Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define key population characteristics like density and distribution.
• Explain the four factors that affect population size.
• Interpret exponential and logistic population growth curves.
• Understand the concept of carrying capacity and limiting factors.
• Relate these concepts to South African environmental issues.

Lesson summary

Population ecology is the study of populations in relation to their environment, including environmental influences on population density and distribution, age structure, and variations in population size. Understanding population ecology is crucial because it helps us predict how populations will change over time and how they interact with their environment. This is particularly relevant in South Africa, where we face challenges such as managing populations of endangered species (like rhinos), controlling invasive species (like the Port Jackson willow), and understanding the impact of human population growth on our natural resources (water scarcity, deforestation).

Lesson notes

2.1 Defining Populations and Their Characteristics: Population: A group of individuals of the same species living in the same area at the same time. It's vital to remember the three components: species, location, and time. For example, the number of African wild dogs in the Kruger National Park in

2

0

2

3. We wouldn't say 'the population of all animals in Kruger,' because it includes numerous species.

Population Density: The number of individuals per unit area or volume. Density helps us understand how crowded a population is.

Formula: Population Density = Number of Individuals / Area (or Volume)

Example: If there are 500 springboks in a 10 km² area, the population density is 50 springboks/km².

Population Distribution: The pattern of spacing among individuals within the boundaries of the population.

There are three main types: Clumped: Individuals aggregate in patches (most common).

Example: A herd of elephants around a waterhole during the dry season. They cluster for resources (water).

Uniform: Individuals are evenly distributed. This is often driven by competition or territoriality.

Example: Certain seabird colonies, where nesting sites are evenly spaced.

Random: Unpredictable spacing. This occurs in the absence of strong attractions or repulsions.

Example: Dandelions scattered in a field if wind dispersal is truly random. 2.2 Factors Affecting Population Size: Population size is dynamic and changes over time due to four key factors: Birth Rate (Natality): The number of births in a population per unit time. Higher birth rates increase population size.

Death Rate (Mortality): The number of deaths in a population per unit time. Higher death rates decrease population size.

Immigration: The influx of new individuals from other areas. Immigration increases population size. Think about people moving into South Africa from neighboring countries.

Emigration: The movement of individuals out of a population. Emigration decreases population size. Think about skilled South Africans moving overseas seeking better opportunities. The per capita rate of increase (r) gives us a snapshot of how the population is growing (or shrinking): r = birth rate – death rate. If r is positive, the population is growing. If r is negative, the population is declining. 2.3 Population Growth Models: Exponential Growth: Population increase under ideal conditions (unlimited resources). It results in a J-shaped curve. This type of growth cannot be sustained indefinitely.

Equation: dN/dt = r max N, where: dN/dt = rate of population increase r max = intrinsic rate of increase (maximum potential growth rate under ideal conditions) N = population size

Example: Imagine introducing a small number of rabbits to an island with abundant food and no predators. The rabbit population might initially grow exponentially.

Logistic Growth: Population growth slows as it approaches carrying capacity. It results in an S-shaped curve. This model is more realistic because it considers limiting factors.

Equation: dN/dt = r max N(K-N)/K, where: K = Carrying Capacity (the maximum population size the environment can sustainably support).

Example: Think of a fish population in a dam. Initially, the fish population grows rapidly. As the population increases, competition for food and space intensifies, slowing down the growth rate. Eventually, the population reaches a point where the birth rate equals the death rate, and the population size stabilizes at the carrying capacity (K). 2.4 Carrying Capacity (K): The maximum population size that a particular environment can sustain, given the available resources (food, water, shelter, etc.). Limiting factors determine carrying capacity. 2.5 Limiting Factors: Factors that restrict population growth.

They can be: Density-Dependent: The effect intensifies as population density increases.

Examples: Competition for resources (food, water, mates, shelter)

Predation: Predators may focus on prey that are more abundant and easier to find.

Disease: Diseases spread more easily in dense populations.

Accumulation of toxic wastes: Waste products build up more quickly in dense populations.

Stress: high population densities can cause stress leading to reduced birth rate.

Density-Independent: The effect is not related to population density.

Examples: Natural disasters (floods, droughts, fires, volcanic eruptions) Unusual weather (severe frost, heat waves) Human activities (deforestation, pollution) 2.6 Human Impact on the Environment in South Africa: Deforestation: Clearing forests for agriculture, urbanization, and timber. Consequences include soil erosion, loss of biodiversity, reduced carbon sequestration, and changes in local climate.

Example: Deforestation in KwaZulu-Natal to make way for sugar cane farms has led to soil degradation and habitat loss.

Pollution: Water Pollution: Contamination of water bodies with pollutants (sewage, industrial waste, agricultural runoff).