Understanding Populations: The Dynamics of Life 

 

What is a Population?

A population is a group of organisms of the same species, living in the same area at the same time.
For example:

• A herd of zebras in a grassland.
• A school of fish in the ocean.

How Populations Fit Into Communities and Ecosystems

• Community: All the different populations of species in an area (e.g., zebras, lions, grass).
• Ecosystem: The community of organisms interacting with their environment (e.g., grassland, including soil, air, and water).

 

Factors Affecting Population Growth

Factor	Explanation
Food Supply	More food = faster growth; less food = slower growth or population decline.
Competition	Organisms compete for resources like food, water, and space. Intense competition slows growth.
Predation	More predators reduce population size, fewer predators allow prey to increase.
Disease	Diseases can spread quickly in crowded populations, reducing numbers.

 

Population Growth: The Sigmoid (S-Shaped) Curve 

Populations often grow in four distinct phases when resources are limited. This growth is represented by a sigmoid (S-shaped) curve.

Phases of the Sigmoid Curve:

1. Lag Phase

   • What happens: Population grows slowly as individuals adjust to the environment.
   • Reason: Few individuals = limited reproduction and establishment.

2. Exponential (Log) Phase

   • What happens: Rapid population growth due to abundant resources.
   • Reason: Birth rate > death rate, minimal competition or predation.

3. Stationary Phase

   • What happens: Population size stabilizes.
   • Reason: Birth rate = death rate. Resources become limiting (e.g., food or space).

4. Death Phase

   • What happens: Population declines as resources become too scarce.
   • Reason: Death rate > birth rate due to starvation, disease, or competition.

Graph of a Sigmoid Curve

A typical sigmoid curve looks like this: