Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A stable ecosystem can continue for thousands of years. It needs a steady supply of energy, recycling of nutrients, genetic diversity and a climate within the tolerance limits of its species. Sealed mesocosms are small model ecosystems used to test this: matter is recycled inside, but light energy must enter.
Stability can be lost. Beyond a tipping point, a change continues by itself and the ecosystem moves to a new state, as may happen if too much Amazon forest is cleared. Removing a keystone species can also cause collapse.
Human harvesting is sustainable only if the resource is replaced as fast as it is removed. Pollution by nutrients, persistent toxins and plastics also damages ecosystems.
In eutrophication, students say the algae use up the oxygen. Living algae produce oxygen in the light. The oxygen is used by the bacteria that decompose the dead algae.
Answer: 20 000 times more concentrated, by biomagnification.
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