Population Dynamics

This lesson covers population growth curves, carrying capacity, density-dependent and density-independent factors, and predator-prey relationships, as required by the Edexcel A-Level Biology specification (9BI0), Topic 10 -- Ecosystems.

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Population Growth

A population is all the organisms of one species living in a particular area at a particular time. The size of a population is determined by the balance between factors that increase it (births, immigration) and factors that decrease it (deaths, emigration).

$\text{Population change} = (\text{Births} + \text{Immigration}) - (\text{Deaths} + \text{Emigration})$Population change=(Births+Immigration)−(Deaths+Emigration)

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Population Growth Curves

Exponential (J-shaped) Growth

When a population colonises a new, resource-rich environment with no limiting factors, it can grow exponentially -- the rate of increase is proportional to the current population size.

Characteristics:

• Unlimited resources available
• No predators, disease, or competition
• Birth rate greatly exceeds death rate
• The population doubles in fixed time intervals
• This cannot continue indefinitely in nature

Logistic (S-shaped / Sigmoid) Growth

In reality, resources are limited. The logistic growth curve has four phases: