Ecology: Populations, Communities & Energy Transfer

Complete Study Guide

Topics Covered

1. Populations and Communities (C4.1)
2. Transfers of Energy and Matter (C4.2)
3. HL Extension — Diversity Indices & Statistical Tests
4. Exam Strategy & Common Mistakes
5. Mixed Practice — Exam Style

Aligned to IB Biology HL 2025 syllabus — C4.1, C4.2

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Watch: Energy Flow Through Ecosystems

Stephanie Castle · 10 min · Trophic levels, food chains, energy transfer efficiency, producers and consumers

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Section 1: Populations and Communities (C4.1)

Ecology is the study of relationships between living organisms and their environment. Understanding how populations grow, interact, and form communities is central to IB Biology HL and appears heavily on both Paper 1 and Paper 2.

1.1 Sampling Techniques

Ecologists cannot count every individual in a population, so they use sampling techniques to estimate population size.

Quadrats are square frames (typically $0.25 \text{ m}^2$ or $1 \text{ m}^2$) placed randomly within a study area. The number or percentage cover of a species is recorded in each quadrat, and the results are used to estimate population density across the whole area.

Transects are lines laid across an area where environmental conditions change (e.g. from shoreline to inland). Organisms are sampled at regular intervals along the line. A line transect records species touching the line; a belt transect uses quadrats placed along the line for more detailed data.

Capture-Mark-Recapture (Lincoln Index)

For mobile animals, the capture-mark-recapture method is used:

1. Capture a sample of individuals, count them, mark them, and release them
2. After a period (allowing marked individuals to mix with the population), capture a second sample
3. Count the total caught and the number of marked individuals in the second sample

The Lincoln Index formula estimates population size:

$N = \frac{n_1 \times n_2}{m_2}$

Where:

• $N$ = estimated population size
• $n_1$ = number captured and marked in the first sample
• $n_2$ = total number captured in the second sample
• $m_2$ = number of marked individuals recaptured in the second sample

1.2 Population Growth

Populations can grow in two characteristic patterns:

Exponential (J-curve) growth occurs when resources are unlimited. The population grows at an ever-increasing rate with no upper limit. This is rare in nature but can occur temporarily when a species colonises a new habitat or when a limiting factor is removed.

$\frac{dN}{dt} = rN$

Where $r$ is the intrinsic rate of natural increase and $N$ is the population size.

Logistic (S-curve) growth is the more realistic model. Growth starts exponentially but slows as the population approaches the carrying capacity ($K$) of the environment. Environmental resistance (limited food, space, predation, disease) acts as a brake on growth.

$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$

1.3 Factors Affecting Population Size

Population size changes according to four factors:

$\text{Population change} = (B + I) - (D + E)$

Where $B$ = births (natality), $I$ = immigration, $D$ = deaths (mortality), $E$ = emigration.

Density-dependent factors become more intense as population density increases:

• Competition for food, water, space, mates
• Predation (more prey attracts more predators)
• Disease (spreads more easily in dense populations)
• Parasitism

Density-independent factors affect populations regardless of density:

• Natural disasters (floods, fires, volcanic eruptions)
• Climate change / extreme weather
• Human activities (habitat destruction, pollution)

1.4 Community Interactions

Interspecific competition occurs between individuals of different species that share a resource. Intraspecific competition occurs between individuals of the same species — this is often more intense because individuals require exactly the same resources.

1.5 Ecological Niches

The fundamental niche is the full range of environmental conditions and resources a species could potentially occupy in the absence of competition and predation.

The realized niche is the actual niche a species occupies in practice, which is narrower than the fundamental niche due to interspecific competition, predation, and other biotic interactions.

1.6 Ecological Succession

Succession is the gradual change in species composition and community structure over time in a given area.

Primary succession starts on bare, lifeless substrates (e.g. bare rock after volcanic eruption, newly formed sand dunes). There is no pre-existing soil.

1. Pioneer species (lichens, mosses) colonise bare rock
2. They weather the rock and add organic matter when they die, forming a thin soil layer
3. Small plants (grasses, ferns) can now grow in the thin soil
4. Soil deepens over time, supporting shrubs and then trees
5. A climax community is eventually reached — a stable, self-sustaining community in equilibrium with the climate

Secondary succession occurs in areas where a community has been disturbed but soil remains (e.g. after a forest fire, abandoned farmland). It proceeds faster because soil, seeds, and root systems are already present.

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Section 2: Transfers of Energy and Matter (C4.2)

2.1 Food Chains and Food Webs

A food chain shows a single pathway of energy transfer from one organism to the next. A food web is a network of interconnected food chains showing the feeding relationships in a community.

