Ecological systems (ecosystems)

# Ecological systems (ecosystems) ## Objectifs et prérequis This chapter aims to provide Lower Sixth students with a comprehensive understanding of ecological systems (ecosystems), their structure, functioning, and interactions. Students will learn to define ecosystems, identify their components, analyze energy flow and nutrient cycling, evaluate productivity, and relate these concepts to biomes and ecosystem services. Prerequisites include foundational knowledge from Meteorology, Climatology, Hydrology, Geomorphology, and Biogeography, particularly regarding climate zones, soil types, water cycles, and vegetation patterns in Cameroon and Africa. ## Introduction Ecology is the scientific study of interactions between organisms and their environment. An ecosystem is a functional unit comprising a community of living organisms (biotic components) interacting with their non-living (abiotic) environment. These interactions involve energy flow, nutrient cycling, and dynamic equilibrium. Understanding ecosystems is crucial for sustainable resource management, biodiversity conservation, and addressing environmental challenges such as deforestation, desertification, and climate change—issues highly relevant to Cameroon’s diverse landscapes, from the tropical rainforests of the South to the Sahelian savannas of the North. ## I. TU 80. Lesson 17: Meaning of ecology and ecosystems (Definition, components and characteristics of ecosystems) **Context**: Builds on prior knowledge of biogeography and physical geography to introduce the core concept of ecosystems as integrated systems. **Definitions and Components**: - **Ecology**: The branch of biology that studies the relationships between living organisms and their physical environment. - **Ecosystem**: A community of living organisms (biotic) interacting with each other and with their non-living (abiotic) environment as a functional unit. - **Biotic components**: Producers (autotrophs, e.g., plants), consumers (heterotrophs, e.g., herbivores, carnivores), decomposers (detritivores and saprotrophs, e.g., fungi, bacteria). - **Abiotic components**: Non-living factors such as sunlight, temperature, water, soil, air, and minerals. **Characteristics of Ecosystems**: 1. **Structure**: Defined by the composition and arrangement of biotic and abiotic components. 2. **Function**: Involves processes like energy flow, nutrient cycling, and productivity. 3. **Dynamic equilibrium**: Ecosystems maintain stability through feedback mechanisms, though they can change over time (succession). 4. **Openness**: Exchange of energy and matter with surrounding ecosystems. 5. **Hierarchy**: Exist at multiple scales (e.g., pond, forest, biome, biosphere). **Vocabulary to retain**: Biotic, abiotic, producer, consumer, decomposer, autotroph, heterotroph, dynamic equilibrium. **Encadré à retenir**: > An ecosystem is a community of living organisms interacting with their non-living environment as a functional unit. **Example of exam question with plan**: *Question*: Define an ecosystem and explain its biotic and abiotic components using examples from the Korup National Park in Cameroon. *Plan*: I. Definition of an ecosystem II. Biotic components: producers (e.g., *Triplochiton scleroxylon*), consumers (e.g., forest elephants, chimpanzees), decomposers (e.g., termites, fungi) III. Abiotic components: high rainfall (>2500 mm/year), constant temperature (24–27°C), nutrient-poor lateritic soils IV. Interdependence: How biotic and abiotic components interact to maintain ecosystem function ## II. TU 81. Lesson 18: The eco-system as a system **Context**: Applies systems theory to ecology, emphasizing interconnectedness and feedback mechanisms. **Key Concepts**: - An ecosystem operates as a **system** with inputs, outputs, processes, and feedback loops. - **Inputs**: Solar energy, water, nutrients, atmospheric gases. - **Outputs**: Heat energy, water vapor, biomass, waste products. - **Processes**: Photosynthesis, respiration, decomposition, nutrient uptake. - **Feedback mechanisms**: - *Negative feedback*: Stabilizes the system (e.g., increased plant