Ecology: Ecosystems and Food Chains

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Ecology: Ecosystems and Food Chains covers the relationships between organisms and their environment. An ecosystem is a community of living organisms interacting with their physical environment. It includes both biotic factors (living things: plants, animals, fungi, bacteria) and abiotic factors (non-living: sunlight, temperature, water, soil, minerals).

Key Definitions:

• Habitat: The natural home of an organism (e.g., rainforest, freshwater pond, savanna)
• Population: A group of organisms of the same species living in the same area
• Community: All populations of different species living in a habitat
• Biosphere: The region of Earth where life exists

Food Chains and Energy Flow: Energy flows through ecosystems in one direction: Sun → Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers → Decomposers

In a Nigerian savanna, a typical food chain might be: Grass → Grasshopper → Lizard → Snake → Hawk

In a freshwater pond: Phytoplankton → Water Flea → Small Fish → Large Fish → Kingfisher

The 10% Rule: Only about 10% of energy is passed from one trophic level to the next. The rest is lost as heat, used in respiration, or egested as waste.

Food Webs: Organisms usually eat multiple species, forming interconnected food webs rather than simple linear chains. This makes ecosystems more stable.

Key Equations:

• Net Primary Productivity (NPP) = Gross Primary Productivity − Respiratory losses
• Biomass = dry weight of organic matter per unit area

⚡ WAEC Tip: Questions on ecology are very common in WAEC. Remember that energy flows in ONE direction but chemicals cycle. Decomposers break down dead matter, releasing nutrients back to the soil — they are NOT part of the regular food chain but are essential to ecosystem function.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of ecology.

Ecosystem Structure and Components

An ecosystem has two main components:

1. Abiotic Components:

   • Light energy (solar radiation)
   • Temperature (affects enzyme activity and metabolism)
   • Water (essential for all metabolic reactions)
   • Soil minerals and nutrients (N, P, K, Ca, Mg)
   • Atmospheric gases (O₂, CO₂, N₂)
   • pH levels

2. Biotic Components:

   • Producers (Autotrophs): Plants, algae, cyanobacteria — make food via photosynthesis
   • Consumers (Heterotrophs):
     • Primary consumers (Herbivores): Feed on producers
     • Secondary consumers (Carnivores): Feed on herbivores
     • Tertiary consumers (Top carnivores): Feed on other carnivores
     • Omnivores: Feed on both plants and animals
   • Decomposers: Fungi and bacteria — break down dead organic matter

Pyramids of Numbers, Biomass, and Energy

The pyramid of numbers shows the number of organisms at each trophic level. In a grassland: many grasses → many grasshoppers → fewer lizards → even fewer snakes → few hawks.

The pyramid of biomass shows the total dry mass at each level. Usually pyramid-shaped, but can be inverted in some aquatic ecosystems (e.g., many small zooplankton supported by fewer phytoplankton).

The pyramid of energy is ALWAYS pyramid-shaped because energy is lost at each level. Energy pyramids cannot be inverted.

Carbon Cycle: CO₂ is fixed by photosynthesis → passed through food chain → returned via respiration, decomposition, combustion → reabsorbed by plants. In Nigeria, deforestation and burning of fossil fuels have disrupted the carbon cycle, contributing to climate change.

Nitrogen Cycle: N₂ → Ammonia (nitrogen fixation by Rhizobium bacteria in legume root nodules) → Nitrites → Nitrates (nitrification by Nitrosomonas and Nitrobacter) → absorbed by plants → returned via denitrification by Pseudomonas bacteria.

⚡ Common Student Mistakes: Confusing “population” with “community.” A population is one species; a community is multiple species together. Also confuse “habitat” with “ecosystem” — habitat is the location; ecosystem includes both habitat AND the organisms within it.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Ecological Interactions and Ecosystem Stability

Types of Ecological Relationships:

1. Predation: One organism kills and consumes another (lion and zebra)
2. Competition: Organisms compete for limited resources (two species of grass competing for water)
3. Symbiosis:
   • Mutualism: Both benefit (+/+) — e.g., lichen (fungi + algae); pollination by bees and flowers
   • Commensalism: One benefits, other unaffected (+/0) — e.g., barnacles on whales
   • Parasitism: One benefits, other harmed (+/−) — e.g., tapeworm in human intestine; malaria parasite in humans

Population Dynamics:

• Exponential growth: When resources are unlimited (J-curve)
• Logistic growth: Population stabilises at carrying capacity (S-curve)
• Carrying capacity (K): Maximum population size an environment can support
• Population density: Number of organisms per unit area

Ecological Succession:

• Primary succession: Starts on bare rock/void — no soil (e.g., volcanic islands)
• Secondary succession: Starts where soil already exists (e.g., after fire, abandoned farmland)
• Pioneer species: First colonisers (lichens, mosses) — tolerant of harsh conditions
• Climax community: Stable final stage (e.g., tropical rainforest in Nigeria)

Biomes of Nigeria:

• Tropical rainforest: High rainfall (>2000 mm/year), biodiversity hotspot, in southern Nigeria
• Guinea savanna: Grassland with scattered trees, central Nigeria
• Sudan savanna: drier, shorter grasses, northern Nigeria
• Mangrove swamp: Coastal areas, Niger Delta

Human Impact on Ecosystems:

• Deforestation: Loss of habitat, biodiversity decline, soil erosion. Nigeria has lost about 60% of its original forest cover.
• Desertification: Advancement of desert conditions into savanna regions
• Water pollution: Oil spills in Niger Delta affecting aquatic ecosystems
• Overfishing: Depletion of fish stocks in Nigerian waters
• Climate change: Rising temperatures affecting crop yields and shifting ecological zones

Conservation in Nigeria:

• National Parks: Yankari, Gashaka-Gumti, Okomu Forest Reserve
• Endangered species: Cross River gorilla, Nigeria-Cameroon chimpanzee, African elephant
• CITES: International treaty protecting endangered species from trade

Ecological Terms for WAEC:

• Ecosystem: Community + abiotic environment
• Ecological niche: The role and position of a species in its ecosystem (what it does, where it lives, how it interacts)
• Specialisation: Adaption to a narrow ecological niche
• Indicator species: Species that indicate environmental quality (e.g., lichens indicate air quality)

Food Chain Example (Nigerian pond): Phytoplankton (producer) → Water flea (primary consumer) → Dragonfly larva → Small fish → Kingfisher (tertiary consumer)

Decomposition Process: Dead organic matter → Decomposers (fungi, bacteria) produce enzymes → Break down proteins, carbohydrates, fats → Release CO₂, NH₃, minerals → Nutrients recycled to soil and water

⚡ WAEC Examination Patterns: Expect questions on drawing and interpreting pyramids of numbers/biomass/energy. Be prepared to explain why energy pyramids are always pyramid-shaped. Know the differences between producers, consumers, and decomposers. Understand how human activities affect Nigerian ecosystems.