Populations grow, crash, compete, cooperate. The maths of ecology starts here.
Every sub-topic below feeds at least one of these questions.
Populations and Communities.
The required syllabus content for C4.1, in order. Each card is one lesson-sized checkpoint.
All individuals of the same species living in a defined area at the same time.
Distinguish population, community, ecosystem
IB Biology DP | C4.1 Populations and Communities | SL & HL
IB Biology DP | C4.1 Populations and Communities | SL & HL
Carrying Capacity (K) — C4.1.5
Density-dependent factors & negative feedback
Sigmoid (S-shaped) Growth — Logistic
Compare sigmoid vs exponential growth
IB Biology DP | C4.1 Populations and Communities | SL & HL
Species 1: Legumes (Fabaceae): peas, beans, clover, soy
Species 1: Orchids (Orchidaceae) — tiny seeds with no endosperm
Species 1: Hard corals (reef-building) — Scleractinia
IB Biology DP | C4.1 Populations and Communities | SL & HL
IB Biology DP | C4.1 Populations and Communities | SL & HL
IB Biology DP | C4.1 Populations and Communities | SL & HL
IB Biology DP | C4.1 Populations and Communities | SL & HL
The release of chemical compounds by a plant that inhibit the germination or growth of neighbouring plants.
Antibiotics — Penicillium/penicillin
The nested hierarchy of ecology: population → community → ecosystem.
All individuals of the same species in one place at one time. Members can interbreed to produce fertile offspring. Reproductive isolation between populations of the same species can lead to speciation.
All populations of different species living and interacting in the same area. Includes interactions: competition, predation, mutualism, parasitism.
A community plus the non-living environment with which it interacts — soil, water, climate, nutrients.
Quadrats for stationary organisms; Lincoln index for mobile ones.
For mobile species:
No population grows forever. Density-dependent factors create negative feedback that stabilises populations around carrying capacity (K).
As population grows toward K, the per-capita availability of resources falls. Disease spreads more easily. Predators concentrate where prey are dense. Intraspecific competition intensifies. Births fall, deaths rise — bringing the population back down. K is not fixed: if resources change, K changes.
Two ways populations grow — one bounded by K, one not. In nature, exponential growth is short-lived.
Lag — small population; slow growth as organisms acclimatise.
Exponential — abundant resources; rapid growth; births >> deaths.
Deceleration — resources become limiting; growth slows.
Plateau — at K; births ≈ deaths.
Each individual contributes the same offspring. Population doubles repeatedly. Only sustained while resources unlimited — early colonisation, invasive introductions, bacterial culture, population recovery from a crash.
Categorised by who benefits and who's harmed.
| Relationship | A | B | Example |
|---|---|---|---|
| Intraspecific competition | − | − | Male deer fighting for mates |
| Interspecific competition | − | − | Grey vs red squirrels in UK |
| Predation | + | − | Lynx and snowshoe hare |
| Mutualism | + | + | Clownfish & sea anemone |
| Commensalism | + | 0 | Epiphyte on tree |
| Parasitism | + | − | Tapeworm in a mammal |
Three IB-named cases of obligate mutualism, each crucial to an entire ecosystem.
Legumes (peas, beans, clover, soy) host Rhizobium bacteria in root nodules. Bacteria fix N₂ → NH₄⁺ using the nitrogenase enzyme — providing nitrogen the plant needs for amino acids. Plant provides carbohydrates and a protected, oxygen-free environment for nitrogenase.
Orchid seeds have no endosperm — they cannot germinate without fungal partners. Fungi provide water, minerals, organic carbon during germination. Once mature, the plant returns carbohydrates from photosynthesis.
Reef-building corals host photosynthetic dinoflagellate algae in their tissues. Algae supply up to 90% of coral energy; coral provides shelter, CO₂, and nutrients. Coral bleaching = expulsion of zooxanthellae under heat stress; without them, coral starves.
Non-native species introduced to a new ecosystem often lack natural predators and competitors. They can spread rapidly and devastate native communities.
Introduced from North America. Outcompetes the native red squirrel for food and habitat. Carries squirrelpox virus (lethal to red squirrels, not grey). Red squirrel populations now restricted to isolated refugia.
From Europe/Asia via ballast water. Filters phytoplankton at huge rates, depleting food for native zooplankton and fish larvae. Outcompetes native mussels for space; colonises water pipes and infrastructure. Ecosystem-wide impact.
Are two species found together more (or less) often than expected by chance? The chi-squared test answers this with a single number.
Method:
A significant result tells you the species are associated — but not why. They might compete (negative association), or co-require the same conditions (positive association).
Hudson Bay Company fur records from the 1800s revealed a striking ~10-year cycle of paired population oscillations.
Lynx peaks always follow hare peaks. Neither population goes extinct — they oscillate around each other indefinitely. Modern analysis shows it's not purely top-down — vegetation availability (bottom-up) also matters.
Populations are controlled by both resource availability (bottom-up) and predators (top-down). The Yellowstone wolf reintroduction is a famous trophic cascade.
Primary productivity → herbivores → predators. Adding resources at the base ripples up the food web.
Predators limit prey populations, which limit vegetation. Remove apex predators → prey explode → vegetation collapses.
Wolves reintroduced in 1995 after 70-year absence. Elk had been overpopulated and overgrazed riparian zones. With wolves back, elk changed behaviour ("landscape of fear") — avoided riverbanks. Willows and aspens recovered → beavers returned → beaver dams stabilised rivers → erosion fell → river meanders reduced. Wolves changed the geomorphology of the park.
Some organisms compete by releasing chemicals that harm rivals. Two named examples — one botanical, one microbial.
Juglans nigra produces juglone in roots, hulls and leaves. Juglone inhibits cellular respiration in many sensitive plants — tomatoes, apples, blueberries can't grow underneath. The tree reduces its own competition.
Penicillium mould produces penicillin — kills competing bacteria by blocking peptidoglycan synthesis. Fleming discovered this in 1928 from clear zones around mould colonies on bacterial culture plates. The first antibiotic, and a form of interspecific chemical competition exploited for medicine.
If you can't define one of these in a sentence, that's where to revise next.