The sudden loss of which organism would necessarily cause the loss of most or all of a food web?

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A keystone species is one whose presence is critical to maintaining the structure and function of an ecosystem. Their removal can cause a cascading effect of extinctions and disrupt the food web. Removing a producer or secondary consumer may affect some parts of the food web, but it is unlikely to cause the loss of most or all of it. Similarly, a mutualistic species may have important interactions with other species but is unlikely to have such a widespread effect on the food web.

The Sudden Loss of Which Organism Would Necessarily Cause the Loss