19.4 Populations Revision Notes
1. Overview
This topic explores how groups of organisms of the same species interact within their environment and with other species. Understanding population dynamics is essential for managing natural resources, predicting the impact of human activity on ecosystems, and implementing effective conservation strategies.
Key Definitions
- Population: A group of organisms of one species, living in the same area, at the same time.
- Community: All of the populations of different species in an ecosystem.
- Ecosystem: A unit containing the community of organisms and their environment (abiotic factors), interacting together.
- Limiting Factor: An environmental factor (such as food or space) that restricts the growth, abundance, or distribution of a population.
- Carrying Capacity: The maximum population size of a species that an environment can sustain indefinitely.
Core Content
Factors Affecting Population Growth
The rate at which a population grows is determined by the balance between the birth rate and the death rate. Several key factors limit how fast a population can increase:
- Food Supply: More food allows for higher survival rates and more successful reproduction.
- Competition: Organisms compete for resources like light, water, and nesting sites. Competition can be between members of the same species (intraspecific) or different species (interspecific).
- Predation: An increase in the number of predators will increase the death rate of the prey population.
- Disease: Pathogens spread more easily in crowded populations, increasing the death rate.
The Sigmoid Curve of Population Growth
When a population is introduced to a new environment with limited resources, it follows a specific "S-shaped" growth pattern known as a sigmoid curve.
The Four Phases:
- Lag Phase: The initial period where the population size is small and growth is slow as organisms adapt to their new environment.
- Exponential (Log) Phase: The population grows rapidly. Resources are abundant, and the birth rate significantly exceeds the death rate.
- Stationary Phase: The growth rate slows down and the population size stabilizes. The birth rate equals the death rate.
- Death Phase: The population size begins to decrease. This happens when resources run out or waste products become toxic.
Extended Content (Extended Only)
Explaining the Sigmoid Curve via Limiting Factors
In the extended curriculum, you must explain why the phases of the sigmoid curve occur by referring to limiting factors.
1. Lag Phase
- Reasoning: Growth is slow because there are few reproductive individuals. Organisms may be preparing for growth by synthesizing enzymes or finding mates.
2. Exponential (Log) Phase
- Reasoning: There are no limiting factors yet. The birth rate is at its maximum because food is plentiful, there is little competition, and space is available.
3. Stationary Phase
- Reasoning: The population has reached the carrying capacity of the environment. Limiting factors (e.g., lack of food, accumulation of metabolic waste, or lack of space) mean that the death rate increases until it equals the birth rate.
4. Death Phase
- Reasoning: Limiting factors become severe. For example, in a closed environment (like bacteria in a flask), food runs out and toxic waste (like CO2 or ethanol) builds up to lethal levels. The death rate exceeds the birth rate.
Key Equations
While Biology is less math-heavy than Physics, you may be asked to calculate population changes.
1. Population Growth Rate $$\text{Growth Rate} = (\text{Births} + \text{Immigration}) - (\text{Deaths} + \text{Emigration})$$
2. Percentage Change $$% \text{ Change} = \frac{\text{New Value} - \text{Original Value}}{\text{Original Value}} \times 100$$
- Note: Use this when asked to calculate the percentage increase in a population between two years (e.g., from 52.0 to 100.0 individuals).
Common Mistakes to Avoid
- ❌ Wrong: Defining a population as "all the animals in an area."
- ✅ Right: A population must be specified as "one species" living in the "same area" at the "same time."
- ❌ Wrong: Thinking the "Stationary Phase" means no organisms are being born.
- ✅ Right: In the stationary phase, organisms are still being born, but the birth rate is exactly balanced by the death rate.
- ❌ Wrong: Forgetting the "abiotic" (non-living) part of an ecosystem.
- ✅ Right: An ecosystem includes both the community (biotic) and the environment (abiotic).
Exam Tips
- Command Words:
- "State": Give a brief name or factor (e.g., "State two factors affecting growth" -> "Food and Predation").
- "Describe": Tell the examiner what is happening on a graph (e.g., "The population increases from 10.0 to 200.0 between day 1 and day 5").
- "Explain": Tell the examiner why it is happening (e.g., "The population levels off because food becomes a limiting factor").
- Calculation Practice: Be ready to work with decimals. Past papers frequently use values like 3.0, 25.0, or 52.0. Always show your working to gain marks even if your final answer is slightly off.
- Contexts: Look out for "environmental" contexts (e.g., the growth of fish in a lake) or "wave" contexts (fluctuations in predator-prey cycles).
- Graph Reading: Always check the units on the axes. If the y-axis says "Number of individuals (thousands)," an answer of "5" actually means "5,000."