Key Idea: Topic 3.8 is about how living things are organised into populations and communities, and what decides where they live, how many there are, and how they affect one another. The single thread running through it is interaction. The levels. A population is all of one species in an area; many populations interacting make a community; add the non-living (abiotic) surroundings and you have an ecosystem. Where species live. Each population is found only where conditions suit it — set by abiotic factors (temperature, light, water) and biotic factors (food, competition, predators). How many there are. Populations grow along a sigmoid curve until limiting factors cap them at the carrying capacity. We estimate population size by sampling (quadrats or capture–mark–release–recapture). How species affect each other. Pairs of species sit in named interspecific relationships (mutualism, competition, predation, herbivory, parasitism, pathogenicity); two species cannot share one niche indefinitely (competitive exclusion); and a single keystone species can hold a whole community together. This topic is a regular on Paper 1 (define a term, identify a relationship from its +/− signs, read a growth curve, do a Lincoln-index calculation) and a favourite on Paper 2 / Paper 3 (explain distribution, a limiting factor, an interspecific relationship, or the loss of a keystone species from a data table).
🌍 Populations, communities & ecosystems
Ecology builds up in levels. A population is all the individuals of one species in an area at one time. Put many different populations together — interacting through feeding and competition — and you have a community (the living part only). Add the abiotic (non-living) environment it interacts with, and the community becomes an ecosystem. Two more terms: a habitat is simply the place a species or community lives, and species richness is the number of different species present (a count, not a measure of how many of each). A community is never fixed — over time it can change through succession.
A community in one picture: every labelled organism is a separate POPULATION, and the feeding arrows are the interactions that link them into one interdependent community. Add the abiotic environment around it and you have the ecosystem.
🔒 Interactive diagram
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| Level | What it includes | Living, non-living, or both? |
|---|---|---|
| Population | all individuals of ONE species, one area, one time | living (one species) |
| Community | ALL the interacting populations of different species | living (many species) |
| Ecosystem | a community PLUS its abiotic (non-living) environment | both living and non-living |
| Habitat | the place / type of environment where it lives | the environment / place |
Nesting order: population → community → ecosystem (one species → many populations → community + environment). The classic diagram trap: an oval enclosing autotrophs, heterotrophs AND the abiotic environment is labelling an ecosystem, not just a community — the non-living part is the giveaway.
🌡️ Abiotic & biotic factors and distribution
A species is not found everywhere — it lives only where conditions suit it. Its distribution (where it is found) is set by two kinds of factor. Abiotic factors are the non-living physical and chemical conditions: temperature, light, water, pH, salinity, oxygen, soil nutrients. Biotic factors are the living interactions: food supply, competition, predation, disease. Each species survives only within its range of tolerance for an abiotic factor; beyond the limits of tolerance it is absent. The limiting factor is the one in shortest supply — relieve it and the population responds (e.g. iron for ocean phytoplankton).
| Feature | Abiotic factor | Biotic factor |
|---|---|---|
| Living or non-living? | non-living (physical / chemical) | living (other organisms) |
| Examples | temperature, light, water, pH, salinity, nutrients | food, competition, predation, disease |
| How it acts | sets the range of tolerance / can be limiting | can exclude a species even where conditions suit it |
| Effect | helps set the species' distribution | helps set the species' distribution |
A-BIOTIC = NOT living (heat, light, water, pH). BIOTIC = living (food, competitors, predators, disease). To explain a distribution, give both kinds of factor — and remember a limiting factor is simply the one in shortest supply.
🔢 Estimating population size
Counting every organism is impossible, so ecologists count a small sample and scale up. Sampling must be random to avoid bias. The method depends on whether the organism moves. Non-moving organisms (plants) → random quadrats: count what is inside several quadrats, find the mean per quadrat, then scale up by area. Moving animals → capture–mark–release–recapture: mark a first sample, release it, recapture a second sample, and use the Lincoln index, N = (M × n) ÷ m, where M = number marked first, n = second sample size, and m = marked ones recaptured.
| Method | Best for | How it estimates the population |
|---|---|---|
| Quadrat sampling | non-moving organisms (plants) | mean number per random quadrat, scaled up to the whole habitat area |
| Capture–mark–release–recapture | animals that move | the proportion marked in a second sample → Lincoln index N = (M × n) ÷ m |
Sit still → quadrat. Runs away → recapture. For the Lincoln index, the two marked numbers go together: M (marked at the start) on top, m (marked when you come back) on the bottom — N = (M × n) ÷ m. The estimate is only valid if the marks aren't lost and the population doesn't change between samples.
📈 Carrying capacity & population growth
Plot population size against time and you get the sigmoid (S-shaped) curve, read in four parts: a slow lag, a rapid exponential rise, a transitional slowing, and a flat plateau. Growth is fast in the exponential phase because resources are plentiful and limiting factors are few, so births greatly outnumber deaths. It slows because limiting factors (shortage of food, water or space; disease; predation) make deaths rise until births ≈ deaths — the plateau at the carrying capacity (K), the largest population the habitat can support. Limiting factors come in two kinds: density-dependent (stronger when crowded — competition, disease, predation) and density-independent (the same at any density — drought, fire, extreme weather).
The sigmoid (S-shaped) growth curve: a slow lag start, a rapid exponential rise while resources are plentiful, then a plateau at the carrying capacity (K) where limiting factors make births ≈ deaths.
🔒 Interactive diagram
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| Phase of the curve | What the population is doing | Why |
|---|---|---|
| Lag | grows slowly at first | few individuals are present, so few are reproducing |
| Exponential | grows rapidly | plenty of resources, few limiting factors → births ≫ deaths |
| Transitional | growth slows down | limiting factors start to bite as it gets crowded |
| Plateau | levels off at K | births ≈ deaths — the carrying capacity is reached |
Density-DEPENDENT depends on the crowd (competition, disease get worse the more there are). Density-INDEPENDENT ignores the crowd (a drought or frost doesn't care how many organisms there are). If a curve's flat top (region X) is labelled, the factor causing it is a limiting factor such as competition for food, as the population reaches its carrying capacity where births ≈ deaths.
🤝 Interspecific relationships
An interspecific relationship is a close interaction between two different species. The quickest way to describe one is a pair of signs — for each species, does it benefit (+), is it harmed (−), or is it unaffected (0)? Only two relationships affect both species the same way: mutualism is the only + / + (both benefit) and interspecific competition is the only − / − (both harmed). The other four are all + / − — one wins, one loses — so you tell them apart by the mechanism: a predator kills and eats prey, a herbivore eats a plant, a parasite lives on a host without quickly killing it, and a pathogen causes disease in its host.
The interspecific relationships, each read as a pair of effects — a green + means that species benefits, a red − means it is harmed. Mutualism is the only +/+, competition the only −/−, and predation, herbivory, parasitism and pathogenicity are all +/−.
🔒 Interactive diagram
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| Relationship | Signs | Quick test |
|---|---|---|
| Mutualism | + / + | both species benefit |
| Interspecific competition | − / − | both harmed, same limited resource |
| Predation | + / − | predator kills and eats prey |
| Herbivory | + / − | an animal eats a plant |
| Parasitism | + / − | parasite feeds on a living host |
| Pathogenicity | + / − | pathogen causes disease in a host |
Mutual = mutual benefit (+ / +). Compete and you both come off worse (− / −). If a question says both organisms are harmed, the answer is almost always competition. For an Explain question, name the relationship and give the benefit/harm to each species — the label alone is only half the marks.
🎯 Competitive exclusion, niche & allelopathy
A species' ecological niche is its full role in the community — the conditions it tolerates, the resources it uses, and how it interacts with others. The competitive exclusion principle says that two species needing the same limited resource cannot coexist indefinitely: the better competitor obtains more, grows faster and excludes the other, which dies out locally or shifts to a different niche. Competition shrinks a species' fundamental niche (its full potential, with no competitors) down to a smaller realized niche (what it actually occupies once competitors restrict it). Some organisms compete with chemicals: a plant that releases a chemical to inhibit other plants is showing allelopathy (e.g. the black walnut); a microbe that inhibits other microbes is showing antibiosis (e.g. a mould making an antibiotic).
Alone, a species fills its full FUNDAMENTAL niche; add a competitor for the same resource and it is squeezed into a smaller REALIZED niche. Two species cannot share one niche indefinitely — the better competitor excludes the other.
