Competition and Interdependence

Organisms within an ecosystem do not exist in isolation. They interact with one another in ways that determine their survival and reproduction. This lesson explores the two key ecological relationships you need to understand for AQA GCSE Biology: competition (both interspecific and intraspecific) and interdependence between species.

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What Is Competition?

Competition occurs when two or more organisms require the same resource that is in limited supply. Resources are anything organisms need to survive and reproduce. Competition is one of the most important factors controlling population size.

There are two types of competition:

Type	Definition	Example
Intraspecific competition	Competition between organisms of the same species	Two robin males fighting over territory in the same garden
Interspecific competition	Competition between organisms of different species	Grey squirrels outcompeting red squirrels for food in UK woodlands

Intraspecific competition is often more intense because organisms of the same species have identical resource requirements.

Exam Tip: Make sure you know the difference between intraspecific and interspecific competition. The prefix intra- means "within" (same species), while inter- means "between" (different species). This distinction commonly appears in exam questions.

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What Do Organisms Compete For?

Plants Compete For:

Resource	Why It Is Needed
Light	Essential for photosynthesis; taller plants shade out shorter ones
Water	Needed for photosynthesis, transport of minerals, and maintaining cell turgor
Space	Roots need space to spread; overcrowding limits growth
Mineral ions	Nitrates for protein synthesis, phosphates for DNA and energy transfer, potassium for enzyme function

Animals Compete For:

Resource	Why It Is Needed
Food	Provides energy for life processes; insufficient food means starvation
Territory	A territory provides access to food, mates, and shelter
Mates	Access to mates is essential for reproduction and passing on genes
Water	Especially important in dry habitats; animals may compete for water sources

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The Impact of Competition

When competition is strong, it can affect organisms in several ways:

1. Reduced growth — organisms that cannot get enough resources grow more slowly
2. Reduced reproduction — fewer resources means less energy available for producing offspring
3. Migration — organisms may be forced to move to new areas with less competition
4. Death — in extreme cases, weaker competitors die, reducing population size
5. Extinction — if a species is consistently outcompeted and cannot adapt, it may become locally extinct

Case Study: Red vs Grey Squirrels in the UK

The native red squirrel (Sciurus vulgaris) has been largely replaced by the introduced grey squirrel (Sciurus carolinensis) across most of England and Wales. This is a classic example of interspecific competition:

Factor	Red Squirrel	Grey Squirrel
Body size	Smaller (250–350 g)	Larger (400–600 g)
Diet	Prefers small seeds from coniferous trees	Can digest acorns and a wider range of foods
Habitat preference	Coniferous and mixed woodland	Deciduous and mixed woodland
Disease susceptibility	Highly susceptible to squirrelpox virus	Carries the virus but is not affected
Current UK range	Mostly Scotland and small areas of northern England	Most of England, Wales and parts of Scotland

The grey squirrel outcompetes the red squirrel for food, and also carries the squirrelpox virus which is lethal to red squirrels but harmless to greys. This demonstrates how interspecific competition and disease can work together to cause population decline.

Exam Tip: The red/grey squirrel example is excellent for 6-mark questions. You can discuss interspecific competition, food resources, disease, and habitat loss all within a single case study.

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Interdependence in Communities

Interdependence means that organisms within a community depend on each other for survival. This creates a web of relationships where changing one part affects the whole system.

Key types of interdependence include:

Relationship	Description	Example
Predation	One organism (predator) kills and eats another (prey)	Foxes eating rabbits
Mutualism	Both organisms benefit from the relationship	Bees pollinating flowers (bees get nectar, plants get pollinated)
Parasitism	One organism (parasite) benefits at the expense of another (host)	Fleas living on a dog
Commensalism	One organism benefits while the other is unaffected	Barnacles growing on a whale

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Predator-Prey Relationships

Predator and prey populations are closely linked and tend to follow cyclical patterns:

graph TD
    A[Prey population increases] --> B[More food available for predators]
    B --> C[Predator population increases]
    C --> D[More prey are eaten]
    D --> E[Prey population decreases]
    E --> F[Less food for predators]
    F --> G[Predator population decreases]
    G --> H[Fewer prey are eaten]
    H --> A
    style A fill:#c8e6c9,stroke:#2e7d32
    style C fill:#ffcdd2,stroke:#c62828
    style E fill:#c8e6c9,stroke:#2e7d32
    style G fill:#ffcdd2,stroke:#c62828

Key Features of Predator-Prey Graphs:

• The prey population peaks before the predator population
• The predator population peaks after the prey population
• There is a time lag between changes in prey numbers and the corresponding change in predator numbers
• The two curves oscillate — they rise and fall in a repeating cycle
• Neither population stays at zero for long (if it did, the cycle would end)

Exam Tip: When interpreting predator-prey graphs, always identify which line is the prey and which is the predator. The prey population always peaks first. Examiners often test whether you understand the time lag between the two curves.

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Mutualism

Mutualism is a relationship where both organisms benefit. Key examples for AQA GCSE Biology include:

Pollination

Many flowering plants depend on insects (especially bees) for pollination. The bee receives nectar (a sugary food source) and pollen, while the plant benefits from having its pollen carried to another flower of the same species, enabling fertilisation.

Nitrogen-Fixing Bacteria and Legumes