G11: Ecosystems

📋 What is an Ecosystem?

Every ecosystem has two main components:

Biotic Components (Living)

• Producers — plants and algae that make their own food through photosynthesis, converting sunlight energy into chemical energy stored in glucose
• Consumers — animals that eat other organisms. Primary consumers eat plants; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers
• Decomposers — bacteria and fungi that break down dead organisms and waste material, releasing nutrients back into the soil

Abiotic Components (Non-living)

• Climate — temperature, rainfall, sunlight, wind
• Soil — type, depth, nutrient content, pH, structure
• Water — availability, quality, salinity
• Landforms — altitude, slope, aspect
• Minerals and rocks — underlying geology influences soil and water chemistry

🌿 Producers, Consumers and Decomposers

Producers (Autotrophs)

• Green plants and algae that produce their own food through photosynthesis
• They convert light energy from the Sun into stored chemical energy (glucose)
• They form the base of all food chains — without producers, no consumers could survive
• Example: grass, trees, phytoplankton, seaweed

Consumers (Heterotrophs)

• Primary consumers (herbivores) — eat producers only. Examples: rabbits, caterpillars, deer, zooplankton
• Secondary consumers (carnivores) — eat primary consumers. Examples: foxes, frogs, small fish
• Tertiary consumers (top carnivores) — eat secondary consumers. Examples: hawks, sharks, lions
• Omnivores — eat both plants and animals. Examples: bears, humans, badgers

Decomposers (Saprotrophs)

• Bacteria and fungi that feed on dead organisms and waste material
• They release enzymes that break down organic matter into simpler substances
• They recycle nutrients back into the soil, making them available for plants
• Without decomposers, nutrients would remain locked in dead material and the ecosystem would run out of essential elements
• Examples: earthworms, woodlice, fungi, bacteria

🔗 Food Chains and Food Webs

♻️ Nutrient Cycling

The Carbon Cycle

• Plants absorb CO₂ from the atmosphere through photosynthesis and use it to build carbohydrates
• Carbon is passed along the food chain as animals eat plants and other animals
• All living organisms release CO₂ through respiration
• When organisms die, decomposers break down their bodies and release CO₂ back into the atmosphere
• Combustion (burning fossil fuels and wood) also releases CO₂
• Oceans absorb CO₂ from the atmosphere and slowly release it back

The Nutrient Store

The relative size of these stores varies between ecosystems. In tropical rainforests, most nutrients are stored in the biomass (living plants) because rapid decomposition returns nutrients to plants quickly and the soil is relatively nutrient-poor. In cold climates like the tundra, decomposition is very slow, so nutrients accumulate in the soil and litter.

🔄 How Ecosystems Change

Natural Changes

• Succession — the process by which ecosystems develop and change over time. Primary succession starts on bare rock or sand (e.g. after a volcanic eruption or glacier retreat). Pioneer species (lichens, mosses) colonise first, gradually building up soil. Over hundreds of years, larger plants (grasses, shrubs, then trees) replace the pioneers, eventually forming a climax community — the final, stable ecosystem
• Seasonal changes — temperature, rainfall and daylight hours vary through the year, affecting plant growth and animal behaviour (hibernation, migration, breeding)
• Natural disturbances — fires, floods, volcanic eruptions and storms can destroy parts of an ecosystem, restarting the succession process

Human-Induced Changes

• Deforestation — removing trees destroys habitats, reduces biodiversity, disrupts food webs and nutrient cycles
• Agriculture — converting natural ecosystems to farmland replaces diverse communities with single crops (monoculture), reducing biodiversity
• Urbanisation — building on land destroys habitats and fragments ecosystems
• Pollution — water pollution, air pollution and soil contamination harm organisms at all trophic levels
• Climate change — changing temperature and rainfall patterns alter the distribution of species and the functioning of entire ecosystems
• Introduction of non-native species — can outcompete or prey on native species, disrupting food webs

📈 Ecosystem Scale

Ecosystems exist at many different scales:

❓ Practice Questions

✅ Answers

1. An ecosystem is a community of living organisms (plants, animals and microorganisms) interacting with each other and with the non-living (physical) elements of their environment (soil, climate, water).
2. Biotic components are the living elements of an ecosystem. Examples: plants (producers), animals (consumers), bacteria and fungi (decomposers). Abiotic components are the non-living elements. Examples: temperature, rainfall, soil type, water availability, sunlight, rocks and minerals.
3. Producers (plants and algae) make their own food through photosynthesis, converting sunlight into chemical energy stored in glucose. They form the base of all food chains. Consumers (animals) cannot make their own food and must eat other organisms — primary consumers eat plants, secondary consumers eat primary consumers, and so on. Decomposers (bacteria and fungi) break down dead organisms and waste material, releasing nutrients back into the soil so they can be used again by producers.
4. Food chains rarely exceed 4–5 trophic levels because only about 10% of energy is transferred from one level to the next. The remaining 90% is lost through respiration, waste and heat. By the 4th or 5th trophic level, so little energy remains that it cannot support a further level of consumers. Top predators must be relatively few in number because the energy available to them is so limited.
5. Nutrients are recycled through the ecosystem in a continuous loop. Plants absorb nutrients (e.g. nitrogen, phosphorus) from the soil through their roots. These nutrients are passed along the food chain as animals eat the plants and other animals eat them. When organisms die or produce waste, decomposers (bacteria and fungi) break down the dead organic material, releasing the nutrients back into the soil. Plants can then absorb these nutrients again, completing the cycle. In a balanced ecosystem, the rate of nutrient uptake by plants roughly equals the rate of nutrient return by decomposers.
6. If one species is removed from a food web, it can cause cascading effects throughout the ecosystem. For example, if a top predator like a fox is removed: (1) Its prey (rabbits, mice) would increase in number due to reduced predation; (2) The increased herbivore population would eat more plants, reducing plant biomass; (3) Fewer plants would mean less food and habitat for other species, including insects and birds; (4) Other predators that compete with foxes might increase, while scavengers that fed on fox kills would decline. The interconnected nature of food webs means that removing one species can destabilise the entire ecosystem.

🎯 Exam Tips

• Always define 'ecosystem' if asked — it must include both biotic AND abiotic components
• Know the difference between producers, consumers and decomposers and their roles
• Use the 10% energy transfer rule to explain why food chains are short
• Draw food webs if asked — arrows point from the organism being eaten to the consumer
• When discussing ecosystem change, link cause and effect through the food web
• Remember that decomposers are essential — without them, nutrients would not be recycled

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