Welcome to your complete guide for Unit 6: Population and Natural Resources. This lesson is designed specifically for Ethiopian Grade 11 students. We will break down complex topics like population growth, natural resources, and conservation into simple, easy-to-understand steps.
6.1 Population Ecology
Have you ever wondered how scientists count the number of fish in a lake or wild animals in a forest? They cannot count every single one! Instead, they use Population Ecology.
6.1.1 Population Size, Density, and Dispersal
Population Size (N) is the total number of individuals. Since counting everyone is hard, scientists use sampling methods.
1. Quadrat Method (For plants/slow animals)
Scientists place a square frame (quadrat) on the ground and count organisms inside it. They repeat this many times and calculate the average.
Example: If a field is 50 m², and your quadrat (10 m²) counts 50 plants, the estimated population is proportional.
2. Mark-Recapture Method (For mobile animals)
How do we count birds or lions?
- Catch a sample, mark them (M), and release them.
- Later, catch a second sample (n).
- Count how many in the second sample are already marked (X).
Researchers captured and marked 60 birds (M). Later, they caught 100 birds (n). Out of these 100, 20 were marked (X). What is the total population?
$$ N = \frac{60 \times 100}{20} = \frac{6000}{20} = 300 \text{ birds} $$
Population Growth
Populations change over time. Two main models describe this:
1. Exponential Growth (J-Shaped Curve)
This happens when resources are unlimited. The population grows faster and faster.
Where $r_{max}$ is the maximum growth rate. If $r > 0$, population grows. If $r = 0$, it is stable (Zero Population Growth).
2. Logistic Growth (S-Shaped Curve)
In reality, resources (food, space) are limited. This limit is called Carrying Capacity (K). The population grows fast at first, then slows down as it reaches $K$.
The term $\frac{K – N}{K}$ represents the “room for growth.” When $N$ is close to $K$, growth stops.
A population of bacteria has $r_{max} = 0.5$. The carrying capacity of the environment is 1000. If the current population $N$ is 200, calculate the growth rate.
Use the logistic formula: $\frac{dN}{dt} = rN \left( \frac{K – N}{K} \right)$
$r = 0.5, N = 200, K = 1000$
$$ \frac{dN}{dt} = 0.5 \times 200 \times \left( \frac{1000 – 200}{1000} \right) $$ $$ = 100 \times \left( \frac{800}{1000} \right) = 100 \times 0.8 = 80 $$ The population is adding 80 individuals per time unit.
6.1.2 Population Regulation
What stops populations from growing forever?
- Density-Dependent Factors: Biotic factors that get worse as population gets crowded. Examples: Competition, Disease, Predation.
- Density-Independent Factors: Abiotic factors that affect everyone regardless of density. Examples: Earthquakes, Floods, Temperature.
6.1.3 Demographic Structure
Scientists use Population Pyramids to show age and sex structure.
- Expansive (Broad base): High birth rates, young population (e.g., Ethiopia).
- Constrictive (Narrow base): Low birth rates, aging population.
- Stationary: Stable population, equal widths.
Ethiopia currently has an expansive pyramid, meaning a high percentage of young people.
6.2 Natural Resources
Natural resources are materials found in nature that humans use. They are classified as:
- Renewable: Can be replaced naturally (e.g., Solar energy, Water, Wind, Trees). However, they can be depleted if overused.
- Non-Renewable: Exist in limited amounts or take millions of years to form (e.g., Coal, Oil, Natural Gas, Minerals).
6.3 Conservation in Ethiopia
Conservation is the wise use and protection of natural resources so they last for future generations.
Wildlife Conservation
Ethiopia has established National Parks (e.g., Simien Mountains, Bale Mountains) and Sanctuaries to protect endemic animals like the Walia Ibex and Ethiopian Wolf.
Plant Conservation
- Indigenous Practices: Sacred groves around churches/mosques, taboos against cutting certain trees.
- Modern Practices: Gene banks (storing seeds), Botanical gardens.
Soil and Water Conservation
To stop erosion:
Mechanical: Terraces, bunds, check dams.