growth reduces CO₂, slowing further growth). - *Positive feedback*: Amplifies change (e.g., deforestation → reduced transpiration → less rainfall → more deforestation). **Properties of Ecosystems as Systems**: - **Holism**: The whole is greater than the sum of its parts. - **Interdependence**: Change in one component affects others. - **Emergent properties**: Characteristics not predictable from individual parts (e.g., ecosystem resilience). - **Adaptability**: Capacity to adjust to changes within limits. **Vocabulary to retain**: System, input, output, process, feedback loop, negative feedback, positive feedback, holism, emergent property. **Encadré à retenir**: > Ecosystems function as systems through inputs (solar energy, water), processes (photosynthesis, decomposition), and outputs (heat, biomass), regulated by feedback mechanisms. **Example of exam question with plan**: *Question*: Explain how the Lake Chad basin functions as an ecological system, identifying inputs, outputs, and one negative and one positive feedback mechanism. *Plan*: I. Inputs: Rainfall, river inflow (Chari, Logone), solar energy II. Outputs: Evaporation, outflow via outlets, sediment deposition III. Processes: Photosynthesis in phytoplankton, fish respiration, microbial decomposition IV. Negative feedback: Increased evaporation → higher salinity → reduced algal growth → less organic matter → lower evaporation V. Positive feedback: Reduced rainfall → lower water levels → exposed sediments → increased dust → reduced albedo → higher local temps → less rainfall ## III. TU 82. Lesson 19: Examples of ecosystems **Context**: Illustrates ecosystem diversity using local, national, and global examples. **Types of Ecosystems**: 1. **Natural ecosystems**: Minimal human interference. - *Tropical rainforest*: Korup National Park (SW Cameroon) – high biodiversity, multilayer canopy, nutrient cycling via decomposers. - *Savanna*: Waza National Park (Far North Cameroon) – grasslands with scattered trees (e.g., *Acacia*), seasonal fires, large herbivores (elephants, giraffes). - *Freshwater*: Lake Ossa (Littoral Region) – supports fish, crocodiles, waterbirds; fed by Sanaga River. - *Marine*: Cameroonian coastal waters – mangroves (e.g., in Douala Estuary), coral reefs (limited), fisheries. - *Desert*: Sahelian fringe (North) – sparse vegetation (*Acacia tortilis*), adapted fauna (dorcas gazelle). 2. **Artificial (human-modified) ecosystems**: - Agricultural fields (e.g., coffee plantations in West Region). - Urban ecosystems (e.g., Yaoundé city – buildings, pollution, adapted species like pigeons and rats). - Reservoirs (e.g., Lagdo Reservoir – hydropower, fishing, altered river ecology). **Vocabulary to retain**: Natural ecosystem, artificial ecosystem, tropical rainforest, savanna, freshwater ecosystem, marine ecosystem, desert ecosystem. **Encadré à retenir**: > Korup National Park exemplifies a tropical rainforest ecosystem with high biodiversity, multilayer vegetation structure, and rapid nutrient cycling via decomposers. **Example of exam question with plan**: *Question*: Compare the structure and functioning of a savanna ecosystem (Waza National Park) and a tropical rainforest ecosystem (Korup National Park) in Cameroon. *Plan*: I. Climate: Savanna – seasonal rainfall (800–1200 mm), distinct wet/dry seasons; Rainforest – >2500 mm year-round, no dry season II. Vegetation: Savanna – grasses, fire-resistant trees (e.g., *Acacia*); Rainforest – tall evergreen trees, multiple canopy layers III. Fauna: Savanna – large herbivores, predators; Rainforest – high insect diversity, arboreal mammals IV. Soil: Savanna – nutrient-rich laterite; Rainforest – nutrient-poor, nutrients stored in biomass V. Nutrient cycling: Savanna – slower, fire-mediated; Rainforest – rapid, decomposer-driven VI. Human impact: Savanna – overgrazing, desertification risk; Rainforest – logging, poaching ## IV. TU 83. Lesson 20: Interactions within the ecosystem **Context**: Focuses on biotic interactions that shape community structure and ecosystem dynamics. **Types of Biotic Interactions**: 1. **Competition**: Organisms vie for limited resources (light, water, nutrients, space). - *Intraspecific*: Same species (e.g., two maize plants competing for nitrogen). - *Interspecific*: Different species (e.g., *Chromolaena odorata* invading farmland, outcompeting native grasses). 