Abiotic and Biotic Factors

The distribution and abundance of organisms in an ecosystem are affected by both living and non-living factors. Understanding these factors and how to measure populations is essential for ecology questions in the Edexcel GCSE.

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Abiotic Factors

Abiotic factors are non-living physical and chemical conditions that affect organisms in an ecosystem.

Abiotic Factor	How it affects organisms
Temperature	Affects enzyme activity and metabolic rate; most organisms have an optimum temperature range; extreme temperatures denature enzymes
Light intensity	Affects the rate of photosynthesis in plants; plants grow towards light; affects animal behaviour (e.g. nocturnal vs diurnal)
Moisture/water availability	Essential for all life; affects plant growth and distribution; organisms in deserts are adapted to conserve water
Soil pH	Affects mineral solubility and availability to plants; affects which plant species can grow; acidic or alkaline soils suit different species
Soil mineral content	Plants need minerals (nitrates, phosphates, potassium) for growth; poor mineral content limits plant growth
Wind speed	Increases transpiration in plants; affects pollination; strong winds can physically damage organisms
Carbon dioxide concentration	Affects rate of photosynthesis; higher CO₂ generally increases plant growth (up to a point)
Oxygen levels	Aquatic organisms need dissolved oxygen; polluted or warm water has less dissolved oxygen, affecting fish and invertebrate populations

How Changes in Abiotic Factors Affect Communities

A change in an abiotic factor can:

• Change the distribution of a species (e.g. rising temperature may cause species to move to higher altitudes or latitudes)
• Change population size (e.g. drought reduces plant populations)
• Alter competition (e.g. if one species is better adapted to the new conditions, it may outcompete others)

Exam tip: When explaining how an abiotic factor affects a community, always create a logical chain. For example: "An increase in temperature increases the rate of photosynthesis in producers → more food available → primary consumer population increases → secondary consumer population may also increase."

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Biotic Factors

Biotic factors are living factors that affect organisms in an ecosystem.

Biotic Factor	How it affects organisms
Competition	Organisms compete for limited resources (food, water, territory, mates, light); affects population size and distribution
Predation	Predators kill prey for food; affects prey population size
Disease	Pathogens can reduce population size; can spread rapidly through dense populations
Availability of food	Determines the maximum population size an area can support (carrying capacity)

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Competition

Competition occurs when organisms are trying to use the same limited resource.

Types of Competition

Type	Definition	Example	Intensity
Intraspecific	Competition between members of the same species	Two robins competing for a territory	More intense — individuals need exactly the same resources
Interspecific	Competition between members of different species	Robins and blackbirds competing for worms	Less intense — different species may use slightly different resources

What Organisms Compete For

Animals compete for:

• Food
• Territory (space)
• Mates
• Water
• Shelter

Plants compete for:

• Light (for photosynthesis)
• Water (from soil)
• Mineral ions (from soil)
• Space (for roots and shoots)

Exam tip: Intraspecific competition is more intense than interspecific competition because individuals of the same species have identical resource requirements. This is a commonly tested concept.

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Predator–Prey Relationships

The populations of predators and prey are interdependent — changes in one population affect the other, creating a cyclical pattern.

The Cycle

1. Prey population increases (e.g. lots of food available, few predators)
2. More food for predators → predator population increases
3. More predators → more prey are eaten → prey population decreases
4. Less food for predators → predator population decreases
5. Fewer predators → prey population begins to increase again
6. The cycle repeats

Key Features of Predator–Prey Graphs

• Both populations show oscillating (cyclical) patterns
• The predator peak always follows the prey peak (there is a time lag)
• The predator curve lags behind the prey curve — predator numbers change AFTER prey numbers change
• Neither population ever reaches zero in a stable ecosystem

Exam tip: When interpreting predator–prey graphs, always explain the TIME LAG. The predator population does not increase immediately when prey increases — it takes time for predators to reproduce. Similarly, the predator population does not decrease immediately when prey decreases — it takes time for predators to die or emigrate.

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Sampling Organisms

To study ecosystems, scientists need to estimate population sizes and distribution. Since it is usually impossible to count every organism, we use sampling techniques.

Quadrats

A quadrat is a square frame (usually 0.25 m² or 1 m²) placed on the ground to sample organisms in a defined area.

Method for random sampling:

1. Lay out two tape measures at right angles to form axes along two edges of the area
2. Use a random number generator to generate pairs of coordinates
3. Place the quadrat at each set of coordinates
4. Count the organisms or estimate percentage cover within the quadrat
5. Repeat many times (at least 10 quadrats) for reliability
6. Calculate the mean number of organisms per quadrat

Estimating total population:

Total population = mean number per quadrat × total area ÷ area of one quadrat

Example: Mean count of daisies per 0.25 m² quadrat = 6. Total field area = 500 m².

Total population = 6 × (500 ÷ 0.25) = 6 × 2,000 = 12,000 daisies

Percentage Cover

For plants that are difficult to count individually (e.g. grass, moss), estimate the percentage of the quadrat covered by each species.

Transects

A transect is a line along which organisms are sampled. Transects are used to study how species distribution changes along an environmental gradient (e.g. from a path into a field, from shore into water, up a hillside).

Type	Method
Line transect	Stretch a tape measure along the gradient; record every organism touching the line at regular intervals
Belt transect	Place quadrats at regular intervals along the tape measure; record organisms in each quadrat

Belt transects provide more data than line transects and give a better picture of species distribution.

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Practical: Investigating Population Size

Using Quadrats

Aim: Investigate the population size of a common species in a habitat.