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CraftExam
medium difficulty7 min

Population Interactions in Communities

Population interaction is the effect that individuals of one species have on individuals of another species when they live together in a community. The interaction may benefit, harm or have no major effect on either species.

In one line: First mark the signs, then name the interaction.

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Understand it

  • Species in a community interact for food, space, shelter, pollination, dispersal or protection.
  • In mutualism, both species benefit; in competition, both may be harmed due to shared limiting resources; in predation and parasitism, one benefits while the other is harmed; in commensalism, one benefits and the other is unaffected; in amensalism, one is harmed and the other is unaffected.
  • Gause's competitive exclusion principle states that two closely related species competing for the same limiting resource cannot coexist indefinitely in the same niche.
  • This concept is frequently tested through examples and sign notation.

Example

Fig trees and their pollinating wasps show mutualism because the wasp pollinates the fig while using it for reproduction. A tiger preying on deer shows predation, while a tapeworm living in the intestine of a host shows parasitism.

Rule to remember

Interaction signs: mutualism +/+, competition -/-, predation +/-, parasitism +/-, commensalism +/0, amensalism -/0.

Gause's principle applies when two species compete for the same limiting resource in the same niche.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If an orchid grows on a mango tree for support and does not draw nutrition from the tree, identify the interaction.
  2. 2The orchid benefits by getting better light, while the tree is not significantly affected, so it is commensalism.

Method you can reuse

To identify an interaction: determine whether species A benefits, is harmed or is unaffected; do the same for species B; match the signs to the interaction name; add a correct biological example.

Visual tip

A sign-table or interaction matrix is useful. Labels should show species A, species B, effect symbols and examples for each interaction type.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How is commensalism different from mutualism?

Think first, then show answer
In commensalism, one species benefits while the other is neither helped nor harmed. In mutualism, both interacting species benefit from the association.

Common wrong answer

Calling every close association mutualism.

Why it loses marks: One partner may be harmed or unaffected depending on the interaction.

Don't mix this up

Students often confuse predation and parasitism. In predation, the predator usually kills and eats prey; in parasitism, the parasite lives in or on the host and harms it over time.

Exam tip

For interaction questions, write the effect on both species using +, -, or 0, then add a specific example.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask for definitions with examples, sign notation, Gause's competitive exclusion principle, or case-based identification of interaction type.

Answer plan

Name the interaction, write its sign notation, define it in one sentence, give a specific example, and explain the effect on both species. For competition, add Gause's principle when the question mentions same niche or limiting resource.
  1. 1

    Enough for 1 mark

    Population interaction is the effect that individuals of one species have on individuals of another species when they live together in a community. The interaction may benefit, harm or have no major effect on either species.

  2. 2

    Add for 2 marks

    Example: Fig trees and their pollinating wasps show mutualism because the wasp pollinates the fig while using it for reproduction. A tiger preying on deer shows predation, while a tapeworm living in the intestine of a host shows parasitism.

  3. 3

    Add for 3 marks

    Species in a community interact for food, space, shelter, pollination, dispersal or protection.

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