Keystone species in an ecosystem are those :

Options

A. Present in maximum number
B. That are not frequent
C. Attaining a large biomass
D. Contributing to ecosystem properties

Answer

### Step-by-Step Solution:

1. **Understanding Keystone Species**:
- Keystone species are organisms that play a crucial role in maintaining the structure of an ecological community. They have a disproportionately large impact on their environment relative to their abundance.

2. **Regulatory Effect**:
- Keystone species exert an important regulatory effect on other species in the community. This means they help control the population sizes of other species, which in turn affects the overall biodiversity of the ecosystem.

3. **Contribution to Ecosystem Properties**:
- These species contribute to the properties of the ecosystem. Their presence or absence can significantly alter the ecosystem's structure and function.

4. **Impact on Species Diversity**:
- Keystone species maintain high species diversity within a community. They often do this by reducing the density of strong competitors, allowing for a wider variety of species to coexist.

5. **Consequences of Absence**:
- If a keystone species is removed from an ecosystem, it can lead to dramatic changes, potentially causing the ecosystem to collapse or cease functioning effectively.

6. **Evaluating Options**:
- When presented with options regarding the definition of keystone species, we need to identify which option correctly reflects their role. The correct option will highlight their importance in contributing to ecosystem properties and maintaining diversity, rather than simply being abundant or large in biomass.

7. **Final Answer**:
- The correct answer is that keystone species are those that contribute to the ecosystem properties.