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)
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)
High niche overlap → **competition increases** → one species may be outcompeted or forced to change resource use.
📉 Population dynamics (patterns you must recognise)
In graphs: predators **lag behind** prey. Explain the lag using reproduction time and food availability.
🧱 Limiting factors & carrying capacity
If the limiting factor improves → **carrying capacity increases**. If it worsens → **carrying capacity decreases**.
📏 Measuring populations (field methods)
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?