Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Functioning Ecosystem, Autotrophy and Heterotrophy - Food Webs and Tropic Levels
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Functioning Ecosystem, Autotrophy and Heterotrophy - Food Webs and Tropic Levels
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Subject: Biology
Class: Senior Secondary 1
Term: 3rd Term
Week: 2
Theme: The Organism And Its Environment
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Define the terms autotrophy and heterotrophy. Recognise that chemical energy (in the form of carbohydrates, fats, proteins) and nutrients are transferred among producers,consumers and decomposers. State that trophic level refers to the partsof food chain. Correctly define (or describe) food chainsand pyramid of energy/number. Describe the nature of energy transfer or flow in the ecosystem. State that the re is a progressivediminution of energy in the feeding chain. Recongnise a definite change in number of individuals from one feeding level to another,especially between producers and consumers.
→ even fewer lizards (secondary consumers) → very few hawks (tertiary consumers).
Inverted: Occurs when a single large producer supports many smaller consumers.
Example relevant to Nigeria: One large baobab tree (producer) → many insects (primary consumers) → a few insectivorous birds (secondary consumers). Here, the base (producer) is smaller than the next level.
Spindle-shaped: Occurs when a small number of producers support a larger number of primary consumers, which in turn support a smaller number of secondary consumers.
Example relevant to Nigeria: A few large trees (producers) → many birds and insects (primary consumers) → fewer raptors or large predators (secondary consumers). * Recognition of definite change in number: The pyramid of number visually demonstrates that there is a significant decrease in the number of individuals as one moves up the trophic levels, especially from producers to primary consumers, and then to higher-level consumers. This is because each higher-level consumer typically requires many individuals from the trophic level below it to sustain itself. --- 2.1 Autotrophy Definition: Autotrophy (from Greek "auto" meaning self and "trophos" meaning feeder) is the process by which organisms produce their own organic food (energy) from inorganic sources. These organisms are known as autotrophs or producers.
Types: Photoautotrophs: Organisms that use light energy to synthesize organic compounds. This process is called photosynthesis.
Examples relevant to Nigeria: Most plants (e.g., maize, yam, cassava, oil palm, mango tree), algae (e.g., those found in ponds and rivers), cyanobacteria (blue-green algae).
Process (Photosynthesis): Carbon dioxide + Water + Light energy → Glucose + Oxygen. (CO2 + H2O + Light → C6H12O6 + O2).
Chemoautotrophs: Organisms that obtain energy by oxidising inorganic chemical compounds (e.g., hydrogen sulphide, ammonia, iron) to synthesize organic food. This process is called chemosynthesis.
Examples: Certain bacteria (e.g., nitrifying bacteria in soil, sulfur bacteria) found in specialized environments like hydrothermal vents or anaerobic soils. While less common in everyday Nigerian contexts compared to photoautotrophs, they play vital roles in biogeochemical cycles (e.g., nitrogen cycle in agricultural soils). 2.2 Heterotrophy Definition: Heterotrophy (from Greek "hetero" meaning other and "trophos" meaning feeder) is the process by which organisms obtain their food by consuming other organisms or organic matter. These organisms are known as heterotrophs or consumers.
Types (based on diet): Herbivores (Primary Consumers): Organisms that feed exclusively on producers (plants).
Examples relevant to Nigeria: Goats, cows, sheep, grasshoppers, caterpillars, snails, rabbits. Carnivores (Secondary or Tertiary Consumers): Organisms that feed on other animals.
Primary Carnivores (Secondary Consumers): Feed on herbivores (e.g., lizard eating a grasshopper, lion eating a gazelle).
Secondary Carnivores (Tertiary Consumers): Feed on primary carnivores (e.g., snake eating a lizard, hawk eating a snake). Tertiary Carnivores (Quaternary Consumers): Feed on secondary carnivores (less common, e.g., a very large predator eating another predator).
Examples relevant to Nigeria: Lions, leopards, snakes, hawks, frogs, spiders.
Omnivores: Organisms that feed on both plants and animals.