🔒 Interactive diagram
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Competition for a shared resource: Two species need the **same limited resource**. The **better competitor** obtains more of it. The poorer competitor's population **declines**. Ends in **competitive exclusion** (one is excluded).
Chemical competition: An organism **releases a chemical** to harm rivals. **Plant → plant** inhibition = **allelopathy**. **Microbe → microbe** inhibition = **antibiosis**. Reduces competition without direct contact.
Fundamental = the full niche; realized = what's really left after competition. Allelopathy = plants at chemical war (the walnut adds a chemical to the soil); antibiosis = anti-microbe chemicals (think antibiotics from moulds). If a population crashes only when grown with another species, the cause is competition, not predation.
🗝️ Keystone species
A keystone species has a disproportionately large effect on its community — far bigger than its abundance would suggest. Even in small numbers it holds the community's structure together (named after the keystone of an arch: small, but pull it out and the whole arch falls). It is not the same as the most abundant / dominant species. A keystone species works in one of two ways. A keystone predator preys on the strongest competitor, keeping it in check so many other species can coexist — so its presence actually raises biodiversity (the classic sea-star-and-mussels example). An ecosystem engineer physically builds the habitat other species depend on (a beaver's dam creates wetland). Remove a keystone species and its control is lost, so a cascade of changes spreads through the community: the species it held in check takes over, crowds others out, and biodiversity collapses.
| Feature | Keystone species | Dominant (abundant) species |
|---|---|---|
| Numbers | often small | very large |
| Size of effect | disproportionately large for its abundance | large, roughly in proportion to its numbers |
| How it acts | keystone predator OR ecosystem engineer | big effect simply because there is so much of it |
| If removed | a cascade spreads → biodiversity collapses | a big gap, but change in proportion to its numbers |
| Example | sea-star predator; the beaver engineer | grass in a grassland; the commonest tree |
Define a keystone species by its EFFECT, not its numbers — a disproportionately large effect relative to abundance, never just 'the most common species'. Loss of a keystone runs the cascade the wrong way: control removed → one species takes over → competitors crowded out → biodiversity falls.
✍️ Worked examples
IB-style question — name the ecological level
A diagram shows an oval that encloses the autotrophs, the heterotrophs and the abiotic environment of an area. State which ecological level the oval represents, and explain how you can tell. [2]
How to score both marks:
Name the level. The oval represents an ecosystem.
Justify with the abiotic part. It encloses the abiotic (non-living) environment as well as the living autotrophs and heterotrophs. A community would be the living organisms only; because the abiotic environment is also included, the level shown is the ecosystem. (Mark 1: ecosystem. Mark 2: it includes the abiotic / non-living environment as well as the living organisms.)
An ecosystem — the oval includes the abiotic (non-living) environment as well as the living organisms, and that non-living part is what makes it an ecosystem rather than just a community.
IB-style question — list factors that set distribution
Different plant species grow at different heights up a mountainside, from the warm valley to the cold summit. List three factors that could influence where these species are distributed. [3]
How to score all three marks:
Give an abiotic factor. Temperature — it falls with altitude, and each species survives only within its range of tolerance for temperature.
Give a second, distinct factor. Water availability / rainfall (abiotic) — too little water excludes species that need moisture.
Give a third (you can mix in a biotic one). Light intensity or wind exposure (abiotic), or competition (biotic). (Award 1 mark for each distinct, correct factor, up to 3 — repeating the same factor reworded scores once.)
Any three distinct factors, e.g. temperature, water availability and light intensity (or competition) — mixing abiotic and biotic factors is fine.
IB-style question — calculate population size
To estimate a beetle population, 60 beetles were caught, marked and released. Later, a second sample of 80 beetles was caught, of which 20 were marked. Calculate the estimated total population size, showing your working. [2]
Model answer:
Write down the Lincoln index. N = (M × n) ÷ m, with M = 60 (marked first), n = 80 (second sample) and m = 20 (marked ones recaptured).
Substitute and calculate. N = (60 × 80) ÷ 20 = 4800 ÷ 20 = 240 beetles. (Mark 1: correct substitution into N = (M × n) ÷ m. Mark 2: correct answer, 240.)
N = (60 × 80) ÷ 20 = 240 beetles (estimated total population).
