Ecosystem Pressures Part 1

Predict the effects of selection pressures on organisms in ecosystems by examining.

  • Biotic factors
  • Abiotic factors


Ecosytems are made up of both biotic factors and abiotic factors. Biotic factors are factors which relate to living things, where as, abiotic factors are related to non living things in an ecosystem. An ecosystem is a community of organisms which interact with one another as well as the physical environment.

The below image is a marine ecosystem. The abiotic factors include the water, sun, salts, rock, oxygen and carbon dioxide. Where as, the biotic factors are the fish, coral, micro organisms and algae.

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A selective pressure “selects” the traits which are most suited to particular conditions. This is done by the selective pressure killing organisms of a species before they are able to reproduce and pass on the less favorable traits. The frequency of the favorable trait within the population is then increased hence the species evolves to suit the environment. An abiotic factor which acts as a selective pressure is the amount of light available to low light plants which have been naturally selected to be the most suitable species for the low light. A biotic selective pressure for a rhino is the lion hence the rhino adapted a large size, horn and armour which protects it from other predators.


Question 1

Identify three biotic and three abiotic factors in the following ecosystem. (2 marks)

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Question 2 (2 marks)

Define an ecosystem using an example

Question 3 (3 marks)

The prickly pair cactus likely originated in Mexico where it is extremely dry and hot. Due to a particular selective pressure it has adapted to store large amounts of water within it’s leaves as well as developed the ability to limit water loss.

Explain the likely abiotic factor which has attributed to this adaptation.

Question 4 (3 marks)

Identify the biotic selective pressure’s which may have created the following adaptations.

  • Cane toads containing venom secreting poison glands which kill animals which try to eat them
  • Giraffes having long necks
  • Full grown elephants being one of the largest on land animals

Up Next: Ecosystem Pressure Part 2

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