Key Idea: Topic 2.3 explains how species share ecosystems (niches + resource use), how populations interact and change over time, what stops populations growing forever (limiting factors + carrying capacity), and how ecologists estimate population size using sampling.
✅ Niche vs habitat (exam favourite)
- Habitat = address (where a species lives)
- Niche = job (how it survives and interacts)
- A niche includes: food + how obtained, time of activity, role (predator/pollinator/decomposer), tolerance to abiotic factors
- Different niches = less competition (species can coexist)
To describe a niche: give feeding role + interaction + conditions needed (e.g., nocturnal predator that hunts rodents, needs forest cover).
🤝 Population interactions (know the outcomes)
- Predation: predator benefits, prey harmed (often creates predator–prey cycles)
- Herbivory: animals eat plants; can cause overgrazing if herbivores too many
- Parasitism: parasite benefits, host harmed
- Mutualism: both benefit (e.g., pollination)
- Competition: demand for same limited resource; intra (same species) vs inter (different species)
- Disease spreads faster at high density → reduces population size
High niche overlap → competition increases → one species may be outcompeted or forced to change resource use.
📉 Population dynamics (patterns you must recognise)
- Population size changes via births, deaths, immigration, emigration
- Predator–prey cycles show oscillations: prey rises first, predators rise after (time lag)
- Negative feedback helps stabilise populations (like a thermostat)
In graphs: predators lag behind prey. Explain the lag using reproduction time and food availability.
🧱 Limiting factors & carrying capacity
- Limiting factor = the factor that restricts population size/distribution
- Carrying capacity (K) = maximum population an environment can support sustainably
- Liebig’s Law: population is limited by the factor in shortest supply (lowest stave in the barrel)
- Density-dependent factors increase with crowding (competition, disease, predation, stress)
- Density-independent factors affect all populations (drought, storms, fire, extreme temperatures)
- J-curve = exponential boom (often followed by crash); S-curve = logistic growth (levels off near K)
If the limiting factor improves → carrying capacity increases. If it worsens → carrying capacity decreases.
📏 Measuring populations (field methods)
- Usually populations are estimated, not fully counted
- Random sampling reduces bias; systematic sampling uses regular intervals
- Transects show change across a gradient; quadrats sample non-mobile organisms (plants, barnacles)
- Quadrats measure density, % cover, and frequency
- Mobile organisms: capture–mark–release–recapture with the Lincoln Index
Lincoln Index: N = (n₁ × n₂) / m
Know assumptions: population closed, marks not lost, marked mix evenly, samples random.
1) Habitat vs niche? 2) What causes competition? 3) Define carrying capacity. 4) Difference between density-dependent and density-independent factors? 5) When do you use quadrats vs Lincoln Index?