Examples relevant to Nigeria: Humans, chickens, pigs, some bears, chimpanzees.
Detritivores: Organisms that feed on dead organic matter (detritus) like decaying plants and animals. They mechanically break down the detritus into smaller pieces.
Examples relevant to Nigeria: Earthworms, millipedes, termites, vultures, dung beetles.
Decomposers: Organisms (mainly bacteria and fungi) that break down complex organic molecules in dead organisms and waste products into simpler inorganic substances, returning nutrients to the environment. They do not ingest food but secrete enzymes externally.
Examples relevant to Nigeria: Bacteria and fungi found in soil, decaying logs, and compost heaps. They are crucial for nutrient cycling in farmlands and forests. 2.3 Transfer of Chemical Energy and Nutrients Chemical Energy: Energy in an ecosystem originates primarily from the sun, captured by producers during photosynthesis and converted into chemical energy stored in organic molecules (carbohydrates, fats, proteins). This chemical energy is then transferred sequentially from one trophic level to another as organisms consume others.
Nutrients: Essential elements like nitrogen, phosphorus, carbon, and water are also transferred among producers, consumers, and decomposers. Unlike energy, which flows in one direction and is progressively lost, nutrients are cycled within the ecosystem. Decomposers play a critical role in releasing these nutrients from dead organic matter, making them available again for producers. 2.4 Trophic Levels and Food Chains Food Chain: A food chain illustrates the feeding relationship between different organisms in an ecosystem, showing how energy and nutrients are transferred from one organism to another. It typically starts with a producer and moves through a series of consumers.
General structure: Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer Example relevant to Nigeria (Terrestrial): Grass (Producer) → Grasshopper (Primary Consumer) → Lizard (Secondary Consumer) → Hawk (Tertiary Consumer)
Example relevant to Nigeria (Aquatic): Algae (Producer) → Tadpole (Primary Consumer) → Tilapia fish (Secondary Consumer) → Kingfisher (Tertiary Consumer)
Trophic Level: Each step or feeding level in a food chain is called a trophic level. Organisms at the same position in a food chain belong to the same trophic level.
First Trophic Level: Producers (Autotrophs) – e.g., plants, algae. * Second Trophic Level: Primary Consumers (Herbivores) Tertiary Consumer Example relevant to Nigeria (Terrestrial): Grass (Producer) → Grasshopper (Primary Consumer) → Lizard (Secondary Consumer) → Hawk (Tertiary Consumer)
Example relevant to Nigeria (Aquatic): Algae (Producer) → Tadpole (Primary Consumer) → Tilapia fish (Secondary Consumer) → Kingfisher (Tertiary Consumer)
Trophic Level: Each step or feeding level in a food chain is called a trophic level. Organisms at the same position in a food chain belong to the same trophic level.
First Trophic Level: Producers (Autotrophs) – e.g., plants, algae.
Second Trophic Level: Primary Consumers (Herbivores) – e.g., grasshoppers, goats.
Third Trophic Level: Secondary Consumers (Primary Carnivores or Omnivores) – e.g., lizards, humans eating goats.
Fourth Trophic Level: Tertiary Consumers (Secondary Carnivores or Omnivores) – e.g., hawks, humans eating fish.
Fifth Trophic Level: Quaternary Consumers (Tertiary Carnivores) – less common. Decomposers operate at all trophic levels, breaking down dead organic matter from any level. 2.5 Food Web Definition: A food web consists of many interconnected food chains in an ecosystem. It provides a more realistic representation of feeding relationships because most organisms consume, and are consumed by, more than one type of organism.
Importance: Shows the complexity and stability of an ecosystem. A diverse food web is generally more stable because if one food source becomes scarce, consumers have alternative options. 2.6 Energy Transfer or Flow in the Ecosystem Unidirectional Flow: Energy flows unidirectionally, meaning it moves from the sun to producers, then to consumers, and is eventually dissipated as heat. It does not cycle back to the sun or producers in the same way nutrients do. Progressive Diminution of Energy (The 10% Rule / Lindeman's Law): At each transfer from one trophic level to the next, a significant amount of energy is lost. Only about 10% of the energy from one trophic level is typically incorporated into the biomass of the next trophic level. The remaining 90% is lost primarily as heat during metabolic processes (respiration), incomplete consumption, or excretion of waste.
