Key Idea: Topic 2.1 explains how we define and classify life, how we identify organisms, and why populations are found where they are (because of abiotic + biotic limiting factors and tolerance ranges).
✅ Must-know definitions
- Organism
- One individual living thing.
- Species
- Organisms that can interbreed to produce fertile offspring.
- Population
- Members of the same species living in the same area at the same time.
- Habitat
- The place where an organism lives.
- Limiting factor
- Any abiotic or biotic factor that restricts population size, growth, or distribution.
🧬 Classification (why it matters)
- Classification organises biodiversity so scientists can identify, predict traits, and communicate using shared names
- Binomial naming: Genus species (Genus capital, species lowercase, italic/underline)
- Taxonomy ladder: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species
- Limits: classifications can change with new evidence (especially DNA)
🔍 Identification (field skills)
- Identification uses observable features (shape, colour/pattern, leaf/flower traits, beak/fin shape, number of legs/wings)
- Dichotomous keys use paired choices at each step to narrow to a species
- Strengths: simple, quick, low cost, portable, systematic
- Limitations: damaged specimens, immature life stages, similar-looking species, user error, limited to included species
Evaluation question? Give one strength + one limitation of dichotomous keys, with a short example (e.g., damaged wing spots).
👥 Populations (exam favourite)
- Population size changes via births, deaths, immigration, emigration
- Distribution patterns: clumped (most common), uniform, random
- Abundance = how common/rare a population is in an area (often estimated by sampling)
🌡️ Abiotic vs biotic limiting factors
Abiotic (non-living): Temperature (enzyme activity/metabolism). Light (photosynthesis). Water availability (all life processes). pH (enzyme function). Salinity (freshwater vs marine tolerance).
Biotic (living): Producers / consumers / decomposers. Predation. Competition. Disease / parasites.
The most limiting factor is the one that has the greatest effect on population size or distribution.
📈 Tolerance curves (range of tolerance)
- Tolerance curve shows performance vs an abiotic factor (often bell-shaped)
- Optimum = peak (best condition)
- Zone of tolerance = range where species survives & reproduces
- Zones of stress = near edges (survives but low growth/reproduction)
- Upper/lower lethal limits = beyond this, cannot survive
- Outside tolerance → factor becomes a limiting factor → distribution shrinks
Graph questions: read optimum from the peak x-value, then give the tolerance range, and identify stress vs lethal zones.
1) Define species (include fertile offspring). 2) Define population (same species/area/time). 3) Name 3 abiotic + 2 biotic factors. 4) Label a tolerance curve.