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Patterns of Biodiversity: Latitudinal Gradient and Species-Area Relationship

Patterns of biodiversity are regular trends in species richness, especially the increase in diversity from poles to tropics and the increase in species number with sampled area, expressed by the species-area relationship.

In one line: Tropics are old, warm and stable; bigger area means more niches.

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Understand it

  • Two major biodiversity patterns are important for exams.
  • The latitudinal gradient shows that species richness generally increases from high latitudes towards the equator because tropical regions have had more stable climate, longer evolutionary time and greater solar energy input.
  • The species-area relationship shows that larger areas usually contain more species because they include more habitats and niches.
  • On a log scale, the relationship is written as log S = log C + Z log A, where S is species richness, A is area, C is a constant and Z is the slope.
  • For very large areas, the slope becomes steeper.

Example

Tropical rainforests have far more species than temperate forests. A small island may have fewer bird species than a larger island because the larger island offers more habitat types and resources.

Rule to remember

Species-area relationship: log S = log C + Z log A.

S means species richness, A means area, C is the y-intercept constant and Z is the slope of the line on a log-log plot.

See it worked out

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

Worked example

If log S = log C + Z log A is given and the question asks what happens when area increases, state that species richness increases because a larger area usually includes more habitats, resources and ecological niches.

Method you can reuse

  1. 1Identify whether the question is about latitude or area.
  2. 2For latitude, compare tropics and temperate or polar regions and give ecological reasons.
  3. 3For area, write the species-area equation, define S, A, C and Z, then explain that increasing area increases available niches and therefore species richness.

Visual tip

Diagram required: species-area curve and log-log straight line. Key labels: species richness on y-axis, area on x-axis, log S, log A, slope Z, intercept log C, steeper slope for very large regions.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why do tropical regions generally show higher species richness than temperate regions?

Think first, then show answer
Tropical regions generally show higher species richness because they have had more stable climates, longer uninterrupted evolutionary time and high solar energy, which support more niches and speciation.

Common wrong answer

Writing that Z is species richness is wrong; Z is the slope of the line in the log-log species-area relationship.

Don't mix this up

Students often remember the formula but forget what S, A, C and Z represent, or draw the species-area curve as a straight line on the normal scale without mentioning the log transformation.

Exam tip

For species-area questions, always write the formula, define each symbol and mention that Alexander von Humboldt described the relationship between species richness and explored area.

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 formula interpretation, graph labelling, reason for high tropical diversity, or assertion-reason linking stable tropical climate with greater species richness.

Answer plan

State the pattern, write the species-area equation when relevant, define all symbols, draw or describe the graph with labelled axes, and add one reason such as habitat variety, stable climate or evolutionary time.
  1. 1

    Enough for 1 mark

    Patterns of biodiversity are regular trends in species richness, especially the increase in diversity from poles to tropics and the increase in species number with sampled area, expressed by the species-area relationship.

  2. 2

    Add for 2 marks

    Example: Tropical rainforests have far more species than temperate forests. A small island may have fewer bird species than a larger island because the larger island offers more habitat types and resources.

  3. 3

    Add for 3 marks

    The latitudinal gradient shows that species richness generally increases from high latitudes towards the equator because tropical regions have had more stable climate, longer evolutionary time and greater solar energy input.

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