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3.9.1Additional lessons

Ecosystems as Open Systems — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 19 of 101

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Part 1 of 5Open, Closed and Isolated Systems

3.9.1 Ecosystems as Open Systems

Ecologists describe an ecosystem by what crosses its boundary.

System
  • A system is a set of components that interact and are considered together, such as a pond, a forest or the whole of the biosphere.
Open system
  • An open system exchanges both energy and matter with its surroundings, which is true of most ecosystems.
Closed system
  • A closed system exchanges energy but not matter with its surroundings, and Earth as a whole is close to this.
Isolated system
  • An isolated system exchanges neither energy nor matter, and no real ecosystem is truly isolated.

Part 2 of 5Inputs and Outputs of an Ecosystem

Inputs and Outputs of an Ecosystem
Energy input
  • Light energy from the Sun enters and is converted into chemical energy by photosynthesis in producers.
Energy output
  • Energy leaves as heat, produced mainly by respiration, and is radiated back to the atmosphere and space.
Matter input
  • Water, gases and minerals enter through rainfall, wind, rivers and the movement of animals.
Matter output
  • Matter leaves in run-off, harvested crops, migration, gases from respiration and drainage of dissolved ions.

Part 3 of 5Sustainability of Ecosystems

1
Producers absorb nutrients, such as nitrate and phosphate ions, from the soil or water and build them into organic compounds.
2
Consumers obtain these nutrients by feeding, and waste and dead matter pass the nutrients on to decomposers.
3
Decomposers return the nutrients to the soil or water in inorganic form, ready for uptake by producers again.
4
Because nutrients are recycled while energy flows through in one direction, an ecosystem can persist for a very long time, provided the Sun keeps supplying energy.
ItemBehaviour in an ecosystem
EnergyFlows in one direction, entering as light and leaving as heat
NutrientsRecycled between organisms and the physical environment
WaterCycles via evaporation, rainfall and run-off

Part 4 of 5Worked Illustration: A Pond

Energy in
  • Sunlight falls on the pond and is captured by algae and water plants, which supply organic compounds to the rest of the community.
Matter in
  • Streams and rainfall bring water and dissolved ions, while oxygen and carbon dioxide are exchanged with the air.
Matter out
  • Water drains out of the pond, insects with aquatic larvae fly away as adults, and fish may be caught by birds from outside.
Energy out
  • All the organisms release heat through respiration, which warms the water and is lost to the air.
Recycling
  • Bacteria and fungi decompose dead plants and animals on the bottom, returning nitrate and phosphate ions to the water.

Part 5 of 5Exam Checklist & Tips

3.9.1 – Key Points for the Exam

  • Open system A system that exchanges energy and matter with its surroundings.
  • Closed system A system that exchanges energy but not matter.
  • Decomposer An organism that breaks down dead matter and releases nutrients.
  • Sustainability The ability of an ecosystem to continue without outside intervention.
  • Biosphere All the parts of Earth where life exists.

Common Mistakes

  • Saying ecosystems are closed systems because nutrients are recycled;
    • they still exchange matter with surroundings.
  • Thinking energy is recycled, when it flows and is lost as heat.
  • Forgetting that energy leaving the ecosystem is mostly heat.
  • Confusing an isolated system with a closed one.

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