Effects of humans on ecosystems

Biology · Relationships of organisms with one another and with the environment. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

Humans damage ecosystems in several ways. Deforestation is the clearing of forests for timber, farmland, roads and building. It destroys habitats, so biodiversity falls and species can become extinct. Without roots to hold the soil and leaves to break the fall of rain, soil is washed away and flooding increases. Fewer trees also means less carbon dioxide is removed by photosynthesis.

Over-harvesting means taking plants or animals faster than they can reproduce. Introducing a non-native species can harm native species through competition, predation or disease.

Eutrophication is water pollution caused by fertilisers or untreated sewage. You must learn its steps in the correct order.

Rules to remember

Deforestation → loss of habitat → less biodiversity and more extinction.
Deforestation → no roots to hold soil → soil erosion; less water absorbed and more run-off → flooding.
Deforestation → less photosynthesis, and burning or decay of trees → more carbon dioxide in the atmosphere.
Eutrophication 1: nitrate and other ions enter the water → producers (algae) grow rapidly.
Eutrophication 2: algae block light, plants below die, and algae die → more decomposition.
Eutrophication 3: decomposers respire aerobically and use up dissolved oxygen → fish and other animals die.
A non-native species may have no natural predators, so its population grows and it outcompetes or eats native species.

Common mistake

Students write that the algae use up the oxygen. The oxygen falls because decomposers (bacteria) respire aerobically while breaking down the dead algae and plants.

Worked example

Untreated sewage enters a river. Dissolved oxygen is 9 mg/dm³ upstream of the pipe and 2 mg/dm³ just downstream. Explain the fall and its effect on fish.

Sewage contains organic matter, which is food for decomposers such as bacteria.
The bacteria multiply and respire aerobically as they break the sewage down.
Their respiration uses dissolved oxygen, so the concentration falls from 9 to 2 mg/dm³, a drop of 7 mg/dm³.

Answer: Decomposers use up the oxygen; fish that need a high oxygen concentration die or move away.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Ecology

Keep going

← Ecosystems and biodiversity Conservation →Take a Biology test