In some cases, population density is measured in terms of biomass rather than in terms of number because

Options

A. it is a more meaningful measure when the considered organisms vary greately in size
B. it is more convenient when population is huge and counting is impossible or very time consuming
C. it is a relatively constatn measure
D. both (a) and (b)

Explanation

To answer the question "In some cases, population density is measured in terms of biomass rather than in terms of number because...", we can break down the reasoning into several key points:

### Step-by-Step Solution:

1. **Understanding Population Density**:
- Population density typically refers to the number of individuals of a species per unit area or volume. However, in certain contexts, this measure may not adequately represent the ecological significance of different organisms.

**Hint**: Consider what population density usually measures and why that might not always be sufficient.

2. **Biomass as a Measure**:
- Biomass refers to the total mass of living organisms in a given area or volume. It provides a more comprehensive understanding of the ecological role of organisms, especially when size and resource use vary significantly among species.

**Hint**: Think about how the size and mass of organisms can affect their role in an ecosystem.

3. **Example of Forest Ecosystem**:
- In a forest, if there are many small plants (e.g., 200 Parthenium plants) and only one large tree (e.g., a single Banyan tree), counting the number of individuals may suggest that the Parthenium is more significant. However, the Banyan tree's biomass and ecological contribution (like providing shade, habitat, etc.) are far greater.

**Hint**: Reflect on how the presence of a single large organism can have a larger impact than many smaller ones.

4. **Counting Challenges**:
- In cases where populations are very large or dense, such as bacteria in a Petri dish, counting individual organisms can be impractical or impossible. In these situations, measuring biomass becomes a more feasible and meaningful approach.

**Hint**: Consider scenarios where counting individual organisms is not practical.

5. **Conclusion**:
- Therefore, population density is measured in terms of biomass rather than numbers because:
- It provides a more meaningful measure of ecological significance when organisms vary greatly in size.
- It is more convenient when populations are large and counting is impractical.

### Final Answer:
Population density is measured in terms of biomass rather than in terms of number because it is a more meaningful measure when considering organisms that vary greatly in size and is more convenient when populations are large and counting is impossible or very time-consuming.

The correct answer is: **Both A and B** (Option 4).