A: During succession , nutrient availability decreases .
R: NCP (Net community productivity ) decreaes during succession.

Options

A. If both Assertion &Reason are true and the reason is the corect explanation of the assertion , then mark (1)
B. If both Assertion & Reason are true but the reason, is not the correct explanation of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false , them mark (3).
D. If both Asseration and Reason are false statements , then mark (4).

Answer

### Step-by-Step Solution

1. **Understanding the Assertion (A)**: The assertion states that during succession, nutrient availability decreases. This means that as an ecosystem undergoes succession, the amount of nutrients available for organisms, particularly producers, diminishes.

2. **Understanding the Reason (R)**: The reason states that Net Community Productivity (NCP) decreases during succession. NCP is defined as the net primary productivity (NPP) minus the gross secondary productivity (GSP).

3. **Defining Key Terms**:
- **Net Primary Productivity (NPP)**: This is the amount of energy or biomass that remains after producers (like plants) have used some of the energy for their own respiration. It reflects the energy available for the next trophic levels.
- **Gross Secondary Productivity (GSP)**: This is the total energy or biomass that is consumed by heterotrophs (consumers) in an ecosystem.

4. **Analyzing the Relationship**:
- During succession, the diversity of organisms increases, particularly the number of heterotrophs. This leads to an increase in GSP as more energy is consumed by these organisms.
- However, the increase in GSP does not correspondingly increase NPP, which may increase only slightly or remain constant. As a result, if GSP increases significantly while NPP does not, NCP will decrease.

5. **Connecting Assertion and Reason**:
- If NCP decreases, it implies that less energy is available in the biomass of producers. This leads to reduced nutrient recycling and ultimately decreases nutrient availability in the ecosystem.
- Therefore, the assertion that nutrient availability decreases during succession is supported by the reason that NCP decreases during succession.

6. **Conclusion**: Both the assertion and reason are true, and the reason provides an explanation for the assertion. Thus, the answer is that both statements are correct and the reason is a valid explanation for the assertion.