Which of the following statements is incorrect?

Options

A. biomass decreases from first to fourth trophic level
B. energy content gradually increases from first to fourth trophic levels.
C. number of individuals decreases from first trophic level to fourth trophic level
D. Energy content gradually decreases from first to fourth trophic level

Answer

To determine which statement is incorrect regarding trophic levels, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Biomass**:
- Biomass refers to the total mass of living matter at each trophic level. In a typical food chain, biomass tends to decrease as you move from the first trophic level (producers) to the fourth trophic level (top predators).
- **Conclusion**: The statement "biomass decreases from 1st to 4th trophic level" is **correct**.

2. **Energy Content**:
- Energy content in an ecosystem generally decreases as you move up the trophic levels. This is due to the second law of thermodynamics, where energy is lost as heat during metabolic processes and not all energy is transferred to the next trophic level.
- **Conclusion**: The statement "energy content gradually increases from 1st to 4th trophic level" is **incorrect**.

3. **Number of Individuals**:
- Typically, the number of individuals decreases as you move from the first trophic level to the fourth trophic level. For example, there are many plants (producers) but fewer herbivores (primary consumers) and even fewer carnivores (secondary and tertiary consumers).
- **Conclusion**: The statement "number of individuals decreases from 1st trophic level to the 4th trophic level" is **correct**.

4. **Energy Content Again**:
- As previously discussed, energy content does not remain constant and is not accumulated; instead, it decreases as energy is transferred from one trophic level to another.
- **Conclusion**: The statement "energy content gradually decreases from the 1st trophic level to the 4th trophic level" is **correct**.

### Final Answer:
The incorrect statement among the options provided is **option B: Energy content gradually increases from 1st to 4th trophic level**.