CIE iGCSE Biology-19.4 Populations- Study Notes- New Syllabus
CIE iGCSE Biology-19.4 Populations- Study Notes – New syllabus
CIE iGCSE Biology-19.4 Populations- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe a population as a group of organisms of one species, living in the same area, at the same time
- Describe a community as all of the populations of different species in an ecosystem
- Describe an ecosystem as a unit containing the community of organisms and their environment, interacting together
- 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
- 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
- Interpret graphs and diagrams of population growth
Supplement
- 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
What Is a Population?
Definition:
A population is a group of organisms of the same species that live in the same area at the same time and are able to interbreed.
📌 Key Features of a Population:
- All individuals belong to the same species (e.g. all oak trees, all rabbits, all lions).
- They live in a specific area (e.g. a forest, a lake, or a field).
- They exist in the same time period.
- They can reproduce with each other (interbreed).
🧠 Example:
All the zebras living on a grassland in Kenya at the same time make up a zebra population.
What Is a Community?
Definition:
A community is made up of all the populations of different species that live and interact in the same ecosystem.
📌 Key Points:
- A community includes plants, animals, fungi, bacteria, and all other living organisms.
- It consists of many different species living together in the same area.
- Organisms interact through:
- Feeding relationships (predator-prey)
- Competition for resources
- Mutual support (e.g. pollination)
🧠 Example:
In a freshwater pond, the community includes:
- Fish
- Frogs
- Algae
- Aquatic plants
- Insects
- Bacteria
All of these different species form the pond’s community.
🔄 Relationship to Other Terms:
| Term | Meaning |
|---|---|
| Population | All individuals of one species in an area |
| Community | All populations of different species in the same area |
| Ecosystem | A community plus the non-living environment (e.g. water, soil, air) |
What Is an Ecosystem?
Definition:
An ecosystem is a unit that includes a community of living organisms (plants, animals, microbes) and their non-living environment (such as air, water, soil), interacting together.
📌 Key Components of an Ecosystem:
- Biotic factors (living): Plants, animals, fungi, bacteria – the community of organisms.
- Abiotic factors (non-living): Sunlight, temperature, water, soil, air, minerals.
🔄 How It Works:
- Organisms interact with each other (feeding, competing, reproducing).
- They also interact with their environment (using resources, responding to changes).
- Together, these interactions form a balanced system where energy and nutrients flow.
🧠 Example Ecosystems:
| Ecosystem Type | Examples |
|---|---|
| Terrestrial ecosystems | Forests, deserts, grasslands |
| Aquatic ecosystems | Ponds, lakes, coral reefs, oceans |
A pond ecosystem includes fish, frogs, algae, water plants, insects (biotic), and also water, sunlight, oxygen, and soil (abiotic).
🔗 Relationship to Other Terms:
| Level | Includes |
|---|---|
| Individual | One organism |
| Population | All organisms of one species |
| Community | All populations of different species in an area |
| Ecosystem | The community plus the physical environment |
Factors Affecting Population Growth
Population growth refers to the change in the number of individuals in a population over time.
The rate of population growth depends on how well organisms survive and reproduce, which is influenced by the following key factors:
🍽️ Food Supply
- A larger or steady food supply allows organisms to grow, survive, and reproduce.
- If food is limited, individuals may starve, and reproduction slows, reducing growth.
- More food → faster growth
- Less food → slower growth or decline
⚔️ Competition
- Organisms compete for limited resources like food, space, mates, or light (in plants).
- Intraspecific competition = between members of the same species
- Interspecific competition = between different species
- High competition reduces survival and reproduction, slowing growth.
- Low competition → stable growth
- High competition → fewer resources → slower growth
🐅 Predation
- Predators reduce the population of prey species by killing and eating them.
- If predator numbers increase, prey population may fall.
- In turn, predator numbers may drop later if there’s less food.
- Note: Predator-prey relationships often cause population sizes to fluctuate in cycles.
🦠 Disease
- Disease can kill individuals or reduce their ability to reproduce.
- In dense populations, disease spreads more quickly, leading to faster population decline.
- Healthy population → faster growth
- Widespread disease → population crash
🧠 Summary Table:
| Factor | Effect on Population Growth |
|---|---|
| Food supply | More food = faster growth; less food = slower growth |
| Competition | High competition = less growth |
| Predation | More predators = lower prey numbers |
| Disease | Disease reduces survival and reproduction rates |
Sigmoid Population Growth Curve
When a population grows in an environment with limited resources, it follows a sigmoid (S-shaped) growth curve. This curve has four distinct phases:
Lag Phase (Slow Growth)
- Population is small and growth is slow.
- Organisms are adjusting to the environment.
- Time is spent on maturing and reproduction hasn’t peaked yet.
🧠 Few individuals → less reproduction → slow increase
Exponential Phase (Log Phase)
- Rapid population growth begins.
- Plenty of resources (food, space, no competition).
- Birth rate is much higher than death rate.
- Population size doubles rapidly — exponential increase.