Example: If producers capture 10,000 units of energy from the sun: Producers: 10,000 units Primary Consumers (Herbivores): 10% of 10,000 = 1,000 units Secondary Consumers (Carnivores): 10% of 1,000 = 100 units Tertiary Consumers (Top Carnivores): 10% of 100 = 10 units This progressive loss of energy limits the number of trophic levels in an ecosystem (usually 4 or 5). 2.7 Ecological Pyramids Ecological pyramids are graphical representations that show the relationship between different trophic levels in terms of energy, biomass, or number of individuals.
Pyramid of Energy: Definition: Represents the total amount of energy present at each trophic level in a given area over a specific period.
Shape: Always upright and pyramid-shaped.
Reason: Due to the progressive diminution of energy, the energy content is always greatest at the producer level and decreases progressively at higher trophic levels. It can never be inverted because energy is lost at each transfer, meaning higher levels can never contain more energy than lower levels.
Example: A pyramid showing 10,000 kJ at the producer level, 1,000 kJ at the primary consumer level, 100 kJ at the secondary consumer level, and 10 kJ at the tertiary consumer level.
Pyramid of Number: Definition: Represents the number of individual organisms at each trophic level in an ecosystem.
Shape: Can be upright, inverted, or spindle-shaped.
Upright: Most common. Producers are most numerous, and the number of individuals decreases at successive trophic levels.
Example relevant to Nigeria: Many grass plants (producers) → fewer grasshoppers (primary consumers) → even fewer lizards (secondary consumers) → very few hawks (tertiary consumers).
Inverted: Occurs when a single large producer supports many smaller consumers.
Example relevant to Nigeria: One large baobab tree (producer) → many insects (primary consumers) → a few insectivorous birds (secondary consumers). Here, the base (producer) is smaller than the next level.
Spindle-shaped: Occurs when a small number of producers support a larger number of primary consumers, which in turn support a smaller number of secondary consumers.
Example relevant to Nigeria:* A few large trees (producers) 3.1 Introduction (10 minutes)
Teacher Activity: Begins by asking students to name some plants and animals they commonly see in their environment (e.g., maize, goats, chickens, lizards). Prompts students to think about what these organisms eat. "What does a goat eat? What eats the goat?" Introduces the idea that all living things need energy and nutrients to survive, and they get this from their food.
States the topic: Functioning Ecosystem, Autotrophy and Heterotrophy - Food Webs and Trophic Levels. Briefly explains the importance of understanding these relationships for agriculture and environmental conservation in Nigeria.
Student Activity: Respond to teacher's questions, naming local plants and animals and their diets. Engage in brief class discussion about food sources. Listen attentively to the introduction and topic statement. 3.2 Concept Development and Explanation (30 minutes)
Teacher Activity: Autotrophy and Heterotrophy (Objective 1): Defines autotrophy and heterotrophy clearly on the board. Provides examples of each from Nigerian contexts (e.g., maize plant vs. human, cow). Explains photosynthesis as the primary mechanism for photoautotrophs. Discusses chemoautotrophs briefly. Chemical Energy and Nutrient Transfer (Objective 2): Explains that the energy initially captured by producers (e.g., in a maize plant) is transferred to consumers (e.g., a human eating maize, or a cow eating grass). Emphasizes that this energy is in the form of organic molecules (carbohydrates, fats, proteins). Clarifies that nutrients (e.g., nitrogen from soil) are also transferred and cycled, unlike energy. Food Chains and Trophic Levels (Objectives 3 & 4): Defines food chain and draws a simple example on the board using local organisms (e.g., Grass → Grasshopper → Lizard → Hawk). Identifies each component's role (producer, primary consumer, etc.). Defines trophic level and labels each level in the drawn food chain. Encourages students to identify organisms from their environment for each trophic level.
