Distinguish between a dominant and keystone species. Add a note on the crucial role played by the keystone species on the community structure and structure of a food web.

Distinguishing Between Dominant and Keystone Species

Both dominant and keystone species are important components of an ecosystem, but their influence differs significantly.

Feature	Dominant Species	Keystone Species
Abundance	High; often the most numerous or biomass-rich species.	Low; often not the most abundant.
Influence	Influence primarily through competition and resource utilization.	Influence disproportionately large relative to their abundance; affects community structure.
Removal Impact	Removal may lead to shifts in species composition, but the ecosystem generally remains functional.	Removal can cause dramatic changes in community structure, potentially leading to ecosystem collapse.
Role	Contribute to ecosystem processes through sheer presence and consumption.	Maintain biodiversity and ecosystem function through critical interactions.
Example	Trees in a forest (e.g., Oak trees in a temperate forest).	Sea otters, starfish (Pisaster ochraceus), beavers, African elephants.

The Crucial Role of Keystone Species

Impact on Community Structure

Keystone species exert a powerful influence on community structure through various mechanisms:

• Predation: Keystone predators control populations of prey species, preventing any single prey species from becoming dominant and outcompeting others. For example, sea otters (Enhydra lutris) prey on sea urchins. Without sea otters, sea urchin populations explode, leading to overgrazing of kelp forests, transforming them into barren urchin "deserts".
• Mutualism: Some keystone species engage in mutualistic relationships that are essential for the survival of other species. For instance, fig wasps are keystone mutualists in many tropical ecosystems, as they are the sole pollinators of fig trees, which provide food and shelter for a wide range of animals.
• Engineering: Keystone engineers physically modify the environment, creating habitats for other species. Beavers (Castor canadensis) are classic examples, building dams that create wetlands, providing habitat for numerous aquatic and terrestrial species.
• Seed Dispersal: Certain frugivores act as keystone species by dispersing seeds of plants, maintaining forest regeneration and diversity.

Impact on Food Web Structure

Keystone species often occupy critical positions within food webs, influencing energy flow and trophic interactions. Their removal can trigger cascading effects throughout the food web:

• Trophic Cascades: The removal of a keystone predator can lead to a trophic cascade, where the populations of its prey increase, leading to a decrease in the populations of species lower down the food chain. The sea otter-sea urchin-kelp forest example is a classic illustration of a trophic cascade.
• Food Web Complexity: Keystone species often maintain the complexity of food webs by supporting a diverse range of species. Their presence prevents the simplification of food webs, making ecosystems more resilient to disturbances.
• Functional Redundancy: While keystone species are not necessarily functionally redundant (meaning their role cannot be easily filled by another species), their presence often supports functional redundancy within the ecosystem, enhancing its stability.

The loss of a keystone species can lead to a shift in the ecosystem's state, potentially resulting in a less diverse and less productive community. This highlights the importance of identifying and protecting keystone species for effective conservation management.