Keystone species in an ecosystem are those:

Options

A. are present in maximum number
B. are most frequent
C. attain a large biomass
D. contribute to major ecosystem functions

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Keystone Species**: Keystone species are defined as species that have a disproportionately large effect on their environment relative to their abundance. They play a critical role in maintaining the structure of an ecological community.

2. **Evaluating the Options**: The question provides several options regarding the characteristics of keystone species. We need to analyze each option to determine which one correctly describes keystone species.

3. **Analyzing Option 1**: The first option states that keystone species are present in maximum number. This is incorrect because keystone species do not have to be the most numerous species in an ecosystem.

4. **Analyzing Option 2**: The second option claims that keystone species are the most frequent. This is also incorrect as frequency does not define a keystone species.

5. **Analyzing Option 3**: The third option suggests that keystone species attain a large biomass. This is misleading because biomass is often related to population density, and keystone species can be present in low numbers yet still have a significant impact on their ecosystem.

6. **Identifying the Correct Option**: The correct option states that keystone species contribute to major ecosystem functions. This means that their presence or absence can significantly affect the ecosystem's health and stability.

7. **Understanding the Impact of Keystone Species**: The removal of a keystone species can lead to dramatic changes in the ecosystem, potentially leading to the extinction of other species and altering the habitat structure.

8. **Examples of Keystone Species**: Common examples include predators that control herbivore populations, such as wolves in a forest ecosystem, and pollinators like honeybees, which are crucial for the reproduction of many plant species.

9. **Conclusion**: Therefore, keystone species are essential for the functioning of their ecosystems, and their loss can lead to significant ecological consequences.

**Final Answer**: Keystone species in an ecosystem are those that contribute to major ecosystem functions.

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