IB-style question — why a population levels off
When a few organisms colonise a new habitat, the population follows a sigmoid growth curve and eventually levels off at a carrying capacity. Explain why the population stops growing. [3]
How to score all three marks:
Resources become limited. As the population grows and becomes crowded, limiting factors take effect — there is not enough food, water or space for everyone, and waste builds up.
Deaths rise (and births fall). Increased competition, disease and predation raise the death rate, while the birth rate falls.
Births ≈ deaths at K. The population stops growing when births equal deaths — this maximum sustainable size is the carrying capacity (K). (Mark 1: resources / limiting factors. Mark 2: deaths rise / births fall. Mark 3: births ≈ deaths = carrying capacity.)
As the population grows, limited resources and rising competition, disease and predation increase deaths and reduce births until births ≈ deaths — the carrying capacity.
IB-style question — explain a relationship between two species
A hummingbird feeds on the sugary nectar inside a flower; as it feeds, pollen sticks to its head and is carried to the next flower, pollinating it. Explain the type of interspecific relationship between the hummingbird and the flower. [2]
How to score both marks:
Name the relationship. This is mutualism — both species benefit (+ / +).
Justify the benefit to each. The hummingbird gains nectar (food / energy), and the flower gains pollination (so it can reproduce). Because both benefit, it is mutualism. (Mark 1: mutualism. Mark 2: states the benefit to each organism.)
Mutualism — the hummingbird gains nectar (food) and the flower gains pollination; both benefit, so it is + / +.
IB-style question — deduce the cause of a decline
Two protist species both thrive when grown separately on the same bacterial food. Grown together in one flask, one species increases while the other falls to zero. Deduce the cause of the decline. [3]
How to score all three marks:
Identify the interaction. Both species feed on the same food, so they occupy the same niche and compete for that limited resource.
Explain cause and effect. One species is the better competitor, so it obtains more food, grows faster and reproduces more; the other gets too little and its population declines.
Name the principle. Two species cannot share one niche indefinitely — this is competitive exclusion, so the weaker competitor is excluded and falls to zero. (Mark 1: same food / niche → competition. Mark 2: one is the better competitor. Mark 3: competitive exclusion.)
They compete for the same limited food (the same niche); the better competitor gets more, so the other is starved out — competitive exclusion drives it to zero. This is competition, not predation.
IB-style question — consequences of losing a keystone species
In a rocky-shore community, a predatory sea star feeds mainly on mussels. Explain the likely consequences for the community if the sea star is removed. [2]
How to score both marks:
The controlled prey is released. With the sea star gone, the mussels are no longer eaten, so their population grows and they take over the available space on the rock.
Link to the cascade and biodiversity. The mussels out-compete and crowd out other species (algae, barnacles, limpets), so the number of different species falls — biodiversity decreases. (Mark 1: prey / mussels increase and dominate. Mark 2: other species crowded out → biodiversity falls.)
The mussels are no longer controlled, so they multiply and dominate the rock, out-competing other species — the community's biodiversity falls.
✅ Quick self-check
Tap each card to check yourself across all seven micros.
What is the difference between a population, a community and an ecosystem? A population is all the individuals of ONE species in an area. A community is ALL the interacting populations of different species (the living part only). An ecosystem is that community PLUS its abiotic (non-living) environment. Nesting order: population → community → ecosystem.
What sets where a species lives, and what is a limiting factor? Both abiotic factors (non-living conditions: temperature, light, water, pH) and biotic factors (living interactions: food, competition, predation) set a species' distribution. Each species lives only within its range of tolerance; the limiting factor is the one in shortest supply, holding growth back.
How do you estimate population size, and what is the Lincoln index? Sample and scale up. Random quadrats for non-moving organisms (mean per quadrat × number of quadrat areas). Capture–mark–release–recapture for movers: N = (M × n) ÷ m, where M = marked first, n = second sample, m = marked ones recaptured.
Why does a population level off, and what are the two types of limiting factor? Limiting factors raise deaths until births ≈ deaths at the carrying capacity (K), so the sigmoid curve plateaus. Density-dependent factors (competition, disease) get stronger when crowded; density-independent factors (drought, fire, extreme weather) act the same at any density.
