Species-Area Z Value — 0.1-0.2 for Small Areas, 0.6-1.2 for Continents
📚 Practice ConceptEcologists have discovered that the value of Z lies in the range of 0.1 to 0.2, regardless of the taxonomic group or the region (whether it is the plants in Britain, birds in California or molluscs in New York state, the slopes of the regression line are amazingly similar). But, if you analyse the species-area relationships among very large areas like the entire continents, you will find that the slope of the line to be much steeper (Z values in the range of 0.6 to 1.2). For example, for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is found to be 1.15. What do steeper slopes mean in this context?
The Z value is constant at around 0.6-1.2 across all spatial scales, regardless of whether the area is small or continental.
Z = 0.1-0.2 for SMALL areas within a region. Z = 0.6-1.2 for CONTINENTAL scales. Z is SCALE-DEPENDENT, not constant.
Small scale = small slope (0.1-0.2). Continent scale = steep slope (0.6-1.2). Frugivores across continents = 1.15.
- Z range for small areas (within a region) = 0.1 to 0.2 — uniform across taxa and regions.
- Z range for continental scale = 0.6 to 1.2 — much steeper.
- Frugivorous birds and mammals across continents: Z = 1.15 (NCERT exact).
- Steeper slope = species richness rises RAPIDLY with area; captures endemic radiations across biogeographic barriers.
Consider the following statements about the species-area Z value: S1: Within a region, the Z value lies in the range 0.1 to 0.2 regardless of taxonomic group. S2: At continental scales, the Z value is in the range 0.6 to 1.2. S3: For frugivorous birds and mammals in tropical forests of different continents, Z = 1.15. S4: A steeper slope (higher Z) means species richness increases more slowly with area. S5: The Z value is the same at all spatial scales — small areas and continents share identical slopes.
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Correct answer: A — S1, S2 and S3
S1 CORRECT: Small-scale Z = 0.1-0.2 (NCERT, uniform across taxa). S2 CORRECT: Continental Z = 0.6-1.2. S3 CORRECT: Frugivorous birds/mammals across continents Z = 1.15 (NCERT exact). S4 WRONG: Steeper slope = species richness rises MORE RAPIDLY with area, not more slowly. S5 WRONG: Z is SCALE-DEPENDENT — small scales give 0.1-0.2, continental scales give 0.6-1.2. Not the same.
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