BIOLOGY CLASS NOTE: THE ECOLOGY OF POPULATION

Subject: Biology Class: Senior Secondary School 2 (SS 2) Term: Second Term Topic: Ecology of Population (Definition, Characteristics, and Growth Curves)

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1. COMPREHENSIVE CORE CONCEPTS

A. Definition of Population Ecology

In our previous lessons, we explored the ecosystem as a whole. However, to understand how nature works, we must zoom in on specific groups. Population Ecology is the study of how groups of organisms of the same species live, interact, and change over time in a particular habitat.

A population is defined as the total number of individuals of the same species living in a specific geographical area at a particular time, and who are capable of interbreeding. For example, all the Tilapia fish in a local pond in Ibadan form a population. All the maize plants in a school farm in Enugu form another population. Ecology helps us understand why some populations explode in number (like locusts) while others decline toward extinction (like the West African Lion).

B. Population Characteristics

To study a population, scientists (Ecologists) use specific measurable markers. These are the "vital signs" of a population.

i. Population Size and Density

Population Size is the total number of individuals in the population. While size tells us "how many," Population Density tells us "how crowded" they are. Density is calculated as the number of individuals per unit area or volume.

• Formula: $Density = \frac{\text{Total Population Size}}{\text{Total Land Area/Volume}}$
• Example: If there are 50 orange trees in a farm measuring 2 hectares, the density is 25 trees per hectare. High density often leads to increased competition for sunlight and nutrients.

ii. Birth Rate (Natality) and Death Rate (Mortality)

• Birth Rate (Natality): This is the rate at which new individuals are added to a population through reproduction. In human populations, we often measure it as births per 1,000 people per year. High natality leads to population growth.
• Death Rate (Mortality): This is the rate at which individuals are lost due to death. Factors like disease, predation, and old age affect this.
• The Balance: If Birth Rate > Death Rate, the population grows. If Death Rate > Birth Rate, the population shrinks.

iii. Age Distribution

This refers to the proportion of individuals of different age groups within a population. Ecologists usually divide a population into three categories:

1. Pre-reproductive (Young): Individuals too young to breed.
2. Reproductive (Adults): Individuals currently capable of producing offspring.
3. Post-reproductive (Old): Individuals past the age of breeding.

• Significance: A population with a huge number of "Pre-reproductive" individuals (like many developing nations) is likely to grow rapidly in the future.

C. The Population Growth Curve

When a species enters a new environment with plenty of food and no predators, it doesn't just grow randomly. It follows a specific pattern known as the Sigmoid (S-shaped) Growth Curve.

1. Lag Phase: Growth is very slow. The organisms are still adapting to the new environment, finding mates, and preparing to reproduce.
2. Log (Exponential) Phase: Growth is very rapid. Resources are abundant, and birth rates far exceed death rates.
3. Deceleration Phase: Growth begins to slow down as resources (food, space) become limited and waste products accumulate.
4. Stationary Phase: The population levels off. The birth rate equals the death rate. The environment has reached its Carrying Capacity ($K$)—the maximum number of individuals the habitat can sustainably support.

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2. REAL-WORLD EXAMPLES

• The Locust Invasion: In Northern Nigeria, occasionally, the desert locust population explodes. This happens during the "Log Phase" when environmental conditions (rain and vegetation) are perfect. Understanding their growth curve helps the government predict when to spray pesticides.
• Overcrowded Fish Ponds: A fish farmer stocks 1,000 fingerlings in a tiny pond. Because the density is too high, the "Death Rate" increases because of low oxygen and rapid disease spread. The farmer uses population ecology to find the "optimum density" for profit.
• Urban Planning in Lagos: Government planners look at Age Distribution. Since Lagos has a large youthful population, they know they must build more schools now and more housing units for the future when these youths reach the "Reproductive" age.

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3. PRACTICAL APPLICATIONS: STEP-BY-STEP GUIDE

How to Estimate Plant Population Density (The Quadrat Method)

If you want to know the density of "Akintola weed" (Chromolaena odorata) on your school field without counting every single weed, follow these steps:

1. Create a Quadrat: Make a 1m x 1m square frame using four wooden laths or even strings tied to pegs.
2. Random Sampling: Throw the quadrat randomly over your shoulder onto the field.
3. Count: Count the number of the specific plants inside that 1m² square.
4. Repeat: Do this 10 times in different spots on the field.
5. Calculate Average: Add all your counts and divide by 10.
6. Find Density: The result is the average density per square meter ($m^2$). Multiply this by the total area of the field to estimate the total population.

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4. SUGGESTED PROJECT-BASED LEARNING (PBL)

Project Title: Observing the "S-Curve" in Bread Mold Objective: To visualize how a population grows, peaks, and levels off due to resource limitation.

Materials Needed:

• A slice of preservative-free bread (or a piece of yam).
• A clear plastic bag (Ziploc).
• A few drops of water.
• A permanent marker.
• A transparent grid (you can draw 1cm x 1cm squares on a clear plastic sheet).

Procedure:

1. Moisten the bread slightly (do not soak it) and leave it exposed to the air for 30 minutes to catch mold spores.
2. Place the bread in the bag and seal it. Keep it in a dark, warm cupboard.
3. Daily Observation: Every 24 hours, place your transparent grid over the bag. Count how many "squares" are covered by the mold.
4. Record Data: Keep a log: Day 1 (0 squares), Day 2 (1 square), Day 3 (4 squares), etc.
5. Graphing: After 10 days, plot a graph (Number of squares vs. Days).
6. Conclusion: Identify the Lag phase (slow start) and the Log phase (rapid spread) on your graph.

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5. LIFE SKILLS INTEGRATION

• Resource Management: Understanding "Carrying Capacity" teaches you that resources (money, food, time) are finite. Just as a habitat cannot support infinite goats, a budget cannot support infinite expenses.
• Career Connections:
  • Demographers: Use population data to help governments plan for hospitals and electricity.
  • Public Health Officers: Use "Birth/Death rates" to track the success of vaccinations.
  • Wildlife Conservationists: Study population density to prevent endangered species (like the Cross River Gorilla) from going extinct.
  • Agricultural Economists: Use these concepts to maximize crop yields.

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6. STUDENT REFLECTION QUESTIONS

1. If a population of rabbits is in the Stationary Phase, does it mean no new rabbits are being born? Explain your answer.
2. In a village, the "Pre-reproductive" age group makes up 60% of the population. What does this tell you about the village's need for resources in 10 years?
3. Why is it that human populations in many cities are in a "Log Phase" (growing very fast), but eventually, they must reach a "Stationary Phase"? What factors might cause that slowing down?
4. How does understanding "Population Density" help a poultry farmer prevent the spread of bird flu?
5. Reflect on your classroom. If we doubled the number of students in the same room, how would the "Environmental Resistance" (heat, noise, space) change?