2. **Predation**: One organism (predator) kills and eats another (prey). - Example: Leopard preying on antelope in Benoue National Park. 3. **Herbivory**: Animals feeding on plants. - Example: Elephants browsing on *Acacia* trees in savannas. 4. **Parasitism**: One organism (parasite) benefits at the expense of the host. - Example: Mistletoe parasitizing trees in Cameroonian forests. 5. **Mutualism**: Both organisms benefit. - Example: Ants and *Acacia* trees – ants protect tree from herbivores, tree provides shelter and food. - Example: Mycorrhizal fungi and plant roots – fungi aid nutrient uptake, plants supply sugars. 6. **Commensalism**: One benefits, the other is unaffected. - Example: Epiphytic orchids growing on tree branches (no harm to tree). **Effects of Interactions**: - Shape species distribution and abundance. - Drive natural selection and evolution. - Influence ecosystem stability and resilience. - Can lead to keystone species effects (e.g., elephants as ecosystem engineers in savannas). **Vocabulary to retain**: Competition, predation, herbivory, parasitism, mutualism, commensalism, keystone species, intraspecific, interspecific. **Encadré à retenir**: > Mutualism is a symbiotic relationship where both species benefit, such as ants protecting Acacia trees in exchange for food and shelter in Cameroonian savannas. **Example of exam question with plan**: *Question*: Describe three types of biotic interactions in the Korup National Park ecosystem and explain their ecological significance. *Plan*: I. Mutualism: Ants and *Acacia* spp. – protection for tree, food/shelter for ants → enhances tree survival II. Predation: Leopard and forest duiker – regulates prey population, prevents overgrazing III. Competition: Lianas and canopy trees – competition for light → influences forest structure and gap dynamics IV. Significance: These interactions maintain biodiversity, energy flow, and ecosystem stability ## V. TU 84. Lesson 21: Energy flows **Context**: Explains how energy moves through ecosystems, emphasizing the unidirectional flow and energy loss. **Principles of Energy Flow**: - **Source**: Solar energy is the primary source for almost all ecosystems. - **Capture**: Only ~1–2% of solar energy is captured by producers via photosynthesis. - **Transfer**: Energy moves from producers to primary consumers, then to secondary/tertiary consumers. - **Loss**: At each trophic level, ~90% of energy is lost as heat (respiration), movement, and undigested waste (10% rule). - **Unidirectional**: Energy flows in one direction; it is not recycled like nutrients. - **Energy pyramids**: Always upright due to energy loss; biomass and numbers pyramids can be inverted. **Example**: In a Cameroonian savanna: - Grass (producer) captures solar energy → 10,000 kcal - Grasshopper (primary consumer) → ~1,000 kcal - Lizard (secondary consumer) → ~100 kcal - Snake (tertiary consumer) → ~10 kcal - Hawk (quaternary consumer) → ~1 kcal **Vocabulary to retain**: Energy flow, trophic level, 10% rule, producer, primary consumer, secondary consumer, tertiary consumer, energy pyramid, unidirectional flow. **Encadré à retenir**: > Only about 10% of energy is transferred from one trophic level to the next; approximately 90% is lost as heat at each level. **Example of exam question with plan**: *Question*: With the aid of an energy pyramid, explain why food chains rarely exceed four or five trophic levels in ecosystems like the Waza National Park savanna. *Plan*: I. Energy enters via sunlight → captured by grass (producer) II. ~10% transferred to grasshopper (primary consumer) III. ~10% of that to lizard (secondary consumer) IV. ~10% to snake (tertiary consumer) V. ~10% to hawk (quaternary consumer) → only ~1 kcal remains VI. Insufficient energy to support a fifth level → food chains limited to 4–5 levels VII. Energy loss due to respiration, heat, undigested matter ## VI. TU 85. Lesson 22: Trophic levels **Context**: Builds on energy flow to classify organisms by their feeding position. **Definition**: - **Trophic level**: The position an organism occupies in a food chain or food web based on its source of nutrition. **Levels**: 1. **First trophic level (T1)**: Producers (autotrophs) – e.g., grasses, trees, phytoplankton. 