To determine which option is not a functional unit of an ecosystem, we need to analyze each of the given options: productivity, stratification, energy flow, and decomposition.
### Step-by-Step Solution:
1. **Understanding Productivity**:
- Productivity refers to the rate at which energy is produced in an ecosystem, particularly through photosynthesis in plants.
- It includes gross primary productivity (total photosynthesis) and net primary productivity (photosynthesis minus respiration).
- Since productivity is essential for energy generation in ecosystems, it is considered a functional unit.
2. **Understanding Stratification**:
- Stratification refers to the vertical distribution of different species in an ecosystem.
- It describes how various organisms occupy different levels in a habitat (e.g., trees at the top, shrubs in the middle, and herbs at the bottom).
- Unlike the other options, stratification does not directly contribute to the energy flow or nutrient cycling in the ecosystem. Therefore, it is not considered a functional unit.
3. **Understanding Energy Flow**:
- Energy flow describes how energy moves through an ecosystem, starting from producers (plants) to consumers (herbivores and carnivores) and finally to decomposers.
- This process is crucial for maintaining the ecosystem's structure and function, making energy flow a functional unit.
4. **Understanding Decomposition**:
- Decomposition is the process by which dead organic matter is broken down by decomposers (like fungi and bacteria).
- This process recycles nutrients back into the ecosystem, making decomposition a vital functional unit.
5. **Conclusion**:
- After analyzing all options, we find that productivity, energy flow, and decomposition are all functional units of an ecosystem.
- Stratification, while important for understanding species distribution, does not function in the same way as the other three options.
Thus, the answer to the question is **B. Stratification**.