2.2 Energy Flow Through Ecosystems

Energy enters most ecosystems as sunlight and is converted to chemical energy by producers through photosynthesis. Energy flows through trophic levels but is not recycled — it is lost at each transfer.

2.3 Pyramids of Ecology

Pyramids of energy show the rate of energy flow (in $\text{kJ m}^{-2} \text{yr}^{-1}$) at each trophic level. They are always upright because energy is lost at each transfer.

Pyramids of biomass show the total mass of organisms at each trophic level. They are usually upright but can be inverted (e.g. in ocean ecosystems where phytoplankton biomass at any one time is less than zooplankton biomass because phytoplankton reproduce extremely rapidly).

Pyramids of numbers show the number of individuals at each trophic level. These are often irregular (e.g. one oak tree supports millions of caterpillars).

2.4 Productivity

Gross primary productivity (GPP) is the total rate of energy fixation (photosynthesis) by producers in an ecosystem.

Net primary productivity (NPP) is the rate of energy storage in plant biomass after accounting for respiratory losses:

$\text{NPP} = \text{GPP} - R$

Where $R$ is the energy lost through cellular respiration by the producers.

NPP represents the energy available to the next trophic level (primary consumers).

2.5 The Carbon Cycle

Carbon is recycled through ecosystems via biological, geological, and chemical processes.

2.6 The Nitrogen Cycle

Nitrogen is essential for amino acids, proteins, and nucleic acids. Although $\text{N}_2$ makes up 78% of the atmosphere, most organisms cannot use it directly — it must be fixed into usable forms.

2.7 Bioaccumulation and Biomagnification

Bioaccumulation is the build-up of a persistent, non-biodegradable substance (e.g. DDT, mercury, PCBs) within an organism over its lifetime. The substance is absorbed faster than it can be metabolised or excreted.

Biomagnification is the increasing concentration of a substance at each successive trophic level in a food chain. Top predators accumulate the highest concentrations.

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Section 3: HL Extensions — Diversity Indices & Statistical Tests

3.1 Simpson’s Diversity Index

Biodiversity can be quantified using the Simpson’s Diversity Index. This measures species richness (number of species) and evenness (how equally individuals are distributed among species).

$D = 1 - \sum\left(\frac{n}{N}\right)^2$

Where:

• $D$ = diversity index (ranges from 0 to 1; higher values = greater diversity)
• $n$ = number of individuals of each species
• $N$ = total number of all individuals

3.2 Chi-Squared Test ($\chi^2$)

The chi-squared test is used to determine whether there is a statistically significant association between two categorical variables (e.g. species distribution and an abiotic factor like soil type).

$\chi^2 = \sum \frac{(O - E)^2}{E}$

Where:

• $O$ = observed frequency
• $E$ = expected frequency (calculated assuming no association)

3.3 Mark-Recapture Assumptions and Limitations

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Section 4: Exam Strategy & Common Mistakes

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Mixed Practice — Exam Style

1. [Energy Flow] In a food chain, which trophic level contains the most energy?

   A. Tertiary consumers (top predators)

   B. Secondary consumers

   C. Primary consumers (herbivores)

   D. Producers (autotrophs)

2. [Population Growth] A population of bacteria is growing in a flask with limited nutrients. The growth curve is best described as:

   A. Exponential growth only, continuing indefinitely

   B. Logistic growth — initially exponential, then levelling off at carrying capacity

   C. Linear growth at a constant rate

   D. Immediate plateau — growth rate is constant from the start

3. [Nitrogen Cycle] Which process converts atmospheric $\text{N}_2$ into a form usable by plants?

   A. Nitrification

   B. Denitrification

   C. Nitrogen fixation

   D. Ammonification

4. [Ecological Pyramids] Which type of ecological pyramid is always upright?

   A. Pyramid of numbers

   B. Pyramid of biomass

   C. Pyramid of energy

   D. All three types are always upright

5. [Simpson’s Diversity Index] A habitat has 4 species with the following numbers: 90, 5, 3, 2. Another habitat has 4 species with: 25, 25, 25, 25. Which statement is correct?

   A. The first habitat has greater diversity because it has more total individuals

   B. The second habitat has greater diversity because individuals are more evenly distributed among species

   C. Both habitats have equal diversity because they have the same number of species

   D. Diversity cannot be compared without knowing the species names

6. [Succession] Which of the following best describes primary succession?

