C
CraftExam
medium difficulty7 min

Productivity

Productivity is the rate of biomass or organic matter production by organisms in an ecosystem per unit area over a given time period.

In one line: Gross is total made; net is what remains after plant respiration.

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Understand it

  • Productivity measures how efficiently an ecosystem converts resources into organic matter.
  • Gross primary productivity, or GPP, is the total rate at which producers fix energy through photosynthesis.
  • Net primary productivity, or NPP, is the organic matter left after producers use some energy in respiration.
  • This NPP becomes available to herbivores and decomposers.
  • Secondary productivity is the rate of formation of new organic matter by consumers.
  • NPP depends on plant species, photosynthetic capacity, nutrients, temperature, rainfall and sunlight.
  • Exam questions often test the relation between GPP, respiration and NPP, and why productivity differs between ecosystems.

Example

A tropical rainforest usually has high primary productivity because it receives abundant light, warmth and rainfall, while a desert has lower productivity because water availability limits plant growth.

Rule to remember

Core relation: NPP = GPP - respiration loss by producers.

GPP is total photosynthetic production; NPP is stored biomass available to the next trophic level.

See it worked out

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

Worked example

  1. 1If producers fix 1000 units of energy and use 400 units in respiration, NPP is 600 units.
  2. 2This 600 units represents biomass available to consumers and decomposers.

Method you can reuse

  1. 1Define productivity as rate of biomass production.
  2. 2Define GPP as total production by producers.
  3. 3State respiration loss.
  4. 4Write NPP = GPP - respiration.
  5. 5Mention that NPP supports consumers and decomposers.

Visual tip

A flow diagram can show sunlight to producers, GPP, respiration loss and remaining NPP. Labels should include GPP, respiration, NPP, herbivores and decomposers.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is NPP more directly available to consumers than GPP?

Think first, then show answer
NPP is the organic matter remaining after plants use part of GPP for respiration. This remaining biomass is available as food to herbivores and later to decomposers.

Common wrong answer

Saying NPP is greater than GPP.

Why it loses marks: NPP is the part of GPP left after respiration loss.

Don't mix this up

Students often write that GPP and NPP are the same, or forget that respiration loss must be subtracted from GPP to obtain NPP.

Exam tip

When GPP and NPP appear together, always mention respiration. The relation NPP equals GPP minus respiration is the most tested scoring point.

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

Asked as a definition question, relation-based short answer, assertion-reason on GPP and NPP, or factor-based question on why productivity varies across ecosystems.

Answer plan

Define productivity. State GPP as total photosynthetic production. State NPP = GPP - respiration. Add that NPP is available to consumers. Mention one factor affecting productivity, such as light, nutrients, rainfall or temperature.
  1. 1

    Enough for 1 mark

    Productivity is the rate of biomass or organic matter production by organisms in an ecosystem per unit area over a given time period.

  2. 2

    Add for 2 marks

    Example: A tropical rainforest usually has high primary productivity because it receives abundant light, warmth and rainfall, while a desert has lower productivity because water availability limits plant growth.

  3. 3

    Add for 3 marks

    Productivity measures how efficiently an ecosystem converts resources into organic matter.

Practice

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