To solve the question "Which of the following parameters decreases as succession proceeds?", we can analyze each option provided:
1. **Humus Content**:
- During ecological succession, particularly in primary succession, the accumulation of organic matter increases as pioneer species die and decompose, leading to an increase in humus content. Therefore, this parameter does not decrease.
2. **Species Diversity**:
- As succession progresses, the number of species typically increases due to the establishment of new species that can thrive in the changing environment. Therefore, species diversity generally increases, not decreases.
3. **Net Community Productivity**:
- Net community productivity refers to the rate at which plants and other producers create organic material through photosynthesis, minus the rate at which they respire. As succession proceeds, especially in a mature ecosystem, productivity can increase due to a greater variety of species and more complex interactions. Thus, this parameter does not decrease.
4. **Biomass**:
- Biomass refers to the total mass of living matter in a given area. In the early stages of succession, biomass may be low, but as succession progresses, biomass typically increases due to the growth of plants and the establishment of a more complex community. However, in certain contexts, particularly in the later stages of succession, the biomass may stabilize or even decrease due to factors such as resource depletion or competition among species.
Based on this analysis, the parameter that decreases as succession proceeds is **Biomass**.
**Final Answer**: Biomass decreases as succession proceeds.
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