Ecosystems

What Is an Ecosystem?

An ecosystem is a community of living organisms interacting with each other and with their non-living environment in a particular area. Ecosystems range from tiny rock pools to vast tropical rainforests.

Every ecosystem has two components:

• Biotic factors — the living parts (plants, animals, fungi, bacteria, algae)
• Abiotic factors — the non-living parts (temperature, rainfall, sunlight, wind, soil type, pH, oxygen levels)

Examples of ecosystems: tropical rainforests, coral reefs, Arctic tundra, freshwater ponds, deserts, grasslands, oak woodlands.

Habitats and Niches

A habitat is the specific environment where an organism lives. It provides everything the organism needs: food, water, shelter, and suitable conditions for reproduction.

A niche is the role an organism plays within its ecosystem — what it eats, when it is active, how it reproduces, and how it interacts with other species. No two species can occupy exactly the same niche in the same habitat for long (competitive exclusion).

Adaptations

Adaptations are features that help an organism survive and reproduce in its environment. They arise through natural selection over many generations.

Types of Adaptation

Structural (physical) adaptations — features of the body:

• Polar bear: thick white fur (insulation + camouflage), small ears (reduce heat loss), large paws (swimming + walking on ice)
• Cactus: thick waxy stem (stores water), spines instead of leaves (reduce water loss), shallow spreading roots (capture brief rainfall)
• Shark: streamlined body (reduce water resistance), gills (extract oxygen from water)

Behavioural adaptations — things an organism does:

• Migration — birds fly to warmer climates in winter
• Hibernation — bears sleep through winter to conserve energy
• Nocturnal activity — desert animals avoid daytime heat

Physiological adaptations — internal body processes:

• Camels produce very concentrated urine and dry faeces (conserve water)
• Deep-sea fish can withstand enormous pressure
• Arctic fish have antifreeze proteins in their blood

Food Chains

A food chain shows the flow of energy between organisms — who eats whom.

Example: Grass → Grasshopper → Frog → Heron

Role	Description	Example
Producer	Makes its own food via photosynthesis; always a plant, alga, or cyanobacterium	Grass, oak tree, phytoplankton
Primary consumer	Eats the producer; a herbivore	Rabbit, caterpillar, deer
Secondary consumer	Eats the primary consumer; often an omnivore or carnivore	Fox, thrush, small fish
Tertiary consumer	Eats the secondary consumer; often a top predator	Eagle, shark, orca
Decomposer	Breaks down dead organisms and waste; returns nutrients to the soil	Bacteria, fungi

The arrow in a food chain means "is eaten by" or "energy flows to."

Food Webs

A food web is a network of interconnected food chains in an ecosystem. It is more realistic than a single food chain because most animals eat several different things.

Food webs help us understand:

• What happens when one species is removed (knock-on effects)
• Which species are most important to ecosystem stability
• How energy flows through a community

Energy Flow Through Ecosystems

Energy enters ecosystems through photosynthesis — plants convert sunlight into chemical energy (glucose).

As energy passes from one organism to the next, most is lost:

• As heat from respiration
• As movement energy
• As waste (urine, faeces)
• In body parts not eaten

Typically only 10% of energy is passed on to the next trophic level. This is why:

• Food chains rarely have more than 4–5 links (not enough energy for more)
• There are always far more producers than consumers (many plants needed to support few animals)
• Eating lower in the food chain is more energy efficient

Pyramids of Numbers and Biomass

Pyramid of numbers — shows the count of organisms at each level (can be unusual shapes if one tree supports many insects).

Pyramid of biomass — shows the total mass of organisms at each level (almost always a true pyramid shape).

Pyramid of energy — shows the energy available at each level (always a true pyramid; most accurate).

Population and Competition

Populations in an ecosystem are controlled by:

• Interspecific competition — competition between different species for food, space, water
• Intraspecific competition — competition within the same species (often most intense)
• Predation — predators control prey population sizes
• Disease — can reduce populations rapidly
• Abiotic factors — drought, extreme cold

Predator-Prey Cycles

When prey numbers are high, predators have plenty of food so their numbers rise. More predators eat more prey, reducing prey numbers. With less food, predator numbers fall. With fewer predators, prey recover. The cycle repeats.

Example: lynx and snowshoe hare populations in Canada show clear oscillating cycles.

Biodiversity

Biodiversity refers to the variety of life — including:

• Species diversity — the number of different species in an area
• Genetic diversity — the variety of genes within a species
• Ecosystem diversity — the variety of different habitats and ecosystems

High biodiversity makes ecosystems more stable and resilient — if one species declines, others can fill its ecological role.

Threats to Biodiversity

Threat	Example
Deforestation	Rainforest cleared for agriculture destroys habitat
Habitat destruction	Wetlands drained, hedgerows removed
Pollution	Pesticides, plastic, oil spills
Overfishing	Collapse of fish stocks
Invasive species	Grey squirrels outcompeting red squirrels
Climate change	Shifting habitats, coral bleaching

Conservation

Methods to protect biodiversity:

• Nature reserves and national parks — protect habitats
• Captive breeding programmes — breed endangered species in zoos, release to wild
• Seed banks — store seeds of plant species
• International agreements — e.g., CITES (controls trade in endangered species)
• Reforestation — planting trees to restore habitats
• Sustainable fishing — quotas to prevent overfishing

Key Terms

Term	Definition
Ecosystem	Community of organisms interacting with each other and their environment
Habitat	The specific environment where an organism lives
Adaptation	Feature that helps an organism survive in its environment
Producer	Organism that makes its own food through photosynthesis
Consumer	Organism that obtains energy by eating other organisms
Decomposer	Organism that breaks down dead matter and returns nutrients to the soil
Food web	Network of interconnected food chains
Biodiversity	Variety of different species in an ecosystem
Trophic level	Position of an organism in a food chain

Worked Example

Q: The rabbit population in an ecosystem suddenly drops due to disease. Predict the effects on the fox population and the grass.

When rabbit numbers fall:

• Foxes have less food available. Fox numbers will decrease as they struggle to find enough prey.
• Grass is no longer being eaten as heavily by rabbits, so grass numbers will increase.

Over time, if rabbit numbers recover (the disease passes), fox numbers will recover too, and grass will be reduced again as more rabbits eat it.

Common Mistakes

• Thinking all arrows in a food web represent "eats" — the arrow shows the direction of energy flow (from food to consumer), so it points in the direction of "is eaten by."
• Saying decomposers are at the bottom of the food chain — decomposers are not usually shown in food chains, but they play a vital role in recycling nutrients.
• Confusing adaptation with evolution — adaptation describes the features an individual organism has; evolution describes how those features arise over generations in a population.
• Thinking higher biodiversity always means more individuals — a field of just grass has millions of individual plants but very low biodiversity (few species).

Tips and Tricks

• Food chain direction: arrow points from food to the thing that eats it (direction of energy flow).
• At each trophic level, remember only ~10% of energy is passed on — so 1000 kg of grass supports 100 kg of rabbits, which supports 10 kg of foxes.
• Biotic = living; Abiotic = non-living (temperature, rainfall, light, pH, oxygen).
• When answering predator-prey questions, always think about the knock-on effects through the whole food web, not just the next level.