2. **Second trophic level (T2)**: Primary consumers (herbivores) – e.g., grasshoppers, zebras, caterpillars. 3. **Third trophic level (T3)**: Secondary consumers (carnivores that eat herbivores) – e.g., frogs, lizards, small birds. 4. **Fourth trophic level (T4)**: Tertiary consumers (carnivores that eat other carnivores) – e.g., snakes, foxes, hawks. 5. **Fifth trophic level (T5)**: Quaternary consumers (top predators) – e.g., lions, leopards, crocodiles (rare due to energy loss). **Food Chains vs. Food Webs**: - **Food chain**: Linear sequence (e.g., grass → grasshopper → lizard → snake). - **Food web**: Interconnected food chains showing complex feeding relationships (more realistic). - Example: In Lake Ossa, phytoplankton → zooplankton → small fish → Nile perch → bird; also phytoplankton → insects → fish → crocodile. **Vocabulary to retain**: Trophic level, producer, primary consumer, secondary consumer, tertiary consumer, quaternary consumer, food chain, food web, autotroph, heterotroph. **Encadré à retenir**: > Organisms are classified into trophic levels based on their feeding position: producers (T1), primary consumers (T2), secondary consumers (T3), and so on, with energy decreasing at each level. **Example of exam question with plan**: *Question*: Construct a food web for the Lake Ossa ecosystem and identify the trophic levels of three organisms. *Plan*: I. Producers (T1): Phytoplankton, aquatic plants (e.g., *Nymphaea*) II. Primary consumers (T2): Zooplankton, herbivorous insects, tilapia III. Secondary consumers (T3): Small fish (e.g., *Barbus*), frog larvae IV. Tertiary consumers (T4): Nile perch, kingfisher V. Quaternary consumers (T5): African fish eagle, crocodile VI. Food web connections: Phytoplankton → zooplankton → small fish → Nile perch → eagle; also → insects → fish → crocodile VII. Significance: Shows multiple pathways, increases ecosystem stability ## VII. TU 85. Lesson 23: Nutrient cycling **Context**: Contrasts with energy flow by showing how nutrients are recycled within ecosystems. **Definition**: - **Nutrient cycling**: The movement and exchange of organic and inorganic matter back into the production of living matter. **Key Nutrient Cycles**: 1. **Carbon cycle**: - CO₂ absorbed by plants in photosynthesis → converted to glucose. - Released via respiration, decomposition, combustion. - Stores: Atmosphere, oceans, biomass, fossil fuels. - Example: Decomposition of leaf litter in Korup releases CO₂; forests act as carbon sinks. 2. **Nitrogen cycle**: - **Fixation**: Atmospheric N₂ → NH₃ by bacteria (e.g., *Rhizobium* in legume roots) or lightning. - **Nitrification**: NH₃ → NO₂⁻ → NO₃⁻ by soil bacteria. - **Assimilation**: Plants absorb NO₃⁻/NH₄⁺ to make proteins. - **Ammonification**: Decomposers convert organic N → NH₃. - **Denitrification**: NO₃⁻ → N₂ by anaerobic bacteria (returns N to atmosphere). - Example: Legumes in Cameroonian fallow fields fix nitrogen, improving soil fertility. 3. **Phosphorus cycle**: - **Weathering**: Rocks release phosphate (PO₄³⁻). - **Absorption**: Plants take up PO₄³⁻ from soil. - **Consumption**: Animals obtain P via food. - **Decomposition**: Returned to soil via waste and dead matter. - **Sedimentation**: Phosphorus may sink in water bodies and form rocks over time. - No atmospheric phase; slower cycle. - Example: Phosphorus from weathered granite supports plant growth in Adamawa highlands. **Vocabulary to retain**: Nutrient cycling, carbon cycle, nitrogen cycle, phosphorus cycle, fixation, nitrification, assimilation, ammonification, denitrification, weathering. **Encadré à retenir**: > In the nitrogen cycle, atmospheric nitrogen is fixed by bacteria (e.g., Rhizobium) into ammonia, which plants use to synthesize proteins. **Example of exam question with plan**: *Question*: Describe the nitrogen cycle