Habitat, niche and environmental factors
The habitat is the place where an organism lives: a pond, a forest, the soil. Think of it as an address.
The ecological niche is the organism's role in that place: what it eats, what eats it, where and when it is active, and what conditions it needs. Think of it as a job. Many species share one habitat, but each has its own niche.
Things that affect organisms are called environmental factors:
- Abiotic (non-living): temperature, light, water, humidity, wind, soil type, pH, salt level, oxygen.
- Biotic (living): food organisms, predators, parasites, competitors, partners such as pollinators, and humans.
Dimensions of a niche
A niche has several dimensions (sides). The main three are:
- Trophic (food): what it eats and who eats it. Example: a heron eats fish and frogs.
- Spatial (space): where it lives and feeds, such as treetops, ground, deep water or shallow mud.
- Temporal (time): when it is active: day or night, which season.
Ecologists picture the niche as a box (or a many-sided shape) in a space made of all these factors. Two owls and two hawks may hunt the same mice, but owls hunt at night and hawks by day: their temporal dimension is different.
Ecological tolerance
For every factor, a species has a range of tolerance: a lowest and a highest value it can survive.
- Optimum zone: the best values; growth and reproduction are highest.
- Zones of stress: it can live but grows or breeds poorly.
- Limits: beyond the minimum or maximum, it dies.
A species with a wide range (like rats or cockroaches) can live in many places. A species with a narrow range (like reef-building corals, which need warm, clear, salty water between about 20 and 29 °C) is very sensitive. The factor in shortest supply often decides where a species can live: it is the limiting factor.
Potential vs realised niche, overlap and competitive exclusion
The potential niche (also called the fundamental niche) is all the conditions and resources a species could use if it lived alone. The realised niche is the smaller part it actually uses when competitors, predators and parasites are present.
A classic case: on rocky shores, one barnacle species could live high and low on the rocks, but a stronger barnacle crowds it out of the lower zone. So its realised niche is only the upper zone.
Niche overlap happens when two species need some of the same resources. More overlap means stronger competition.
The competitive exclusion principle: two species with exactly the same niche cannot live together for long in the same place. The better competitor wins; the other dies out locally, moves, or changes its niche.
Species that overlap only partly often share out resources (resource partitioning). For example, several warbler species feed in the same tree but on different heights and parts of the branches.
Bioindicators and studying populations in the field
A bioindicator is a species whose presence, absence or health tells us about the environment. Species with a narrow tolerance are best.
- Lichens: many disappear where air has a lot of sulfur dioxide.
- Mayfly and stonefly larvae: live only in clean, oxygen-rich streams.
- Bloodworms and sludge worms: survive in polluted water with little oxygen.
A population is all the members of one species in one area. Its main features are size, density (number per m² or per hectare), distribution (clumped, even or random), age structure, birth and death rates.
Field methods
- Quadrat: a square frame (for example 1 m × 1 m). Count plants in several random quadrats, then scale up to the whole area.
- Transect: a line across a changing area (shore, forest edge); sample at intervals to see how species change with a factor.
- Capture–mark–recapture for animals that move: population ≈ (first catch × second catch) ÷ marked in second catch.
Key formulas and definitions
- Population density = number of individuals ÷ area
- Estimated population (quadrats) = mean number per quadrat × (total area ÷ quadrat area)
- Mark–recapture estimate N = (M × C) ÷ R
- Key terms: habitat, niche, tolerance range, optimum, limiting factor, potential (fundamental) niche, realised niche, competitive exclusion, resource partitioning, bioindicator
Worked examples
1. A pond has frogs, water lilies and dragonflies. Is 'the pond' a niche or a habitat? Give the frog's niche.
Step 1: The pond is the habitat (where they live). Step 2: The frog's niche: eats insects and worms; eaten by herons and snakes; active mostly at dusk and night; needs water to breed; tadpoles eat algae.
2. A fish species survives between 4 °C and 30 °C and grows best at 15–20 °C. A lake warms to 32 °C in summer. What happens?
Step 1: 32 °C is above its maximum of 30 °C. Step 2: The fish cannot survive there in summer; it must move to cooler, deeper water or it dies. Temperature is the limiting factor.
3. Five 1 m² quadrats in a 200 m² field contain 3, 5, 2, 6 and 4 daisies. Estimate the daisy population.
Step 1: Mean = (3 + 5 + 2 + 6 + 4) ÷ 5 = 4 per m². Step 2: Population ≈ 4 × 200 = 800 daisies.
4. 40 beetles are caught and marked. Later 50 are caught, of which 10 are marked. Estimate the population.
Step 1: N = (M × C) ÷ R. Step 2: = (40 × 50) ÷ 10 = 200 beetles.
5. Two similar seed-eating birds live on one island. Over time one species evolves a thicker beak and eats bigger seeds. Explain.
Step 1: Their niches overlapped a lot, so competition was strong. Step 2: Eating different seed sizes reduces overlap (resource partitioning). Step 3: This lets both coexist instead of one excluding the other.
Common mistakes
- Using 'niche' and 'habitat' as the same word. The habitat is where; the niche is how it lives there.
- Thinking the realised niche is bigger than the potential niche. It is always the same or smaller.
- Thinking competitive exclusion means the losing species always dies. It may move away or change its niche.
- Thinking abiotic means 'not important'. It just means non-living; temperature or water often limit life more than anything else.