   A. Regrowth of a forest after a fire, using existing soil and seed bank

   B. Colonisation of bare rock by pioneer species, leading gradually to a climax community

   C. A farmer abandoning a field, which gradually returns to forest

   D. The replacement of one dominant species by another after a disease outbreak

7. [Bioaccumulation] A persistent pesticide is found at highest concentrations in:

   A. Producers (phytoplankton)

   B. Primary consumers (zooplankton)

   C. Secondary consumers (small fish)

   D. Tertiary consumers (fish-eating birds)

8. [Carbon Cycle] Which process removes $\text{CO}_2$ from the atmosphere?

   A. Respiration

   B. Combustion of fossil fuels

   C. Photosynthesis

   D. Decomposition

9. [Community Interactions — Distractor] Two species of finch on an island both eat medium-sized seeds. Over time, one species evolves a larger beak and shifts to eating larger seeds. This is an example of:

   A. Mutualism — both species benefit

   B. Parasitism — one species exploits the other

   C. Resource partitioning driven by interspecific competition — the competitive exclusion principle leads to niche differentiation

   D. Predation — one species preys on the other

10. [Mark-Recapture — Distractor] In a mark-recapture study, a researcher marks 50 fish and releases them. In the second sample, they catch 60 fish, of which 5 are marked. The estimated population size is:

    A. 110

    B. 300

    C. 600

    D. 3000

Show Answers

1. D — Producers. Energy is lost at each trophic level, so producers always contain the most energy. The 10% rule means each successive level has approximately one-tenth of the previous level’s energy.

2. B — Logistic growth. With limited nutrients, the population initially grows exponentially when resources are abundant but eventually levels off as nutrients become limiting and the carrying capacity is reached. A (exponential only) ignores resource limitation.

3. C — Nitrogen fixation. This is the conversion of $\text{N}_2$ gas into $\text{NH}_3$ / $\text{NH}_4^+$ by nitrogen-fixing bacteria (e.g. Rhizobium) or lightning. Nitrification (A) converts $\text{NH}_4^+$ to $\text{NO}_3^-$. Denitrification (B) converts $\text{NO}_3^-$ back to $\text{N}_2$.

4. C — Pyramid of energy. Energy is always lost at each trophic level, so energy pyramids must be upright. Pyramids of numbers can be inverted (e.g. one tree supporting many insects). Pyramids of biomass can be inverted (e.g. ocean ecosystems with fast-reproducing phytoplankton).

5. B — The second habitat has greater diversity. Simpson’s Diversity Index accounts for both species richness AND evenness. The first habitat is heavily dominated by one species (90 out of 100), giving low evenness and a low $D$ value. The second habitat has perfectly even distribution, giving the maximum $D$ for 4 species. Total number of individuals does not directly determine diversity.

6. B — Primary succession starts on bare substrates (rock, lava, new sand) where no soil exists. Pioneer species (lichens, mosses) colonise first, gradually building soil and enabling later species. A and C describe secondary succession (soil already present). D describes a single species replacement, not full succession.

7. D — Tertiary consumers. Biomagnification means persistent, fat-soluble toxins become more concentrated at each successive trophic level. Top predators accumulate the highest concentrations because they consume many organisms that have themselves accumulated the toxin.

8. C — Photosynthesis. Producers absorb $\text{CO}_2$ and fix it into organic compounds ($\text{C}_6\text{H}_{12}\text{O}_6$). Respiration (A), combustion (B), and decomposition (D) all release $\text{CO}_2$ into the atmosphere.

9. C — Resource partitioning. When two species compete for the same resource, the competitive exclusion principle predicts one will be displaced unless they differentiate their niches. Character displacement (evolution of differing beak sizes) reduces competition by allowing each species to exploit different food sources. This is not mutualism (both species don’t benefit from competition) or predation.

10. C — Using the Lincoln Index: $N = \frac{n_1 \times n_2}{m_2} = \frac{50 \times 60}{5} = \frac{3000}{5} = 600$. Note the common distractor D (3000) is obtained by multiplying numerator values without dividing. Always apply the formula carefully.

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May 2026 Prediction Questions

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Question 1 — Energy Flow and Ecological Efficiency [8 marks]

The table shows the energy content (kJ m$^{-2}$ yr$^{-1}$) at each trophic level in a grassland ecosystem.

