What is a keystone species?

Options

A. A species which adds up to only a small proportion of the total biomass of a community, yet has a huge impact on the community's organisation and survival.
B. A common species that has plenty of biomass, yet has a fairly low impact on the community's organisation.
C. A rare species that has minimal impact on the biomass and on other species in the community.
D. A dominant species that constitutes a large proportion of the biomass and ~hjch affects many other species

Answer

### Step-by-Step Solution:

1. **Definition of Keystone Species**: A keystone species is defined as a species that has a disproportionately large impact on its environment relative to its abundance. This means that even if it constitutes a small portion of the total biomass of a community, its presence or absence can significantly affect the structure and function of that community.

2. **Characteristics of Keystone Species**:
- **Low Abundance and Biomass**: Keystone species typically have low numbers and biomass compared to dominant species in the ecosystem.
- **Significant Impact**: Despite their low abundance, they play a crucial role in maintaining the ecological balance. Their activities can influence the population dynamics of other species and the overall health of the ecosystem.

3. **Consequences of Removal**: The removal or significant decrease in the population of a keystone species can lead to drastic changes in the ecosystem. This can result in the decline or extinction of other species, leading to a loss of biodiversity and disruption of community structure.

4. **Examples of Keystone Species**:
- **Starfish (Pisaster)**: An example provided is the starfish Pisaster, which preys on mussels. If this starfish is removed from its habitat, the mussel population can explode, leading to the exclusion of other species that compete for resources.
- **Certain Fish Species**: Some fish species act as keystone species by serving as a critical food source for various animals during times of food scarcity.

5. **Conclusion**: Based on the definition and examples, the correct answer to the question about what constitutes a keystone species is:
- **Option A**: A species which adds up to only a small proportion of the total biomass of the community yet has a huge impact on the community's organization and survival.