and explain its importance for agricultural productivity in the Western Highlands of Cameroon. *Plan*: I. Stages: Fixation (Rhizobium in legumes, lightning), nitrification (NH₃ → NO₂⁻ → NO₃⁻), assimilation (plants uptake NO₃⁻), ammonification (decomposers → NH₃), denitrification (NO₃⁻ → N₂) II. Importance: Legume rotation (e.g., beans, groundnuts) enriches soil with nitrogen → reduces need for fertilizers → supports maize and coffee yields III. Example: Farmers in Bamenda use *Mucuna* cover crops to fix nitrogen before planting maize IV. Consequence of disruption: Overuse of fertilizers → leaching → water pollution (eutrophication) ## VIII. TU 86. Lesson 24: Nutrient cycling in contrasting eco-systems **Context**: Compares nutrient cycling in different ecosystem types to highlight environmental influences. **Comparison Table**: | Feature | Tropical Rainforest (Korup) | Savanna (Waza) | Freshwater (Lake Ossa) | |--------|------------------------------|----------------|-------------------------| | **Soil Nutrients** | Low in soil; stored in biomass | Moderate; enhanced by fire ash | Variable; sediments rich in organic matter | | **Decomposition Rate** | Very rapid (high temp, moisture) | Moderate (seasonal moisture) | Slow in depths, faster in shallows | | **Nutrient Cycling Speed** | Fast; tight cycle | Moderate; seasonal pulses | Slow; nutrients may sink and be lost | | **Key Process** | Rapid litterfall → decomposers → uptake | Fire releases nutrients; rapid uptake in wet season | Sedimentation; uptake by phytoplankton | | **Limiting Nutrient** | Often phosphorus | Nitrogen (in dry season), phosphorus | Phosphorus (in freshwater) | | **Human Impact Effect** | Deforestation → nutrient loss, erosion | Overgrazing → reduced organic matter → lower fertility | Pollution → algal blooms (eutrophication) | **Key Insights**: - In rainforests, nutrients are conserved in living matter; deforestation leads to rapid soil degradation. - In savannas, fire plays a dual role: destructive but also nutrient-releasing. - In freshwater systems, phosphorus is often limiting; excess leads to eutrophication. **Vocabulary to retain**: Biomass, decomposition rate, nutrient retention, limiting nutrient, eutrophication, sedimentation, fire-mediated cycling. **Encadré à retenir**: > In tropical rainforests like Korup, over 90% of nutrients are stored in the living biomass, making the ecosystem highly vulnerable to nutrient loss after deforestation. **Example of exam question with plan**: *Question*: Explain why nutrient cycling is more rapid in a tropical rainforest than in a savanna ecosystem, using examples from Cameroon. *Plan*: I. Temperature and moisture: Rainforest – consistently high (24–27°C, >2500 mm/year); Savanna – seasonal (wet: 200–300 mm/month; dry: <50 mm) II. Decomposer activity: High and constant in rainforest; reduced during dry season in savanna III. Litterfall: Continuous in rainforest; pulsed in savanna (linked to leaf fall in dry season) IV. Soil nutrients: Rainforest – nutrients in biomass, poor soil; Savanna – nutrients in soil, enhanced by fire ash V. Example: In Korup, leaf litter decomposes in weeks; in Waza, grass litter may persist months until rains return VI. Result: Nutrients recycled faster in rainforest → supports year-round growth ## IX. TU 87. Lesson 25: Nutrient cycling and ecosystem management **Context**: Applies nutrient cycling knowledge to sustainable resource management. **Principles of Sustainable Management**: - Maintain nutrient balance: Avoid depletion or excess. - Mimic natural cycles: Use agroforestry, crop rotation, intercropping. - Reduce external inputs: Minimize synthetic fertilizers and pesticides. - Protect decomposers: Preserve soil organic matter and microbial activity. - Prevent erosion: Maintain vegetation cover to retain nutrients. **Cameroonian Examples**: 1. **Agroforestry in the West Region**: Farmers integrate *Faidherbia albida* (apple-ring acacia) with maize. The tree fixes nitrogen, sheds leaves in rainy season (providing mulch), and regrows in dry season (reducing competition). 2. **Crop rotation in the North**: Farmers alternate sorghum with cowpeas (legume) to restore soil nitrogen. 