Abiotic and Biotic Factors

This lesson covers the abiotic (non-living) and biotic (living) factors that affect communities, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe how these factors influence the distribution and abundance of organisms in an ecosystem.

---

Abiotic Factors

Abiotic factors are non-living physical and chemical conditions that influence where organisms can survive and how well they grow.

Key Abiotic Factors

Factor	Description	Effect on organisms
Light intensity	Amount of light available	Affects rate of photosynthesis in plants; determines where plants grow
Temperature	Warmth of the environment	Affects enzyme activity and metabolic rate; extreme temperatures can denature enzymes
Water availability	Amount of water in the habitat	Essential for all life; plants wilt without water; animals need water for metabolic reactions
Carbon dioxide (CO₂) concentration	Level of CO₂ in air or water	Affects rate of photosynthesis; higher CO₂ can increase plant growth
Soil pH	Acidity or alkalinity of soil	Affects nutrient availability and which plants can grow
Wind speed and direction	Movement of air	Affects transpiration rate in plants; influences temperature and evaporation
Mineral content of soil	Availability of nitrates, phosphates, potassium, magnesium	Plants need minerals for growth; deficiency leads to poor growth
Oxygen levels	Dissolved oxygen in water; atmospheric oxygen	Aquatic organisms need dissolved oxygen; affects decomposition rates

---

How Abiotic Factors Affect Distribution

The distribution of organisms (where they are found) is strongly influenced by abiotic factors:

• Arctic regions — few species can survive the extreme cold; those present have thick insulation and low surface-area-to-volume ratios
• Deep oceans — low light means no photosynthesis, so food webs depend on chemical energy (chemosynthesis) or detritus from above
• Deserts — very low water availability limits plant growth, which limits animal populations

graph TD
    A["Abiotic Factor Changes"] --> B["Light decreases"]
    A --> C["Temperature falls"]
    A --> D["Water decreases"]
    B --> E["Less photosynthesis → fewer plants"]
    C --> F["Slower enzyme activity → slower growth"]
    D --> G["Plants wilt → animals lose food/water source"]
    E --> H["Fewer primary consumers"]
    F --> H
    G --> H
    H --> I["Fewer secondary consumers"]

Exam Tip: When explaining how an abiotic factor affects a community, always make a chain of logic: factor changes → effect on a specific organism → knock-on effect on other organisms.

---

Biotic Factors

Biotic factors are living factors that affect the size and distribution of populations.

Key Biotic Factors

Factor	Description	Example
Predation	One organism (predator) kills and eats another (prey)	Foxes eating rabbits; ladybirds eating aphids
Competition	Organisms compete for the same limited resources	Red and grey squirrels competing for food and nesting sites
Disease	Pathogens (bacteria, viruses, fungi, protists) infect organisms	Ash dieback disease reducing ash tree populations
Food availability	Amount of food present in the habitat	Abundant seeds lead to higher bird populations
New predators or competitors	Introduction of a new species to an ecosystem	Grey squirrels outcompeting native red squirrels in the UK

---

Predator-Prey Relationships

The population sizes of predators and prey are closely linked and follow a cyclical pattern:

1. Prey population increases → more food for predators
2. Predator population increases → more prey eaten
3. Prey population decreases → less food for predators
4. Predator population decreases → less predation pressure on prey
5. Prey population increases again → the cycle repeats

graph LR
    A["Prey increases"] --> B["Predator increases (more food)"]
    B --> C["Prey decreases (more predation)"]
    C --> D["Predator decreases (less food)"]
    D --> A

The predator population curve always lags behind the prey population curve because it takes time for predators to respond to changes in prey numbers.

Exam Tip: In a predator-prey graph, the predator curve peaks after the prey curve. The predator line will never peak first.

---

The Effect of Disease on a Community

When a disease affects one species in a community, it can have widespread effects:

• If a plant species is affected, herbivores that feed on it lose a food source
• If a prey species declines due to disease, predators may also decline
• Other species may benefit — reduced competition for resources

Example: The outbreak of myxomatosis in rabbits in the 1950s caused rabbit populations to crash. This led to:

• Overgrowth of grassland (less grazing)
• Decline in predators such as foxes and buzzards
• Increase in other herbivores due to reduced competition

---

Measuring Abiotic Factors

Scientists measure abiotic factors using specific equipment:

Factor	Equipment
Light intensity	Light meter (or light sensor with data logger)
Temperature	Thermometer (or temperature probe)
Soil pH	pH probe or universal indicator with soil-water mixture
Soil moisture	Moisture meter
Wind speed	Anemometer
Dissolved oxygen	Dissolved oxygen meter

---

Summary

• Abiotic factors (non-living) include light, temperature, water, CO₂, pH, wind, minerals and oxygen.
• Biotic factors (living) include predation, competition, disease, food availability and new species.
• Abiotic factors affect distribution by determining which organisms can survive in a habitat.
• Predator-prey populations follow linked cyclical patterns — the predator curve lags behind the prey curve.
• Disease in one species has knock-on effects throughout the community.
• Abiotic factors are measured with specific equipment (light meter, thermometer, pH probe, etc.).

---

Worked Example: Using Quadrat Density to Link Abiotic Factors to Distribution

A student investigates whether light intensity affects the distribution of a woodland moss. They record light and moss cover at five points along a transect from the woodland edge to the interior.