Hello, Future Environmental Manager!
Welcome to the notes for the chapter on Changes in Population Size. Understanding how and why populations—especially the human population—change is absolutely crucial for environmental management.
Why? Because the number of people on Earth directly affects how we use resources, how much pollution we create, and how we manage our environment. Don't worry if some terms look tricky; we'll break everything down using simple steps and real-world examples!
8.2 The Population Growth Curve: Understanding the Limits
To understand human population change, we first look at the typical growth curve of any biological population (like bacteria in a petri dish or deer in a park). This curve helps us understand the concept of limits.
The S-Shaped (Sigmoid) Growth Curve
Most natural populations follow an S-shaped pattern when they colonise a new area. This curve has three main phases:
1. The Lag Phase
- Description: Growth is very slow at the start.
- Why? The population is small, individuals are getting used to the environment, and finding mates/reproducing takes time.
- Analogy: Imagine moving into a new town. It takes a while to settle in and start making friends (reproducing).
2. The Exponential (Log) Phase
- Description: Growth accelerates rapidly. It looks almost vertical on a graph.
- Why? Resources (food, space, water) are plentiful, competition is low, and the birth rate is much higher than the death rate.
- Did you know? This phase is also called 'logarithmic' growth because the population increases by a constant percentage rate.
3. The Stationary Phase (Reaching Carrying Capacity)
- Description: Growth slows down until it reaches zero. The population size remains stable, fluctuating slightly around a maximum level.
- Why? The population has reached the environment’s maximum capacity. Resources become limiting (e.g., food runs out, waste builds up, or disease spreads easily).
Key Concept: Carrying Capacity (K)
The most important concept here is Carrying Capacity (K).
- Definition: The maximum population size that an environment can sustainably support indefinitely, given the available resources and space.
- If a population temporarily exceeds the carrying capacity, the death rate will likely increase sharply due to resource scarcity, bringing the population size back down.
Quick Review: The Growth Curve
Lag (Slow) → Exponential (Fast) → Stationary (Limit reached at K)
Measuring Changes in Human Populations
For humans, population change is determined by two main factors: natural change and migration.
1. Natural Change (Births and Deaths)
We measure natural change using rates expressed per 1000 people in a year.
Key Definitions:
1. Birth Rate (BR):
The number of live births per 1000 people in a population per year.
2. Death Rate (DR):
The number of deaths per 1000 people in a population per year.
Calculating Natural Change
The difference between the Birth Rate and the Death Rate determines the rate of natural change.
- If BR > DR, there is Natural Increase.
- If DR > BR, there is Natural Decrease.
The Natural Increase Rate (NIR) is usually expressed as a percentage:
$$ \text{NIR (\%)} = \frac{(\text{Birth Rate} - \text{Death Rate})}{10} $$
2. Migration
Population change is also affected by people moving in (immigration) or moving out (emigration).
- Immigration: People moving in to a country. (Increases population size).
- Emigration: People moving out of a country. (Decreases population size).
$$ \text{Total Population Change} = (\text{Births} - \text{Deaths}) + (\text{Immigration} - \text{Emigration}) $$
Key Takeaway
Human population size is a balance between births, deaths, and migration.
Factors Affecting Birth and Death Rates
Human populations do not always grow at the same speed. Development status (whether a country is a More Economically Developed Country, MEDC, or a Less Economically Developed Country, LEDC) heavily influences BR and DR.
Factors Affecting Birth Rates (Why people have children)
1. Infant Mortality Rate (IMR)
- In LEDCs, if many babies die young (high IMR), parents often have more children to ensure some survive into adulthood.
- In MEDCs, where healthcare is excellent and IMR is low, BR tends to drop.
2. Education and Status of Women
- When women are educated, they often choose to pursue careers later in life, delaying marriage and having fewer children.
- Access to Family Planning (contraception and sex education) allows couples to decide when and how many children to have.
3. Economic Factors
- In LEDCs (often agricultural), children are seen as economic assets: they can work on the farm and look after parents in old age.
- In MEDCs, children are economic liabilities: they are expensive to raise, educate, and clothe, leading to smaller families.
4. Cultural and Religious Beliefs
- Some religious or cultural groups oppose contraception or encourage large families for tradition, leading to higher BR.
5. Government Policies
- Antinatalist policies (like China's former One-Child Policy) aim to reduce BR.
- Pronatalist policies (like providing benefits for large families in some European countries) aim to increase BR.
Factors Affecting Death Rates (Why people live longer/shorter lives)
1. Healthcare and Medical Advances
- Access to vaccinations, antibiotics, and skilled doctors dramatically reduces deaths, especially from infectious diseases (e.g., tuberculosis, malaria).
2. Sanitation and Clean Water
- In LEDCs, lack of access to clean water and sanitation causes high death rates due to waterborne diseases (e.g., cholera, typhoid).
- Improved infrastructure in MEDCs ensures low rates of these preventable deaths.
3. Food Security and Nutrition
- Consistent access to enough nutritious food means people are stronger, healthier, and less susceptible to disease. Malnutrition increases DR.
4. War, Conflict, and Natural Disasters
- These events lead to immediate loss of life and often destroy infrastructure (like hospitals and water pipes), causing DR to spike dramatically.
Memory Aid: Development and Rates
Generally, as a country develops (LEDC to MEDC):
Birth Rates go Down.
Death Rates go Down (and then level off).
(The first thing to drop is usually the Death Rate due to simple improvements in healthcare/water.)
Factors Affecting Migration
Migration is driven by Push Factors (reasons to leave a place) and Pull Factors (reasons to move to a place).
Push Factors (The Reasons to LEAVE)
These are the negative aspects of the home country or region.
- Economic: High unemployment, poverty, lack of resources.
- Environmental: Drought, natural disasters, environmental degradation (e.g., desertification).
- Social/Political: War, persecution, lack of freedom, poor healthcare/education facilities.
Example: A farmer leaves his home because of prolonged drought (environmental push factor).
Pull Factors (The Reasons to COME)
These are the positive attractions of the destination country.
- Economic: High wages, job opportunities, economic stability.
- Social/Political: Safety, stability, freedom, good standard of living, excellent schools and healthcare.
- Family Ties: Moving to join relatives who migrated previously.
Example: A highly skilled individual moves abroad for a specific, high-paying job (economic pull factor).
Did You Know?
Migration often balances out populations. People usually move from areas with low carrying capacity or high conflict to areas that can support higher populations and offer better quality of life.
Quick Chapter Review: Changes in Population Size
You Should Now Be Able To:
- Describe the three stages of population growth (Lag, Exponential, Stationary) and define the Carrying Capacity (K).
- Calculate Natural Increase using Birth Rates and Death Rates (per 1000).
- Explain why factors like education and poverty lead to differences in Birth Rates across the world.
- Explain why factors like clean water and healthcare lead to differences in Death Rates.
- Identify examples of Push (bad) and Pull (good) factors driving migration.
Keep these notes handy! Mastering these basic rates and factors is the foundation for understanding global population issues.