Trophic Level	Energy (kJ m$^{-2}$ yr$^{-1}$)
Producers	40,000
Primary consumers	4,000
Secondary consumers	480
Tertiary consumers	52

(a) Calculate the trophic efficiency between producers and primary consumers. Show your working. [2 marks]

(b) Trophic efficiency between primary and secondary consumers differs from your answer in (a). Suggest two reasons why energy transfer efficiency varies between trophic levels. [2 marks]

(c) Explain why food chains rarely exceed five trophic levels. [2 marks]

(d) Evaluate the claim that “eating lower on the food chain is always more energy-efficient for humans.” [2 marks]

Model Answer:

(a) Trophic efficiency = (energy at next level / energy at current level) x 100 = (4000 / 40000) x 100 = 10%. (2)

(b) Any two from: different respiratory losses at each level (ectotherms vs. endotherms); different proportions of biomass that are indigestible (e.g. more cellulose in plants than in animal prey); different feeding efficiency or foraging behaviours; differences in reproductive strategy affecting proportion of energy used for growth. (1 mark each, max 2)

(c) Energy is lost at each trophic level — approximately 90% is lost as heat (respiration), waste, and indigestible material (1); so by the time you reach a fifth or sixth trophic level, the available energy is too low to support a viable population of predators (1).

(d) In favour: consuming plants directly (first trophic level) avoids the 90% energy loss at each trophic step, so theoretically more people can be fed from the same land area (1). However, not all land is suitable for crop production — some is only suitable for grazing, so livestock can convert non-human-edible plant matter into food; also, human nutritional requirements include specific amino acids and micronutrients that may not all be efficiently obtained from plants alone (1). Accept any well-reasoned balanced evaluation.

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Question 2 — Nitrogen Cycle and Human Impact [7 marks]

(a) Draw and annotate a diagram of the nitrogen cycle, including the following processes: nitrogen fixation, nitrification, denitrification, decomposition/ammonification, and assimilation by plants. [4 marks]

(b) Excessive use of nitrogen fertiliser in agriculture can lead to eutrophication in nearby water bodies. Outline the sequence of events by which this occurs and its ecological consequences. [3 marks]

Model Answer:

(a) Award 1 mark each for any four of: Nitrogen fixation — $\text{N}_2$ in atmosphere converted to $\text{NH}_3$ / $\text{NH}_4^+$ by Rhizobium bacteria in root nodules or free-living bacteria like Azotobacter / by lightning (1); Nitrification — $\text{NH}_4^+$ converted to $\text{NO}_2^-$ then $\text{NO}_3^-$ by nitrifying bacteria (1); Assimilation — plants absorb $\text{NO}_3^-$ / $\text{NH}_4^+$ through roots and incorporate nitrogen into amino acids and nucleotides (1); Decomposition/ammonification — decomposers break down organic nitrogen in dead organisms into $\text{NH}_4^+$ (1); Denitrification — $\text{NO}_3^-$ converted to $\text{N}_2$ by denitrifying bacteria in anaerobic conditions, returning $\text{N}_2$ to the atmosphere (1). Max 4.

(b) Nitrates leach from fields into waterways (1); algae and cyanobacteria grow rapidly in response to the nutrient influx (algal bloom) (1); algal bloom reduces light penetration, causing submerged plants to die; decomposers break down dead organic matter using aerobic respiration, depleting dissolved oxygen (BOD increases); fish and invertebrates die from oxygen depletion (1).

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Question 3 — Biodiversity and Invasive Species [6 marks]

(a) Define biodiversity, distinguishing between species richness and species evenness. [2 marks]

(b) Explain how the introduction of an invasive species can reduce native biodiversity, using a named example. [2 marks]

(c) A researcher uses Simpson’s Diversity Index ($D$) to compare two habitats before and after the introduction of an invasive species. Predict how $D$ would change and explain your reasoning. [2 marks]

Model Answer:

(a) Biodiversity is the variety of life in an area (1). Species richness is the total number of different species present. Species evenness refers to how equally individuals are distributed among those species — a community where all species are equally abundant has greater evenness (1).

(b) Any named invasive species with correct explanation, e.g. cane toads (Rhinella marina) introduced to Australia killed native predators (quolls, goannas) that attempted to eat them because the toads are toxic — native predators lacked evolutionary adaptations to recognise or tolerate the toxin (1, 1). Accept: grey squirrels outcompeting red squirrels in the UK for food and nest sites, also spreading squirrelpox virus; water hyacinth (Eichhornia crassipes) blocking light and oxygen in waterways. Must name the species and explain the mechanism (2).

(c) $D$ would decrease (become closer to 0) (1); because the invasive species typically dominates numerically (reducing evenness) and may cause local extinction of native species (reducing richness) — both of which lower the $D$ value in Simpson’s index (1).

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IB Biology HL — Ecology: Populations, Communities & Energy Transfer — Complete Study Guide — 2025 Syllabus — Good luck!