This is the final and one of the most important lessons, covering ecology, environmental science, and our impact on the planet. It explains how life is organized, how energy flows, how essential elements are recycled, and the challenges of pollution and sustainable living.

Part 1: How Life is Organized

1. Organizational Levels

Life is organized in a hierarchy, from simple to complex.

• Individual: A single organism (e.g., one elephant).

• Population: A group of individuals of the same species living in the same area (e.g., all the elephants in Yala National Park).

• Community: All the different populations (plants, animals, microbes) living and interacting in the same area (e.g., all the elephants, deer, leopards, and trees in Yala).

• Ecosystem: The community of living things plus their non-living (abiotic) environment (e.g., the Yala community plus the soil, water, and sunlight).

• Biosphere: The part of the Earth that supports life, including the land (lithosphere), water (hydrosphere), and air (atmosphere).

2. Population Growth

• Typical Growth Curve (S-shaped): Natural populations grow slowly at first, then rapidly (exponential phase), and finally level off as they reach the environment's carrying capacity (the maximum population size the environment can support).

• Human Growth Curve (J-shaped): Due to advances in medicine and agriculture, the human population is still in the rapid, exponential growth phase.

3. Energy and Nutrient Flow

• Food Chain: A simple, linear path of energy flow (e.g., Grass → Grasshopper → Toad → Snake).

• Food Web: A complex network of many interconnected food chains.

• Trophic Levels: The position an organism occupies in a food chain.

  • Producers (Autotrophs): Make their own food, usually through photosynthesis (e.g., plants).

  • Primary Consumers (Herbivores): Eat producers (e.g., grasshopper).

  • Secondary Consumers (Carnivores/Omnivores): Eat primary consumers (e.g., toad).

  • Tertiary Consumers: Eat secondary consumers (e.g., snake).

• Decomposers: Break down dead organic matter and return nutrients to the soil (e.g., bacteria, fungi).

4. Ecological Pyramids

• Pyramid of Numbers: Shows the number of individuals at each trophic level. Can be inverted (e.g., one big tree supporting many insects).

• Pyramid of Biomass: Shows the total dry mass (biomass) at each trophic level. Usually upright, but can be inverted in some aquatic ecosystems.

• Pyramid of Energy: Shows the energy transferred between trophic levels. It is ALWAYS upright.

  • The 10% Rule (Crucial Concept): Only about 10% of the energy from one trophic level is transferred to the next. The other 90% is lost, mostly as heat during metabolic processes. This is why food chains are rarely longer than 4 or 5 levels.

Part 2: Recycling in Nature (Biogeochemical Cycles)

1. The Carbon Cycle

• Key Processes:

  • Photosynthesis: Plants take CO₂ from the atmosphere.

  • Respiration: All living things release CO₂.

  • Combustion: Burning fossil fuels and wood releases CO₂.

  • Decomposition: Decomposers release CO₂ as they break down dead organisms.

2. The Nitrogen Cycle

• Key Processes:

  • Nitrogen Fixation: Converting atmospheric nitrogen (N₂) into usable forms like ammonia (NH₃). This is done by lightning and special nitrogen-fixing bacteria (like Rhizobium in the root nodules of leguminous plants).

  • Nitrification: Bacteria convert ammonia into nitrates (NO₃⁻), which plants can absorb.

  • Denitrification: Other bacteria convert nitrates back into atmospheric nitrogen (N₂).

Part 3: Environmental Pollution

Key Problems and Their Causes:

• Acid Rain: Caused by sulfur dioxide (SO₂) and nitrogen oxides (NOx) from burning fossil fuels, which mix with rainwater to form sulfuric and nitric acids. It damages forests, aquatic life, and buildings.

• Global Warming (The Greenhouse Effect): Caused by an excess of greenhouse gases (especially CO₂, but also methane CH₄ and CFCs) that trap heat in the atmosphere, leading to rising global temperatures.

• Ozone Layer Depletion: Caused by CFCs (from old refrigerators and aerosol cans) which break down the protective ozone (O₃) layer, allowing harmful UV radiation to reach Earth.

• Eutrophication: Occurs when fertilizers (rich in nitrates and phosphates) wash into lakes and rivers, causing a massive "bloom" of algae. When the algae die and decompose, they use up all the oxygen in the water, killing fish and other aquatic life.

• Biomagnification: The concentration of non-biodegradable toxins (like DDT or heavy metals) increases at each successive trophic level. An organism at the top of the food chain will have the highest concentration of the toxin.

Part 4: Sustainable Development

• Definition: Using resources to meet our current needs without compromising the ability of future generations to meet their own needs.

• Key Strategies:

  • Sustainable Agriculture: Using methods like polyculture (growing multiple crops), biological pest control, and organic fertilizers.

  • Waste Management (4Rs): Reduce, Reuse, Replace, Recycle.

  • Energy Management: Using energy efficiently and shifting to renewable energy sources (solar, wind, hydro).

  • Reducing "Food Miles" & "Carbon Footprint": Consuming local products to reduce the energy spent on transportation and reducing the total CO₂ emitted by our activities.

Exam Tips & Tricks

1. Definitions are Key: Be able to clearly define terms like population, ecosystem, biomagnification, and sustainable development.

2. The 10% Rule: This is a very common topic. Be ready to explain why energy decreases up the food chain and why energy pyramids are always upright.

3. Cycles are for Diagrams: Practice drawing simplified diagrams of the Carbon and Nitrogen cycles. Focus on the main processes (e.g., photosynthesis, respiration, nitrogen fixation).

4. Pollution = Cause + Effect: For acid rain, global warming, and ozone depletion, you need to know the main chemical/pollutant responsible and the major negative consequences.

Local Context: Be aware of environmental issues in Sri Lanka, such as Chronic Kidney Disease (CKD) and its potential links to agrochemicals and heavy metals.