Lesson Notes By Weeks and Term v5 - Grade 11

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Performance objectives

• Define key terms in population ecology.
• Explain factors that influence population growth.
• Describe exponential and logistic growth curves.
• Analyse the impact of human activities on populations.
• Suggest sustainable solutions for environmental issues.

Lesson summary

Population ecology examines how populations of organisms interact with their environment. Understanding population dynamics is crucial because it allows us to predict how populations will change over time, which has significant implications for resource management, conservation efforts, and even public health. In South Africa, where we face challenges such as rapid urbanization, habitat loss, and invasive species, population ecology provides essential tools for managing our natural resources sustainably and mitigating the negative impacts of human activities.

Lesson notes

2. 1. Basic Concepts in Population Ecology Population: A group of individuals of the same species living in the same area at the same time. It's important to stress that all three criteria must be met.

Population Size: The total number of individuals in a population. Determining population size can be done through: Direct Counting: Counting every individual (feasible in small areas with easily visible organisms).

Sampling: Counting individuals in a representative sample and extrapolating to the entire area.

Quadrat Method: Used for plants and sessile (non-moving) animals. Squares of a known area (quadrats) are placed randomly or systematically within the study area. The number of individuals within each quadrat is counted, and the average density per quadrat is calculated. This average is then multiplied by the total area to estimate the total population size.

Example: Imagine studying the density of Aloe vera plants in a nature reserve. You lay down 10 quadrats of 1m x 1m size randomly throughout the reserve. After counting, you find the following numbers of Aloe vera plants in each quadrat: 2, 3, 1, 4, 2, 2, 3, 0, 3,

2. The average number of plants per quadrat is (2+3+1+4+2+2+3+0+3+2)/10 = 2.2 plants/m². If the total area of the reserve is 10,000 m², the estimated population size is 2.2 plants/m² 10,000 m² = 22,000 plants. Capture-Recapture Method (Mark-and-Recapture): Used for mobile animals. A sample of individuals is captured, marked (e.g., with tags), and released. Later, another sample is captured. The proportion of marked individuals in the second sample is used to estimate the total population size.

Formula: Population size (N) = (Number of individuals captured and marked in the first sample Total number of individuals captured in the second sample) / Number of marked individuals recaptured in the second sample.

Example: 50 dassies (rock hyrax) are captured, marked, and released. A week later, 60 dassies are captured, and 10 of them are marked. The estimated population size is (50 60) / 10 = 300 dassies.

Population Density: The number of individuals per unit area or volume. Important for resource availability. High density can lead to increased competition.

Population Distribution (Dispersion): The spatial arrangement of individuals within a population.

Clumped: Individuals are aggregated in patches (e.g., herds of springbok). Often due to resource availability or social behaviour.

Uniform: Individuals are evenly spaced (e.g., territorial birds). Often due to competition for resources.

Random: Individuals are randomly distributed (e.g., plants with wind dispersal of seeds).

Age Structure: The proportion of individuals in different age groups. This can predict future population growth. A population with a large proportion of young individuals is likely to grow rapidly. 2.

2. Factors Affecting Population Growth Birth Rate (Natality): The number of births per unit time (e.g., per year).

Death Rate (Mortality): The number of deaths per unit time.

Immigration: The movement of individuals into a population.

Emigration: The movement of individuals out of a population.

Population Growth Rate (r): r = (Birth Rate + Immigration) - (Death Rate + Emigration) If r > 0: Population is growing. If r < 0: Population is declining. If r = 0: Population is stable (zero population growth).

Example: A population of 1000 elephants in Kruger National Park has 100 births and 50 deaths in a year. 20 elephants immigrate, and 10 emigrate. The growth rate (r) is (100 + 20) - (50 + 10) =

6

0. The population growth rate is 60/1000 = 0.06 or 6%. This means the elephant population has increased by 6% in a year. 2.

3. Population Growth Models Exponential Growth: Population increases rapidly without any limits. This occurs under ideal conditions with unlimited resources. Represented by a J-shaped curve. This is not sustainable in the long term.

Logistic Growth: Population growth slows down as it approaches the carrying capacity. Resources become limited, and competition increases. Represented by an S-shaped curve.

Carrying Capacity (K): The maximum population size that an environment can sustain given the available resources (food, water, shelter, space). 2.

4. Limiting Factors Factors that restrict population growth.

These can be: Density-Dependent Factors: Factors whose effects become more intense as the population density increases.

Examples: Competition: For resources like food, water, and mates.

Predation: Predators may focus on a more abundant prey species.

Disease: Disease spreads more easily in dense populations.

Parasitism: Parasites spread more easily in dense populations.

Density-Independent Factors: Factors whose effects are not related to population density.

Examples: Natural Disasters: Floods, droughts, fires, and volcanic eruptions.

Climate Change: Changes in temperature and rainfall patterns.

Human Activities: Deforestation, pollution, and habitat destruction.