📈 Ideal conditions → fast growth → steep curve
Stationary Phase (Carrying Capacity Reached)
- Resources become limited (food, space, shelter).
- Birth rate ≈ Death rate
- Population growth slows and stabilizes at the carrying capacity.
⚖️ Growth levels off → stable population size
Carrying capacity = maximum individuals the environment can support.
Death Phase (Decline) (optional but common if conditions worsen)
- If resources become too scarce or waste builds up, population starts to decline.
- Death rate exceeds birth rate.
- Often caused by overcrowding, lack of food, disease, or pollution.
🔻 Environmental stress → declining population
📊 Summary Table:
| Phase | Description |
|---|---|
| Lag Phase | Slow start, few individuals, low reproduction |
| Exponential Phase | Rapid growth, ideal conditions, fast population increase |
| Stationary Phase | Growth levels off, limited resources, population stabilizes |
| Death Phase | Population declines due to resource depletion or disease |
📌 Final Note:
This S-shaped curve helps ecologists understand how populations grow, and why they can’t grow indefinitely in a limited environment.
Interpreting Population Growth Graphs & Diagrams
What Do These Graphs Show?
Graphs of population growth usually plot:
- Population size on the Y-axis
- Time on the X-axis
They help us understand how populations change over time and how factors like food, competition, and predators affect growth.
📈 1. Sigmoid (S-Shaped) Curve – Limited Resources
🧠 Key Phases to Identify:
| Phase | What It Looks Like on the Graph |
|---|---|
| Lag phase | Flat or slow rise at the beginning |
| Exponential phase | Steep upward curve |
| Stationary phase | Curve flattens at the top |
| Death phase (if present) | Curve begins to fall (decline) |
Interpretation Example:
A sharp rise followed by a plateau means the population is likely reaching carrying capacity due to limited resources.
🐺 2. Predator-Prey Population Graph
These graphs show two curves – one for prey, one for predator.
🔄 Key Pattern:
- Prey numbers rise → Predator numbers increase (more food)
- Too many predators → Prey decrease
- Fewer prey → Predators decline
- Cycle repeats
Prey curve always peaks before the predator curve.
Interpretation Tip:
The predator population follows the prey population but never exceeds it for long.
🌊 3. Fluctuating Population Graphs
These show ups and downs due to changing environmental factors like seasons, food supply, or disease.
Interpretation Tips:
- Sharp drops: Could be due to disease, harsh weather, or human hunting
- Sudden rises: Favourable conditions like good rainfall or abundant food
- Long-term stability: Population is near carrying capacity
🧠 General Tips for Interpreting Graphs:
| Look For… | It Might Mean… |
|---|---|
| Steep upward slope | Rapid population growth (exponential phase) |
| Flattening curve | Reached carrying capacity (stationary phase) |
| Declining curve | Resource shortage, disease, predation, or human activity |
| Repeating wave-like patterns | Predator-prey or seasonal population cycles |
Final Summary:
To interpret population graphs, always:
- Identify phases or trends
- Link patterns to real biological factors like food, predators, or space
- Understand why population size is increasing, stabilising, or declining
Factors Influencing Each Phase of the Sigmoid Curve
The sigmoid curve describes how a population grows in an environment where resources are limited. It has four distinct phases, each influenced by biological and environmental factors.
Lag Phase – Adjustment Period
- What Happens: Population is small. Individuals are adjusting to the environment. Growth is slow.
- Why? Time is needed for maturity and reproduction. Conditions may be unfamiliar; mates may be scarce.
- Limiting factors: Adaptation time, low reproductive rate, few individuals
Exponential (Log) Phase – Rapid Growth
- What Happens: Population grows rapidly. Birth rate exceeds death rate. Resources are plentiful.
- Why? Little competition, ideal conditions, maximum reproductive potential.
- Limiting factors: Minimal or absent at this stage
Stationary Phase – Carrying Capacity Reached
- What Happens: Growth levels off. Birth rate equals death rate. Population reaches environmental limit.
- Why? Resources become limited. Competition, disease, and waste accumulation increase.
- Limiting factors: Food shortage, space, disease, predation, waste build-up
Death Phase – Population Decline
- What Happens: Population declines. Death rate exceeds birth rate. Conditions deteriorate.
- Why? Resources are exhausted, stress rises, disease or pollution may increase.
- Limiting factors: Strong and overwhelming → unsustainable population
🧠 Final Summary Table:
| Phase | Key Factor(s) | Limiting Factors? |
|---|---|---|
| Lag Phase | Adjustment, slow reproduction | Low mates, unfamiliar environment |
| Exponential Phase | Fast reproduction, ideal conditions | Few or no limiting factors |
| Stationary Phase | Carrying capacity reached, stable size | Food, space, disease, competition |
| Death Phase | Environmental decline, high stress | Severe limitations, collapse risk |
📌 Final Note:
Limiting factors are the main reason populations cannot grow indefinitely. They regulate population size and ensure ecological balance within ecosystems.