Ecosystems

What is an Ecosystem?

An ecosystem is the interaction of a community of living organisms (biotic factors) with the non-living parts of their environment (abiotic factors). Abiotic factors include light intensity, temperature, moisture, soil pH, and wind speed. Biotic factors include predation, competition, disease, and food availability.

A habitat is where an organism lives. A population is all the organisms of one species in a habitat. A community is all the organisms of all species living in a habitat. An ecosystem includes both the community and the abiotic factors.

Food Chains and Food Webs

A food chain shows the flow of energy from one organism to the next. It always starts with a producer (a green plant or algae that photosynthesises). Producers are eaten by primary consumers (herbivores), which are eaten by secondary consumers (carnivores), and so on. Each stage is called a trophic level.

In reality, most organisms eat more than one thing, so food chains interconnect to form food webs. Food webs give a more accurate picture of feeding relationships in an ecosystem. If one species is removed from a food web, it affects many other species.

Energy is transferred along food chains, but only about 10% passes from one trophic level to the next. The rest is lost through respiration (as heat), movement, and in waste products like urine and faeces. This limits the length of food chains to typically 4-5 trophic levels.

Predator-Prey Relationships

Predator and prey populations are interdependent. When the prey population increases, there is more food for predators, so the predator population increases. More predators eat more prey, so the prey population decreases. With less food, the predator population then decreases, allowing the prey population to recover. This creates a cycle.

When plotted on a graph, predator and prey populations show linked cycles, with the predator curve slightly behind the prey curve. This is a very common exam question, so make sure you can describe and explain the pattern.

Adaptations

Organisms have features that help them survive in their environment. Structural adaptations are physical features, like the thick fur of arctic foxes or the large ears of desert foxes for heat loss. Behavioural adaptations include migration, hibernation, and nocturnal activity. Functional adaptations are internal processes, like the ability of camels to tolerate high body temperatures.

Extremophiles are organisms adapted to survive in extreme environments such as deep-sea vents (high pressure, high temperature), salt lakes, or hot springs. Bacteria are the most common extremophiles.

Biodiversity

Biodiversity is the variety of all living organisms in an ecosystem. High biodiversity makes an ecosystem more stable because if one species is affected, others can fill its role. Low biodiversity makes ecosystems fragile.

Human activities that reduce biodiversity include deforestation, pollution, overfishing, agriculture (monoculture), and urbanisation. We can maintain biodiversity through breeding programmes, seed banks, wildlife reserves, reintroduction of hedgerows, and reducing deforestation.

Key Exam Tips

• Always start food chains with a producer, never the sun
• Use arrows to show the direction of energy transfer, not who eats whom
• Remember: only ~10% of energy passes between trophic levels
• Predator curves lag behind prey curves on population graphs
• Know the difference between abiotic and biotic factors