Ch 4 · Populations · IGCSE Biology 0610
Topic 4 of 4 in Organisms and their Environment — IGCSE Biology 0610 — 47 lessons.
Introduction
Put a few yeast cells into a flask of sugar solution, count them every hour, and you will draw one of the most important graphs in ecology: the S-shaped sigmoid growth curve. Every population, from bacteria in a dish to rabbits on an island, grows in this pattern when resources are limited. The human population is the exception that proves the rule: we keep pushing back our limits.
Key ideas — Population, community and ecosystem
Three definitions are examined word for word.
🧑🏫 Teacher's explanation
Think of a population like a single team in a football league. Every player is of the same type (human/player) and they are all on the pitch at once. A community is the entire league—all the different teams playing together in the same stadium. An ecosystem is the stadium itself, including the grass, the weather, the fans, and the lights, all interacting to make the game possible.
📖 Definitions
🔍 Worked example — Easy
A student observes a pond containing 50 frogs, 200 dragonfly larvae, and 15 water lilies. Identify the community in this pond.
🔍 Worked example — Standard
A population of bacteria grows from 100 cells to 10,000 cells in 5 hours. Calculate the percentage increase in the population.
🔍 Worked example — Exam-style
A population of deer follows a sigmoid growth curve. Describe the reasons for the 'stationary phase' in this curve.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one limiting factor that could cause the population to enter the stationary phase. [1] (b) Explain why the population size does not continue to increase indefinitely. [2] [3 marks]
Key ideas — Factors affecting population growth
Whether a population grows, stays steady or shrinks depends on the balance between births and deaths. The syllabus names three factors affecting the rate of population growth:
🧑🏫 Teacher's explanation
Think of a population like a bank account. If you deposit more money (births) than you spend (deaths), your balance grows. If you spend more than you deposit, your balance shrinks. The 'rate' of growth is simply how fast that balance is changing over time.
📖 Definitions
🔍 Worked example — Easy
A population of rabbits starts with 50 individuals. After one year, the population has grown to 150 individuals. Calculate the percentage increase in the population size.
🔍 Worked example — Standard
A population of bacteria is growing in a petri dish. Describe the effect on the population growth rate if the supply of glucose (food source) is suddenly halved.
🔍 Worked example — Exam-style
An ecosystem contains a population of mice and a population of owls (predators). Explain the relationship between the population sizes of these two species over time.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the phase of the sigmoid growth curve where the population size remains constant. [1] (b) Explain why the population growth rate decreases as the population size increases, referring to limiting factors. [3] [4 marks]
Key ideas — The sigmoid growth curve
When a population starts growing in a new environment with limited resources, a graph of population size against time shows four phases:
🧑🏫 Teacher's explanation
Imagine you are throwing a party in a small room. At first, only a few friends arrive, so the room feels empty and nothing changes much (Lag phase). Then, suddenly, everyone arrives at once, and the room fills up incredibly fast (Exponential phase). Soon, the room is so crowded that no one else can fit in, and people are bumping into each other or leaving because it's too hot (Stationary phase). Eventually, if the party keeps going without new guests, people get tired and leave, and the room empties out (Death phase).
📖 Definitions
🔍 Worked example — Easy
A population of bacteria starts with 100 cells. During the exponential phase, the population doubles every 20 minutes. Calculate the population size after 60 minutes.
🔍 Worked example — Standard
A population of rabbits is growing in a field. In the stationary phase, the birth rate is 5 per month and the death rate is 5 per month. If the population is currently 500, what is the net change in population size after 12 months?
🔍 Worked example — Exam-style
A graph shows a population of yeast in a flask. The population increases from 2 × 10^3 to 2 × 10^8 cells. Calculate the percentage increase in population size.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Human population growth
For most of human history the world population grew very slowly and stayed below one billion. Since about 1800 it has risen almost exponentially, passing 8 billion in the 2020s.
🧑🏫 Teacher's explanation
Think of a population like a bank account. If you only spend what you earn, your balance stays the same. But if you earn interest and reinvest it, your money starts growing faster and faster every year. This is 'exponential growth'. In biology, when resources like food and space are unlimited, every new individual added to the population can then produce their own offspring, leading to a massive explosion in numbers.
📖 Definitions
🔍 Worked example — Easy
Calculate the percentage increase in the human population between 1975 and 2000 using the provided data: 1975 = 4.0 billion, 2000 = 6.1 billion.
🔍 Worked example — Standard
A population of bacteria is growing exponentially. If the population is 500 at t=0 and 2000 at t=2 hours, what is the growth factor per hour?
🔍 Worked example — Exam-style
A population graph shows a sigmoid (S-shaped) curve. The population reaches a maximum level and stays constant. Identify the phase and explain why the growth rate becomes zero.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A student added yeast cells to a flask of glucose solution and counted the cells every two hours for 48 hours. The number rose slowly for 4 hours, then very rapidly until 24 hours, stayed constant until 36 hours, and then fell.
🔍 Worked example — Standard
A population of rabbits shows a sigmoid growth curve. During the stationary phase, the birth rate is 5 rabbits per month and the death rate is 5 rabbits per month. Describe the state of the population size.
🔍 Worked example — Exam-style
A population of yeast is grown in a closed flask. The growth curve shows a rapid increase (exponential phase) followed by a plateau (stationary phase). Explain why the population enters the stationary phase using the concept of limiting factors. [Supplement]
⚠️ Common errors
Common mistakes
Practice
Answers