Keystone species play a crucial role in maintaining the structure and stability of an ecosystem. Their presence or absence significantly impacts other species and the overall ecological balance. By regulating populations of prey or competitors, keystone species help sustain biodiversity. For example, a predator can control herbivore populations, preventing overgrazing and allowing various plant species to thrive. The loss of a keystone species could lead to dramatic shifts in the ecosystem, highlighting its essential role in ecological health.