Keystone species in an ecosystem are those:

Options

A. The number is maximum in an ecosystem
B. They aren't required in an ecosystem
C. They have the largest biomass in an ecosystem .
D. They are main contributors to ecosystem properties

Answer

To solve the question about keystone species in an ecosystem, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Keystone Species**:
- Keystone species are defined as species that have a disproportionately large effect on their environment relative to their abundance. This means that their presence or absence can significantly impact the structure and function of the ecosystem.

2. **Regulatory Effect**:
- Keystone species exert important regulatory effects on other species within the community. This means they help maintain the balance of the ecosystem by influencing the population sizes of other species.

3. **Contribution to Ecosystem Properties**:
- These species contribute to the overall properties of the ecosystem. They play a crucial role in maintaining the health and stability of the ecosystem.

4. **Maintaining Species Diversity**:
- Keystone species help maintain higher species diversity in a community. They do this by reducing the density of strong competitors, which allows other species to thrive.

5. **Evaluating Options**:
- Now, we need to evaluate the provided options in the question:
- Option 1: "The number is maximum in an ecosystem." (Incorrect)
- Option 2: "They aren't required in an ecosystem." (Incorrect)
- Option 3: "They have the largest biomass in the ecosystem." (Incorrect)
- Option 4: "They are the main contributors to ecosystem properties." (Correct)

6. **Conclusion**:
- Based on the understanding of keystone species and their roles, the correct answer is that keystone species are the main contributors to ecosystem properties.

### Final Answer:
Keystone species in an ecosystem are those that are the main contributors to ecosystem properties.