Variation in Population Size (AQA A-Level Biology): Revision Notes
Variation in Population Size
What is a population?
A population consists of all organisms of the same species living in a particular habitat at the same time. The population size refers to the total number of individuals within this group. Populations within ecosystems are constantly changing - they are dynamic systems that fluctuate in both size and composition over time.
All populations within a community interact with one another, creating complex relationships that influence how population sizes vary.
The interactions between different populations in a community create a web of relationships that can have cascading effects throughout the entire ecosystem. Understanding these connections is crucial for predicting how changes in one population might affect others.
Plotting growth curves
When populations grow slowly over extended periods, standard graphs effectively show population changes. However, when populations experience rapid growth over short timeframes, conventional plotting methods become impractical.
Worked Example: Bacterial Population Growth
Consider bacterial reproduction where cell numbers double every hour:
- Starting point: 50 cells
- After 1 hour: 100 cells
- After 2 hours: 200 cells
- After 5 hours: 1,600 cells
- After 10 hours: 51,200 cells
Plotting such dramatic increases on a standard scale makes it difficult to distinguish early growth phases from the massive numbers reached later.
Using logarithmic scales
Scientists solve this problem by using logarithmic scales to plot population growth. When bacterial numbers are converted to logarithms and plotted against time, all data points can be clearly represented on the same graph. This approach reveals that rapid initial growth typically slows after several hours as conditions become less favourable.
The logarithmic transformation creates a linear relationship during periods of exponential growth, making patterns easier to analyse and interpret.
Understanding population size changes
Population growth initially appears limitless when all necessary resources are abundant and environmental conditions remain optimal. During this phase, there are no limiting factors restricting growth.
However, conditions inevitably change as populations expand:
- Essential nutrients become depleted as more organisms consume available resources
- Overcrowding prevents adequate light penetration to deeper levels (in aquatic environments)
- New competing species may be introduced, either naturally or through human activity
- Seasonal changes bring lower temperatures and reduced light availability
As these challenges accumulate, population growth slows and may eventually stop entirely. Population size may even decline during particularly harsh conditions, such as winter months, before stabilising at a relatively constant level.
Carrying capacity
No population can maintain indefinite growth because environmental factors eventually limit expansion. Every population reaches a maximum sustainable size called the carrying capacity. This represents the largest number of individuals that an environment can support over an extended period.
Critical Concept: Carrying Capacity
The carrying capacity is not a fixed number - it can change based on environmental conditions, resource availability, and seasonal factors. A habitat that supports 100 individuals in summer might only support 60 individuals in winter due to reduced food availability.
The carrying capacity is determined by various limiting factors including food availability, water access, oxygen levels, shelter provision, disease presence, and predation pressure.
Abiotic factors affecting population size
Abiotic factors are non-living environmental conditions that significantly influence population sizes. These physical and chemical factors create the basic conditions within which organisms must survive and reproduce.
Temperature
Each species has an optimum temperature range where it performs most effectively. As environmental temperature deviates from this optimal range, the species becomes less able to maintain essential life processes.
Temperature affects population size by influencing:
- Metabolic rates and energy efficiency
- Reproductive success and breeding cycles
- Food availability and foraging effectiveness
- Survival rates, particularly during extreme weather events
Species can only survive within specific temperature ranges, and these tolerances vary considerably between different organisms. As temperatures move further from optimal conditions, population sizes typically decline due to increased mortality and reduced reproductive success.
Climate change is causing many species to experience temperatures outside their optimal ranges more frequently, leading to significant population changes worldwide. Some species are adapting by shifting their geographic ranges to find more suitable conditions.
Key Points to Remember:
- Population size is dynamic and constantly changing due to various environmental factors
- Logarithmic scales are essential for plotting rapid population growth accurately
- Limiting factors eventually restrict all population growth, preventing indefinite expansion
- Carrying capacity represents the maximum sustainable population size in any environment
- Abiotic factors like temperature create the basic conditions that determine whether populations can survive and thrive