Biological: Planting trees (afforestation), the “Green Legacy” initiative.
6.4 Impact of Traffic Accidents
Roads crossing wildlife habitats cause animal deaths. Solutions include wildlife underpasses and driving carefully in protected areas.
Key Exam Notes & Formulas
1. Population Density Formula
$$ D = \frac{N}{A} $$ (D = Density, N = Total Number, A = Area)2. Mark-Recapture Formula
$$ N = \frac{M \times n}{X} $$ (M = Marked first time, n = Total second capture, X = Marked recaptured)3. Exponential Growth
$$ \frac{dN}{dt} = r_{max}N $$- Unlimited resources.
- J-shaped curve.
4. Logistic Growth
$$ \frac{dN}{dt} = r_{max}N \left( \frac{K – N}{K} \right) $$- Limited resources (Carrying Capacity K).
- S-shaped curve.
Important Definitions
- Carrying Capacity (K): Maximum population size an environment can sustain.
- Demography: Study of human population statistics (birth rates, death rates).
- Conservation: Protection and wise management of natural resources.
- Zero Population Growth: When birth rate equals death rate ($r=0$).
Common Mistakes in Exams
- Confusing Density-Dependent vs Independent: Remember, Dependent involves living things fighting each other (competition/disease). Independent involves the weather/nature killing everyone equally (flood/fire).
- Logistic Formula: Students often forget to subtract $N$ from $K$ in the bracket. If $N > K$, the growth becomes negative (population declines).
Ethiopia Context
- Census Methods: De Facto (counted where found on census night) vs De Jure (counted at usual residence).
- Conservation: Ethiopia uses “In situ” (parks) and “Ex situ” (gene banks) conservation.
Unit 6 Challenge Questions
Test your knowledge with these difficult questions. Try to solve them before looking at the answers!
Question 1: Calculation (Logistic Growth)
In a protected forest, the carrying capacity ($K$) for a certain antelope species is 500. The current population ($N$) is 100. The intrinsic rate of increase ($r_{max}$) is 0.2 per year. Calculate the population growth rate ($\frac{dN}{dt}$) for this year.
Use the logistic growth equation: $\frac{dN}{dt} = r_{max}N \left( \frac{K – N}{K} \right)$
Given: $K=500, N=100, r_{max}=0.2$.
$$ \frac{dN}{dt} = 0.2 \times 100 \times \left( \frac{500 – 100}{500} \right) $$ $$ = 20 \times \left( \frac{400}{500} \right) $$ $$ = 20 \times 0.8 = 16 $$ The antelope population will increase by 16 individuals this year.
Question 2: Multiple Choice
Which of the following is a density-independent factor that regulates population size?
A. Competition for food
B. Predation
C. Forest Fire
D. Disease outbreak
Explanation: Density-independent factors affect populations regardless of how crowded they are. A fire kills individuals whether the population is dense or sparse. Competition, predation, and disease are density-dependent because they worsen with crowding.
Question 3: Short Answer (Demography)
Describe the shape of Ethiopia’s population pyramid and explain what this indicates about the country’s birth rate and future population growth.
Ethiopia has an expansive (or pyramidal) population pyramid with a very wide base and narrow top. This indicates a high birth rate and a high proportion of young people. It suggests that the population will continue to grow rapidly in the future due to the large number of individuals entering or approaching reproductive age.
Question 4: Calculation (Mark-Recapture)
Ecologists want to estimate the number of frogs in a pond. They capture 40 frogs, mark them, and release them. One week later, they capture 60 frogs. Of these 60, 10 have the mark. Estimate the total population size ($N$).
Formula: $N = \frac{M \times n}{X}$
$M$ (Marked initially) = 40
$n$ (Total in 2nd catch) = 60
$X$ (Marked in 2nd catch) = 10
$$ N = \frac{40 \times 60}{10} = \frac{2400}{10} = 240 $$ The estimated frog population is 240.
Question 5: Fill in the Blanks
The maximum population size that an environment can sustain is called ________. When the population reaches this size, the growth rate levels off, creating an ________-shaped curve in the logistic growth model.
1. Carrying Capacity (K)
2. S