19.4 Populations

2026 Syllabus Objectives

  1. Describe a population as a group of organisms of one species, living in the same area, at the same time
  2. Describe a community as all of the populations of different species in an ecosystem
  3. Describe an ecosystem as a unit containing the community of organisms and their environment, interacting together
  4. Identify and state the factors affecting the rate of population growth for a population of an organism, limited to food supply, competition, predation and disease
  5. Identify the lag, exponential (log), stationary and death phases in the sigmoid curve of population growth for a population growing in an environment with limited resources
  6. Interpret graphs and diagrams of population growth
  7. Explain the factors that lead to each phase in the sigmoid curve of population growth, making reference, where appropriate, to the role of limiting factors

Key Ecological Terms 🌍

https://cambridgeassistant.com

Population

A population is defined as a group of organisms of one species, living in the same area, at the same time.

Key characteristics:

  • All members belong to the same species
  • They occupy the same geographical area
  • They exist at the same time
  • Members can potentially interbreed

Community

A community consists of all of the populations of different species in an ecosystem.

Key characteristics:

  • Contains multiple species
  • Includes all populations living together
  • Organisms interact with each other through feeding relationships, competition, and other ecological interactions

Ecosystem

An ecosystem is a unit containing the community of organisms and their environment, interacting together.

Key characteristics:

  • Includes both living components (biotic factors) - the community
  • Includes non-living components (abiotic factors) - the environment
  • Organisms and environment interact with each other
  • Energy flows and nutrients cycle within the ecosystem

Population Growth and the Sigmoid Curve 📈

https://cambridgeassistant.com

Understanding Population Growth Through Experiments

Scientists study population growth by observing organisms that reproduce quickly and are easy to grow, such as bacteria and yeast. These experiments help us understand population dynamics in all organisms, including humans.

The Yeast Population Experiment

When a few yeast cells are placed into a container of nutrient broth, they feed on the broth, grow, and reproduce. By counting the yeast cells every few hours, we can observe a characteristic S-shaped curve called a sigmoid growth curve.

📌 Key Definition: A sigmoid growth curve is an S-shaped curve showing the change in the size of a population through all the phases in population growth.

The Four Phases of Population Growth

The sigmoid growth curve consists of four distinct phases:

1. Lag Phase ⏱️

Definition: The lag phase is the stage at the start of a population growth curve where the population remains small and grows only very slowly.

Time period in yeast experiment: First 12 hours

Characteristics:

  • Population growth is slow
  • There are not many cells present to reproduce
  • Organisms need time to adjust to the new conditions
  • Birth rate is low

Explanation: During this phase, the small number of organisms are adapting to their new environment. They must synthesize enzymes, absorb nutrients, and prepare for reproduction. Since there are few individuals capable of reproducing, population growth remains minimal.

2. Exponential (Log) Phase 🚀

Definition: The exponential phase (also called the log phase) is the stage in a population growth curve where the population grows at its maximum rate; birth rate exceeds death rate.

Time period in yeast experiment: 12-30 hours

Characteristics:

  • Population grows very rapidly
  • Growth follows exponential pattern: 2→4→8→162 \rightarrow 4 \rightarrow 8 \rightarrow 16
  • No limiting factors are restricting growth
  • Resources (food, space) are abundant
  • Birth rate is at its maximum
  • Death rate is minimal

Explanation: Once organisms have adapted to their environment, they reproduce at their maximum possible rate. Each cell divides to form 2, then 4, then 8, then 16. The only factor limiting growth is the time it takes for cells to grow and divide. There are no environmental constraints holding them back.

3. Stationary Phase ⚖️

Definition: The stationary phase is the stage in a population growth curve where the population remains roughly constant; birth rate equals death rate.

Time period in yeast experiment: 30-54 hours

Characteristics:

  • Population size levels off and remains constant
  • Birth rate equals death rate
  • Limiting factors begin to affect the population
  • Competition for resources increases

Explanation: As the population becomes larger, organisms can no longer reproduce as fast. Several factors contribute to this:

  • Food supply becomes limited - competition for nutrients increases
  • Waste products accumulate - in the yeast experiment, ethanol builds up and becomes toxic
  • Space becomes limited
  • Cells are dying off as fast as new ones are being produced

The population size stabilizes because:

birth rate=death rate\text{birth rate} = \text{death rate}

4. Death Phase ☠️

Definition: The death phase is the final stage in a population growth curve where the population falls; death rate exceeds birth rate.

Time period in yeast experiment: 54+ hours onwards

Characteristics:

  • Population size decreases
  • Death rate exceeds birth rate
  • Resources are severely depleted
  • Toxic waste products accumulate
  • Environmental conditions deteriorate

Explanation: Eventually, conditions become so unfavorable that organisms die faster than they can reproduce. In the yeast experiment, this occurs because food has been exhausted and toxic ethanol has accumulated to lethal levels. The number of living yeast cells in the population starts to fall.

The population size decreases because:

death rate>birth rate\text{death rate} > \text{birth rate}

Sign in to view full notes