has 500,000 kJ m⁻² yr⁻¹ of solar energy input, and producers convert 1.2% of this to biomass. Calculate the maximum energy available to secondary consumers in this ecosystem, showing your working.

Worked Solution

1. Calculate energy stored in producer biomass: $0.012\times 500,000=6,000$ kJ m⁻² yr⁻¹ (1 mark)
2. Apply 10% transfer to primary consumers: $0.1\times 6,000=600$ kJ m⁻² yr⁻¹ (1 mark)
3. Apply 10% transfer to secondary consumers: $0.1\times 600=60$ kJ m⁻² yr⁻¹ (1 mark) Final answer: 60 kJ m⁻² yr⁻¹. You will receive marks for correct intermediate steps even if your final answer is wrong, so always show your working.

Question 2 (4 marks)

Outline three ways that human activities disrupt the nitrogen cycle, and state one consequence of each disruption.

Worked Solution (any 3 of the following, 1 mark per activity + 1 mark per matching consequence):

1. Activity: Excessive use of synthetic nitrogen fertilizers on agricultural land. Consequence: Nitrate runoff into rivers and lakes causes eutrophication, leading to algal blooms, oxygen depletion, and mass death of aquatic organisms.
2. Activity: Planting large areas of monoculture crops without nitrogen-fixing root nodules. Consequence: Depletes soil nitrogen levels over time, reducing soil fertility and crop yields unless more fertilizers are added.
3. Activity: Burning fossil fuels and biomass. Consequence: Releases nitrogen oxides into the atmosphere, which contribute to acid rain, damaging plant leaves and lowering soil and freshwater pH.
4. Activity: Clearing natural vegetation for agriculture or urban development. Consequence: Reduces the amount of nitrogen assimilated by plants, leading to increased soil erosion and loss of nitrogen from the soil ecosystem.

Question 3 (5 marks)

The population of grey wolves in a national park was 220 in 2022. In 2023, 48 pups were born, 29 wolves died of disease or old age, 7 wolves entered the park from a neighbouring region, and 3 wolves left the park to establish new territories. (a) Calculate the net change in population size and the 2023 population (3 marks). (b) State one density-dependent and one density-independent factor that could limit the wolf population in the park (2 marks).

Worked Solution

(a) 1. Use the population growth formula $\Delta N=(B+I)-(D+E)$ (1 mark) 2. Substitute values: $B+I=48+7=55$, $D+E=29+3=32$, so $\Delta N=55-32=+23$ (1 mark) 3. 2023 population = $220+23=243$ (1 mark) (b) Density-dependent factor: Availability of prey (e.g. deer) / incidence of infectious disease / intraspecific competition for territory (1 mark, any valid answer) Density-independent factor: Extreme winter cold / wildfire in the park / legal human hunting quotas (1 mark, any valid answer)

9. Quick Reference Cheatsheet

10. What's Next

This Ecology topic directly connects to two later IB Biology HL syllabus units: first, Evolution (Topic 5), where ecological selection pressures drive natural selection and speciation; second, Option C: Ecology and Conservation, where you will explore advanced conservation management strategies and the socio-economic drivers of ecosystem degradation in more detail. Understanding ecosystem dynamics is also a core foundation for your Internal Assessment (IA) if you choose to investigate an ecological question for your research project, which counts for 20% of your final HL grade. To reinforce your understanding, test yourself with more IB Biology HL Ecology practice questions, or review specific subtopics you found challenging using our targeted flashcards and past paper analysis tools. If you have any questions about this guide, tricky exam questions, or any other IB Biology HL content, you can ask Ollie, our AI tutor, at any time on the homepage for personalized support and instant feedback.