3. **Composting in urban and peri-urban areas**: Household waste converted to compost for urban gardens in Yaoundé and Douala, reducing landfill use and enriching soil. 4. **Buffer zones along rivers**: Vegetated strips (e.g., along Sanaga River) filter runoff, trapping sediments and nutrients before they enter waterways. 5. **Controlled burning in savannas**: Early dry season fires reduce fuel load, prevent destructive late fires, and recycle nutrients without destroying soil structure. **Vocabulary to retain**: Sustainable management, agroforestry, crop rotation, intercropping, composting, buffer zone, controlled burning, nutrient balance. **Encadré à retenir> > Agroforestry systems like Faidherbia albida-maize intercropping in Cameroon’s West Region enhance soil fertility by fixing nitrogen and providing organic mulch. **Example of exam question with plan**: *Question*: Discuss two sustainable agricultural practices in Cameroon that promote nutrient cycling and explain how they work. *Plan*: I. Agroforestry with Faidherbia albida: - Tree fixes nitrogen via root nodules - Sheds leaves in rainy season → nutrient-rich mulch - Deep roots access groundwater → reduces competition with crops - Provides fodder, fuelwood, carbon sequestration II. Legume-cereal rotation (e.g., maize-cowpea): - Cowpea (legume) fixes atmospheric nitrogen → enriches soil - Subsequent maize crop uses nitrogen → higher yields without fertilizer - Breaks pest and disease cycles - Improves soil structure and organic matter III. Both practices reduce reliance on synthetic inputs, maintain soil health, and support long-term productivity ## X. TU 88. Lesson 26: Productivity of ecosystems **Context**: Measures how much biomass ecosystems produce over time. **Definitions**: - **Gross Primary Productivity (GPP)**: Total amount of carbon fixed by photosynthesis in a given area over time (e.g., g C/m²/year). - **Net Primary Productivity (NPP)**: GPP minus energy used by plants for respiration (NPP = GPP – Rₚ). Represents biomass available for consumers. - **Net Ecosystem Productivity (NEP)**: NPP minus heterotrophic respiration (decomposers and consumers); indicates whether ecosystem is a carbon sink (NEP > 0) or source (NEP < 0). - **Secondary Productivity**: Rate of biomass accumulation by consumers. **Factors Affecting Productivity**: - **Climate**: Temperature, precipitation, sunlight. - **Soil**: Nutrient availability, pH, depth. - **Vegetation**: Leaf area index, species type, growth form. - **Disturbance**: Fire, logging, grazing. **Productivity in Cameroonian Ecosystems** (approximate NPP values): - Tropical rainforest (Korup): 2000–3500 g C/m²/year (among highest globally) - Savanna (Waza): 500–1500 g C/m²/year - Freshwater (Lake Ossa): 200–800 g C/m²/year (phytoplankton-driven) - Desert fringe: <200 g C/m²/year **Vocabulary to retain**: Gross Primary Productivity (GPP), Net Primary Productivity (NPP), Net Ecosystem Productivity (NEP), secondary productivity, biomass, carbon fixation, respiration. **Encadré à retenir**: > Net Primary Productivity (NPP) equals Gross Primary Productivity (GPP) minus plant respiration and represents the biomass available for herbivores and decomposers. **Example of exam question with plan**: *Question*: Define Net Primary Productivity (NPP) and explain why tropical rainforests in Cameroon have higher NPP than savannas. *Plan*: I. Definition: NPP = GPP – Rₚ (energy used by plants in respiration); represents net biomass accumulation II. Climate: Rainforest – high year-round rainfall (>2500 mm), constant temp (24–27°C) → continuous photosynthesis Savanna – seasonal rainfall (800–1200 mm), long dry season → photosynthesis limited to wet months III. Vegetation: Rainforest – evergreen, high leaf area index, multi-layer canopy → maximizes light capture Savanna – deciduous or drought-deciduous trees, grass layer dies back in dry season IV. Soil: Rainforest – nutrients in biomass, rapid cycling supports growth Savanna – seasonal nutrient pulses (fire-mediated), lower overall availability V. Result: Korup NPP ~3000 g C/m²/year; Waza NPP ~1000 g C/m²/year → rainforest produces 3x more biomass annually ## XI. TU 89. Practical work 2: Measurement of ecosystem productivity **Context**: Hands-on activity to estimate NPP using field methods. **Objective**: To estimate the Net Primary Productivity of a grassland or savanna ecosystem using the harvest method. **Method**: 1. Select a representative 1 m² plot in a homogeneous area (e.g., Waza National Park savanna). 