You're referring to "keystone species," which are organisms that have a disproportionately large impact on their environment relative to their abundance. Their presence and role in the ecosystem are crucial for maintaining the structure and health of the ecological community.
Examples:
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Wolves in Yellowstone National Park: The reintroduction of gray wolves to Yellowstone in 1995 is a classic example of a keystone species. Wolves help regulate the population of large herbivores like elk. By controlling the elk population, wolves allow for the regeneration of vegetation, especially in riparian (riverbank) areas. This, in turn, benefits other species, including beavers, birds, and various plant species, creating a healthier ecosystem.
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Sea Otters in Coastal Ecosystems: Sea otters play a critical role in maintaining the health of kelp forest ecosystems. They prey on sea urchins, which are herbivores that can overgraze kelp. When sea otter populations decline, sea urchin populations can explode, leading to "urchin barrens" where kelp forests are devastated. By keeping sea urchin populations in check, sea otters help sustain diverse marine life that depends on kelp forests for habitat and food.
The loss of these keystone species can lead to significant shifts in ecosystem structure and function, highlighting their importance in maintaining biodiversity and ecological balance.