The species-area relationship, expressed as S = CA^Z, is a fundamental equation in ecology. Here, S is the number of species, A is the area, and C and Z are constants. The Z value is particularly important as it determines the slope of the curve when the relationship is plotted on a log-log scale. Understanding the properties and ecological significance of the Z value is essential for biodiversity studies, conservation planning, and biogeography.

Evaluating the Statements About ‘Z’

Let’s analyze each statement:

A. Z value is typically not greater than 0.4 across all ecosystem types.

• Correctness: This statement is partially correct. In most continental areas and for many taxa, Z values range from 0.15 to 0.35. However, in highly isolated systems like true islands, Z can be higher (sometimes up to 0.4 or more). So, while Z is often not greater than 0.4, it is not a strict rule “across all ecosystem types.”

B. Z value is positively related to a species’ dispersal capability, with flying and wind-dispersed organisms having the highest values.

• Correctness: This statement is incorrect. In fact, Z value is negatively related to dispersal capability. Organisms with high dispersal abilities (like birds or wind-dispersed plants) tend to have lower Z values because they can colonize new areas more easily, making species accumulation with area less steep. Conversely, poor dispersers have higher Z values.

C. Z value, which represents the slope in the relationship, declines with area, especially when large landmasses such as continents are considered.

• Correctness: This statement is correct. On small, isolated patches (like islands), Z values are higher. On larger scales, such as continents, Z values tend to be lower, reflecting a shallower slope in the species-area relationship.

D. The Z value is the exponent in the power model and can be used to estimate the proportion of area required to represent a given proportion of species present in any land class.

• Correctness: This statement is correct. Z is indeed the exponent in the power model and is used in conservation planning to estimate how much area is needed to conserve a certain proportion of species.

Matching With the Options

• A and B: Only A is mostly correct, B is incorrect.

• A and D: A is mostly correct, D is correct.

• B and C: B is incorrect, C is correct.

• C and D: Both C and D are correct.

Conclusion

The combination that represents all correct statements is:

(4) C and D

Summary Table

Correct answer:
(4) C and D