Food Web: Explains that real ecosystems are more complex than simple food chains. Draws a simple food web on the board by connecting multiple food chains from the initial example (e.g., adding a rat eating grass and a snake eating the rat/lizard). Energy Transfer/Flow and Diminution (Objectives 5 & 6): Explains the unidirectional flow of energy. Introduces the concept of progressive diminution of energy (10% rule). Uses a simple numerical example (e.g., 10,000 units at producer level) to demonstrate energy loss at each trophic level. Explains that this energy loss is primarily due to metabolic activities (respiration) and incomplete consumption. Pyramids of Energy and Number (Objectives 4 & 7): Defines ecological pyramids. Draws and explains the Pyramid of Energy, emphasizing its always upright shape and the reason (diminution of energy). Draws and explains the Pyramid of Number, showing examples of upright, inverted (e.g., a large tree supporting many insects), and spindle shapes using Nigerian examples, highlighting the change in number of individuals.
Student Activity: Actively listen, take notes, and ask questions for clarification. Provide local examples for producers and consumers when prompted. Copy drawn food chains and food webs into their notebooks. Attempt to draw their own simple food chain with local organisms. Participate in discussions about energy flow and the 10% rule. Draw and label the different types of ecological pyramids as explained. 3.3 Group Work / Collaborative Learning (15 minutes)
Teacher Activity: Divides the class into small groups (4-5 students).
Assigns each group a task: Group 1 & 2: Create a food chain (at least 4 trophic levels) for a terrestrial Nigerian ecosystem (e.g., farmland, forest) and identify the trophic level of each organism.
Group 3 & 4: Create a food chain (at least 4 trophic levels) for an aquatic Nigerian ecosystem (e.g., pond, river, coastal area) and identify the trophic level of each organism.
Group 5 & 6: Construct a simple food web that incorporates at least two food chains created by other groups, and identify which organisms are producers, primary consumers, etc. Circulates among groups, providing guidance and clarifying misconceptions. *Student
Agriculture and Food Security in Nigeria: Application: Understanding food webs and energy flow helps Nigerian farmers make informed decisions. Knowing that herbivores (e.g., cattle, goats) are primary consumers means that a larger base of producers (grass, forage crops) is needed to sustain them. Similarly, understanding the energy loss (10% rule) explains why it is more energy-efficient for humans to consume producers (e.g., yam, rice, cassava) directly than to eat meat from animals that have consumed those producers. This knowledge can guide policy on food production and dietary recommendations to ensure food security for a growing population.
Local Context: Farmers dealing with pest infestations (e.g., grasshoppers on maize or cassava mealybug) are dealing with disruptions in food chains. Understanding these relationships helps in applying sustainable pest control methods that do not inadvertently harm beneficial organisms or disrupt the entire food web. Environmental Conservation and Biodiversity Management: Application: Knowledge of trophic levels and food webs is fundamental to conservation efforts in Nigerian national parks (e.g., Yankari, Cross River National Park) and wildlife reserves. Removing a top predator (e.g., lion, leopard) can lead to an increase in its prey (e.g., antelope), which then overgraze producers, causing ecosystem imbalance. Conversely, the extinction of a primary producer can collapse an entire food web.
Local Context: Protecting endangered species (like the Cross River Gorilla or the Nigerian Chimpanzee) involves understanding their place in the food web and ensuring the availability of their food sources and the health of their entire ecosystem. Conservation education for local communities often involves explaining these interdependencies. Impact of Pollution and Sustainable Resource Use: Application: Pollutants (e.g., pesticides, industrial waste) can enter food chains and webs. For example, a persistent pesticide sprayed on crops can accumulate in producers, then concentrate (biomagnification) in higher-level consumers, affecting human health or apex predators.
Local Context: This helps explain the dangers of indiscriminate use of agrochemicals in Nigerian farmlands or the impact of oil spills in the Niger Delta, where pollutants enter the aquatic food web (e.g., affecting plankton, fish, and ultimately the people who consume them). It emphasizes the need for responsible waste management and sustainable fishing/farming practices to protect both the environment and human well-being. ---