Ecosystems and Communities

GCSE Biology · Ecology

Key Ecology Terms

Term	Definition
Ecosystem	The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment
Habitat	The place where an organism lives (e.g. pond, woodland, rock pool)
Population	All the organisms of one species living in a habitat
Community	All the populations of different species living and interacting in a habitat
Abiotic factors	Non-living factors (e.g. temperature, light, water, pH, wind, CO₂ concentration, soil minerals)
Biotic factors	Living factors (e.g. food availability, predation, competition, disease, new predators)

Interdependence

Organisms in an ecosystem depend on each other — this is called interdependence. If one species is removed, it affects others.

Example: If bees decline → fewer plants pollinated → less food for herbivores → less food for predators → ecosystem destabilised.

Competition

Organisms compete for limited resources.

Animals compete for:

Food and water
Territory and shelter
Mates

Plants compete for:

Light (for photosynthesis)
Water and mineral ions (from soil)
Space (to spread roots and leaves)

Intraspecific competition = within the same species (more intense because individuals need the same resources)

Interspecific competition = between different species

Predator-Prey Relationships

The populations of predators and prey are linked and cycle over time:

1. Prey population increases (plenty of food, few predators)

2. More prey = more food for predators → predator population increases

3. More predators = more prey eaten → prey population decreases

4. Less prey = less food for predators → predator population decreases

5. Fewer predators → prey population starts to increase again

6. The cycle repeats

The predator curve follows the prey curve with a time lag.

Adaptations

Adaptations are features that help organisms survive in their environment. They evolve by natural selection over many generations.

Structural adaptations (physical features):

Polar bears: thick fur, white colour (camouflage), thick layer of blubber (insulation), large feet (spread weight on ice)
Cacti: thick stem (stores water), spines instead of leaves (reduce water loss), deep roots

Behavioural adaptations:

Migration (birds flying south in winter)
Hibernation (hedgehogs in winter — reduces energy needs)
Nocturnal behaviour (avoiding heat/predators)

Functional (physiological) adaptations:

Desert rats producing very concentrated urine (conserves water)
Bacteria in hot springs having heat-resistant enzymes
Some organisms produce poisons or venom

Extremophiles — organisms adapted to live in extreme conditions (e.g. deep-sea vents, hot springs, very salty water, high pressure).

Required Practical: Sampling Using Quadrats and Transects

Aim: Estimate the population size of a species in a habitat, or investigate how distribution changes across an area.

Using quadrats (random sampling):

1. Place quadrats randomly in the area (use random number generators for coordinates)

2. Count the number of organisms of the target species in each quadrat

3. Calculate the mean number per quadrat

4. Estimate total population: mean per quadrat × (total area ÷ quadrat area)

Using a transect (systematic sampling):

1. Lay a tape measure in a straight line across the area being studied (e.g. from a pond to dry land)

2. Place quadrats at regular intervals along the transect

3. Record the species present in each quadrat

4. This shows how species distribution changes across the area (e.g. in response to light or moisture)

Improving reliability:

Use a large number of quadrats
Sample randomly to avoid bias
Repeat the investigation and calculate means
Use the same quadrat size

Measuring Abiotic Factors

Factor	How to measure
Temperature	Thermometer
Light intensity	Light meter
pH	pH probe or universal indicator
Wind speed	Anemometer
Moisture level	Moisture meter in soil

Exam Tips

Interdependence means a change in one population affects others — always give a chain of effects in your answer
In predator-prey graphs, the predator peak comes after the prey peak (time lag)
For the quadrat practical, always emphasise random placement to avoid bias
Know examples of structural, behavioural, and functional adaptations — use specific named organisms