A keystone species is a species that isn't necessarily abundant, but has a dramatic effect on the overall function of an ecosystem. An example of this would be the honey bee. Honeybees are responsible for the large majority of plant pollination. Without bees to pollinate these plants, not only will plant species die off, but so will the herbivores that rely on those plants as their main food source. Additionally, this will affect secondary consumers who rely on those herbivores as a primary food source. This domino effect continues throughout the food chain and eventually destroys an ecosystem entirely. As you can see, the removal of the small and non-abundant honey bee can wreak havoc on an ecosystem therefore, it is considered a keystone species.
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