2. Clip all vegetation at ground level within the plot. 3. Separate into fractions: leaves, stems, reproductive parts (if any). 4. Weigh fresh biomass (g). 5. Dry samples in oven at 60–80°C until constant weight (to remove water). 6. Record dry weight (g/m²). 7. Repeat in 5–10 plots for average. 8. Estimate NPP: Assuming biomass represents standing crop, and using turnover rate (e.g., 2–4 harvests/year for grasslands), calculate annual NPP. - *Example*: If dry biomass = 200 g/m² and turnover = 3 times/year → NPP ≈ 600 g/m²/year. 9. Compare with literature values for similar ecosystems. **Limitations**: - Destructive method. - Does not account for below-ground biomass (roots) without coring. - Short-term measurement may not reflect annual variation. - Grazing or herbivory may remove biomass before sampling. **Vocabulary to retain**: Harvest method, dry biomass, standing crop, turnover rate, below-ground biomass, oven-drying, representative plot. **Encadré à retenir> > The harvest method estimates Net Primary Productivity by measuring dry biomass per unit area and multiplying by the estimated number of growth cycles per year. **Example of exam question with plan**: *Question*: Describe the harvest method for measuring Net Primary Productivity in a savanna ecosystem and state two limitations of this technique. *Plan*: I. Select 5–10 random 1 m² plots in a uniform grassland area II. Clip all vegetation at ground level in each plot III. Separate into leaves, stems, inflorescences IV. Weigh fresh biomass, then dry in oven at 70°C for 48 hours V. Record dry weight per plot; calculate average dry biomass (g/m²) VI. Estimate annual NPP: Multiply by estimated number of harvests/year (e.g., 3 for tropical savanna) VII. Limitations: - Destructive – removes vegetation, cannot resample same plot - Underestimates total NPP if below-ground biomass (roots) is not measured VIII. Application: Used in Waza to compare productivity under different grazing intensities ## XII. TU 90. Lesson 27: Biomes **Context**: Defines biomes as large-scale ecological units shaped by climate. **Definition**: - **Biome**: A large community of vegetation and wildlife adapted to a specific climate, characterized by dominant plant forms and adjusted to regional climate conditions. **Major Biomes (zonal)**: 1. **Tropical rainforest**: High temp, high rainfall year-round → broadleaf evergreen forests (e.g., Korup, Congo Basin). 2. **Tropical seasonal forest/savanna**: High temp, seasonal rainfall → grasslands with scattered trees (e.g., Waza, Serengeti). 3. **Desert**: Low rainfall (<250 mm/year), high temp → sparse vegetation, adapted fauna (e.g., Sahelian fringe, Sahara). 4. **Mediterranean**: Mild wet winters, hot dry summers → sclerophyllous shrubs (not prominent in Cameroon). 5. **Temperate forest**: Moderate temp, seasonal rainfall → deciduous or coniferous trees (not in Cameroon). 6. **Boreal forest (taiga)**: Cold, long winters → coniferous forests (not in Cameroon). 7. **Tundra**: Very cold, short growing season → low vegetation, permafrost (not in Cameroon). **Cameroon’s Biomes**: - **South and East**: Tropical rainforest (Guineo-Congolian). - **Center and West**: Transition zone – tropical semi-deciduous forest and derived savanna. - **North**: Tropical savanna (Sudanian) and Sahelian steppe (fringe). **Vocabulary to retain**: Biome, tropical rainforest, tropical savanna, desert, sclerophyllous, deciduous, coniferous, Guineo-Congolian, Sudanian, Sahelian. **Encadré à retenir> > Cameroon spans three main biomes: tropical rainforest in the south, tropical savanna in the center and north, and Sahelian steppe in the extreme north. **Example of exam question with plan**: *Question*: Identify the three main biomes found in