How do you describe and name an interspecific relationship? Use a pair of signs (+ / − / 0). Mutualism is the only + / + (both benefit); competition is the only − / − (both harmed). Predation, herbivory, parasitism and pathogenicity are all + / − — tell them apart by the mechanism. For 'explain', name it AND give the effect on each species.
What is competitive exclusion, and how do allelopathy and antibiosis differ? Two species needing the same limited resource cannot share one niche indefinitely; the better competitor excludes the other (competition shrinks the fundamental niche to a smaller realized niche). Allelopathy = a plant releasing a chemical that inhibits other plants; antibiosis = a microbe inhibiting other microbes.
What is a keystone species, and what happens if it is lost? A species with a disproportionately large effect for its abundance — a keystone predator (controls the top competitor, raising biodiversity) or an ecosystem engineer (a beaver builds wetland habitat). Remove it and a cascade spreads: the freed species takes over, crowds others out, and biodiversity collapses.
Visual recap: exponential growth when resources are plentiful, then a levelling-off at the carrying capacity (K) — the largest population the habitat can support.
🔒 Interactive diagram
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Visual recap: name the relationship from its pair of signs first (mutualism +/+, competition −/−, the rest +/−), then use the mechanism — eating prey, eating a plant, living on a host, or causing disease — to pin down which +/− one it is.
🔒 Interactive diagram
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Alone, a species fills its full FUNDAMENTAL niche; add a competitor for the same resource and it is squeezed into a smaller REALIZED niche. Two species cannot share one niche indefinitely — the better competitor excludes the other.
🔒 Interactive diagram
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Visual recap of the levels: the producers and consumers shown here are all populations sharing a habitat → together the community; add the abiotic surroundings → the ecosystem.
🔒 Interactive diagram
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Exam Tips
- Keep the levels precise: population = ONE species; community = many INTERACTING populations (living only); ecosystem = community + the abiotic environment. The oval that also encloses non-living factors is an ecosystem.
- To explain a distribution, give BOTH abiotic (temperature, water, light) and biotic (food, competition, predation) factors; a limiting factor is the one in shortest supply — in a data question, find the single factor whose change made growth rise.
- Pick the sampling method by movement: non-moving → quadrats; moving → capture–mark–release–recapture. For the Lincoln index put the marked numbers right: M on top, m on the bottom (N = (M × n) ÷ m), and always show the substitution AND the final number.
- For a growth curve, the flat top (region X) is caused by a limiting factor at the carrying capacity, where births ≈ deaths. Don't swap density-DEPENDENT (competition, disease — worse when crowded) with density-INDEPENDENT (drought, fire — same at any density).
- Read every interspecific relationship as a pair of signs: mutualism +/+, competition −/−, the rest +/−. 'Both harmed' → competition. For an 'Explain the relationship' mark, name it AND state the benefit/harm to EACH species.
- For competitive exclusion, the two scoring points are 'same niche / same resource' AND 'one species is excluded — cannot coexist indefinitely'. A population that crashes only when grown WITH another is competition, not predation. Allelopathy = plant→plant; antibiosis = microbe→microbe.
- Define a keystone species by its disproportionately large EFFECT, not its numbers. For 'consequences of losing one', give the cause→effect chain: control removed → one species takes over → competitors crowded out → biodiversity falls. The beaver is the go-to ecosystem-engineer example.
Key Idea: Life is organised into populations (one species), communities (many interacting populations, living only) and ecosystems (a community plus its abiotic environment); a habitat is the place it lives and species richness is the number of species. Where a species lives is set by abiotic and biotic factors, within its range of tolerance, with the limiting factor the one in shortest supply. How many there are follows the sigmoid curve to a carrying capacity where births ≈ deaths, capped by density-dependent or density-independent limiting factors — and we estimate the number by quadrats or the Lincoln index, N = (M × n) ÷ m. How species affect each other is captured by a pair of + / − signs: mutualism (+/+), competition (−/−), and predation, herbivory, parasitism, pathogenicity (all +/−). Two species can't share one niche indefinitely (competitive exclusion), some compete with chemicals (allelopathy / antibiosis), and a single keystone species — a predator or an ecosystem engineer — can hold the whole community together, so losing it sends a cascade through the web and biodiversity collapses.