**Keystone Species:**

- The correct answer is b) Keystone species.

**Definition of Keystone Species:**

- Keystone species are those species of plants or animals that play a crucial role in controlling the relative abundance of other species in a community. They have a disproportionate impact on the ecosystem, despite being relatively low in abundance or biomass.

**Role of Keystone Species:**

- Keystone species have a significant influence on the structure and function of their ecosystem. Their presence or absence can lead to dramatic changes in the diversity and composition of the community.

**Control of Relative Abundance:**

- Keystone species control the relative abundance of other species through various mechanisms:

1. Predation Control: Some keystone species act as top predators and help regulate the population size of their prey species. By keeping the prey population in check, they prevent any one species from dominating the community.

2. Ecosystem Engineering: Certain keystone species can modify the physical environment, creating and maintaining habitats that support a wide range of other species. For example, beavers build dams that create wetlands, benefiting numerous plant and animal species.

3. Mutualistic Interactions: Some keystone species engage in mutualistic relationships with other organisms. They provide essential services or resources that are crucial for the survival and abundance of the associated species. For instance, certain plants act as keystone species by providing food or shelter for pollinators, thereby influencing the abundance and distribution of pollinator-dependent plants.

**Importance of Keystone Species:**

- Keystone species are vital for the stability and resilience of ecosystems. Their removal or decline can lead to cascading effects, causing imbalances and potentially disrupting the entire community.

- By controlling the abundance of other species, keystone species help promote biodiversity and maintain a healthy and functioning ecosystem.

- Understanding the role of keystone species is crucial for effective conservation and management strategies, as their preservation is necessary to ensure the stability and sustainability of ecosystems.

- Examples of keystone species include sea otters, wolves, prairie dogs, and certain plant species like mangroves and seagrasses.

In conclusion, keystone species play a vital role in controlling the relative abundance of other species in a community. Through mechanisms such as predation control, ecosystem engineering, and mutualistic interactions, they have a disproportionate impact on the structure and function of ecosystems. Their preservation is essential for maintaining biodiversity and the overall health of ecosystems.

A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch. While the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity. It became a popular concept in conservation biology, alongside flagship and umbrella species. Although the concept is valued as a descriptor for particularly strong inter-species interactions, and it has allowed easier communication between ecologists and conservation policy-makers, it has been criticized for oversimplifying complex ecological systems.