Cameroon and describe one distinguishing climatic and vegetative feature of each. *Plan*: I. Tropical rainforest (South/East): - Climate: High temperature (24–27°C), high rainfall (>2000 mm/year, no dry season) - Vegetation: Broadleaf evergreen trees, multi-layer canopy, high biodiversity (e.g., Korup) II. Tropical savanna (Center/North): - Climate: Seasonal rainfall (800–1500 mm), distinct wet (Apr–Oct) and dry (Nov–Mar) seasons - Vegetation: Grasses with fire-resistant trees (e.g., *Acacia*, *Butyrospermum*), deciduous in dry season III. Sahelian steppe (Extreme North): - Climate: Low rainfall (<600 mm/year), high evaporation, long dry season - Vegetation: Sparse grasses and thorny shrubs (e.g., *Acacia tortilis*), adapted to drought and grazing ## XIII. TU 91. Lesson 28: Characteristics of tropical biomes **Context**: Focuses on the structure, function, and adaptations of tropical ecosystems. **Characteristics of Tropical Rainforests**: - **Climate**: Constant high temperature (24–27°C), high annual rainfall (>2000 mm, often >2500 mm), no real dry season. - **Soil**: Often oxisols or ultisols – deeply weathered, low nutrient retention (nutrients in biomass), acidic. - **Vegetation structure**: - Emergent layer: Tallest trees (40–50 m, e.g., *Ceiba pentandra*). - Canopy: Continuous layer (25–35 m), dense, absorbs 70% of sunlight. - Understory: Shrubs, young trees, low light. - Forest floor: Sparse vegetation, thick leaf litter, rapid decomposition. - **Biodiversity**: Highest on Earth – 50% of terrestrial species in <10% of area. - **Adaptations**: - Plants: Drip tips on leaves, buttress roots, lianas, epiphytes (orchids, bromeliads). - Animals: Arboreal locomotion, camouflage, specialized diets (e.g., fruit bats, toucans, chimpanzees). - **Nutrient cycling**: Rapid; >90% of nutrients in living biomass; decomposers (fungi, termites) essential. - **Productivity**: Very high NPP (2000–3500 g C/m²/year). **Characteristics of Tropical Savannas**: - **Climate**: High temperature, seasonal rainfall (500–1500 mm), pronounced wet and dry seasons. - **Soil**: Often alfisols or vertisols – better nutrient retention than rainforest soils; enriched by fire ash. - **Vegetation structure**: - Continuous grass layer (mainly C4 grasses like *Hyparrhenia*). - Scattered trees and shrubs (often deciduous, fire-resistant, e.g., *Acacia*, *Combretum*, *Terminalia*). - Trees may have thick bark, deep roots, ability to resprout after fire. - **Adaptations**: - Plants: Deep roots, water-storing stems, fire-resistant bark, deciduous habit in dry season. - Animals: Migration (e.g., kob antelope), seasonal breeding, water dependency. - **Fire**: Natural and human-induced; maintains grassland by preventing woody encroachment. - **Productivity**: Moderate NPP (500–1500 g C/m²/year); peaks in early wet season. **Vocabulary to retain**: Emergent layer, canopy, understory, forest floor, drip tip, buttress roots, liana, epiphyte, C4 grass, fire-resistant bark, oxisol, ultisol, alfisol, vertisol. **Encadré à retenir> > Tropical rainforests exhibit multilayered vegetation structure (emergent, canopy, understory, forest floor) and have over 90% of their nutrients stored in living biomass rather than soil. **Example of exam question with plan**: *Question*: Explain how the vegetation structure and adaptations of plants in tropical rainforests enable them to thrive in high-rainfall, low-light understory conditions. *Plan*: I. Canopy absorbs 70% of sunlight → understory receives <3% light II. Understory adaptations: - Large, thin leaves to maximize light capture - Drip tips to shed water and prevent fungal growth - Shade tolerance – low photosynthetic compensation point - Slow growth, long lifespan III. Canopy adaptations: - Small, thick leaves to reduce water loss - Epiphytes (e.g., orchids) grow on branches to access light - Lianas climb trees to reach canopy IV. Soil nutrients: Stored in biomass → rapid decomposition by fungi and termites supports growth V. Example: In Korup, understory herbs like *Palisota* have large leaves with drip tips; canopy trees like *Irvingia* have buttress roots for stability in shallow soil