Energy enters an ecosystem as sunlight and is lost as heat. This heat cannot be recycled. The flow of energy through an ecosystem is therefore in one direction, that is, it is linear.
The flow of nutrients within an ecosystem is not linear, but cyclic.
All nutrient cycles have one simple sequence:
- The nutrient is taken up by producers as simple, inorganic molecules
- The produce incorporates the nutrient into complex organic molecules
- When the produce is eaten, the nutrient passes into consumers
- It then passes along the food chain when these animals are eaten by other consumers
- When the producers and consumers die, their complex molecules are broken down by saprobiotic microorganisms that release the nutrient it its simplest form